// Copyright 2021 The Jujutsu Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![allow(missing_docs)]

use std::collections::HashMap;
use std::convert::Infallible;
use std::ops::Range;
use std::path::Path;
use std::rc::Rc;
use std::str::FromStr;
use std::sync::Arc;
use std::{error, fmt};

use itertools::Itertools;
use once_cell::sync::Lazy;
use pest::iterators::{Pair, Pairs};
use pest::pratt_parser::{Assoc, Op, PrattParser};
use pest::Parser;
use pest_derive::Parser;
use thiserror::Error;

use crate::backend::{BackendError, BackendResult, ChangeId, CommitId, ObjectId};
use crate::commit::Commit;
use crate::git::{self, get_local_git_tracking_branch};
use crate::hex_util::to_forward_hex;
use crate::index::{HexPrefix, PrefixResolution};
use crate::op_store::WorkspaceId;
use crate::repo::Repo;
use crate::repo_path::{FsPathParseError, RepoPath};
use crate::revset_graph::RevsetGraphEdge;
use crate::store::Store;

/// Error occurred during symbol resolution.
#[derive(Debug, Error)]
pub enum RevsetResolutionError {
    #[error("Revision \"{name}\" doesn't exist")]
    NoSuchRevision {
        name: String,
        candidates: Vec<String>,
    },
    #[error("An empty string is not a valid revision")]
    EmptyString,
    #[error("Commit ID prefix \"{0}\" is ambiguous")]
    AmbiguousCommitIdPrefix(String),
    #[error("Change ID prefix \"{0}\" is ambiguous")]
    AmbiguousChangeIdPrefix(String),
    #[error("Unexpected error from store: {0}")]
    StoreError(#[source] BackendError),
}

/// Error occurred during revset evaluation.
#[derive(Debug, Error)]
pub enum RevsetEvaluationError {
    #[error("Unexpected error from store: {0}")]
    StoreError(#[source] BackendError),
    #[error("{0}")]
    Other(String),
}

#[derive(Parser)]
#[grammar = "revset.pest"]
pub struct RevsetParser;

#[derive(Debug)]
pub struct RevsetParseError {
    kind: RevsetParseErrorKind,
    pest_error: Option<Box<pest::error::Error<Rule>>>,
    origin: Option<Box<RevsetParseError>>,
}

#[derive(Debug, Error, PartialEq, Eq)]
pub enum RevsetParseErrorKind {
    #[error("Syntax error")]
    SyntaxError,
    #[error("'{op}' is not a postfix operator")]
    NotPostfixOperator {
        op: String,
        similar_op: String,
        description: String,
    },
    #[error("'{op}' is not an infix operator")]
    NotInfixOperator {
        op: String,
        similar_op: String,
        description: String,
    },
    #[error(r#"Revset function "{name}" doesn't exist"#)]
    NoSuchFunction {
        name: String,
        candidates: Vec<String>,
    },
    #[error("Invalid arguments to revset function \"{name}\": {message}")]
    InvalidFunctionArguments { name: String, message: String },
    #[error("Invalid file pattern: {0}")]
    FsPathParseError(#[source] FsPathParseError),
    #[error("Cannot resolve file pattern without workspace")]
    FsPathWithoutWorkspace,
    #[error("Redefinition of function parameter")]
    RedefinedFunctionParameter,
    #[error(r#"Alias "{0}" cannot be expanded"#)]
    BadAliasExpansion(String),
    #[error(r#"Alias "{0}" expanded recursively"#)]
    RecursiveAlias(String),
}

impl RevsetParseError {
    fn new(kind: RevsetParseErrorKind) -> Self {
        RevsetParseError {
            kind,
            pest_error: None,
            origin: None,
        }
    }

    fn with_span(kind: RevsetParseErrorKind, span: pest::Span<'_>) -> Self {
        let err = pest::error::Error::new_from_span(
            pest::error::ErrorVariant::CustomError {
                message: kind.to_string(),
            },
            span,
        );
        RevsetParseError {
            kind,
            pest_error: Some(Box::new(err)),
            origin: None,
        }
    }

    fn with_span_and_origin(
        kind: RevsetParseErrorKind,
        span: pest::Span<'_>,
        origin: Self,
    ) -> Self {
        let err = pest::error::Error::new_from_span(
            pest::error::ErrorVariant::CustomError {
                message: kind.to_string(),
            },
            span,
        );
        RevsetParseError {
            kind,
            pest_error: Some(Box::new(err)),
            origin: Some(Box::new(origin)),
        }
    }

    pub fn kind(&self) -> &RevsetParseErrorKind {
        &self.kind
    }

    /// Original parsing error which typically occurred in an alias expression.
    pub fn origin(&self) -> Option<&Self> {
        self.origin.as_deref()
    }
}

impl From<pest::error::Error<Rule>> for RevsetParseError {
    fn from(err: pest::error::Error<Rule>) -> Self {
        RevsetParseError {
            kind: RevsetParseErrorKind::SyntaxError,
            pest_error: Some(Box::new(err)),
            origin: None,
        }
    }
}

impl fmt::Display for RevsetParseError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(err) = &self.pest_error {
            err.fmt(f)
        } else {
            self.kind.fmt(f)
        }
    }
}

impl error::Error for RevsetParseError {
    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        if let Some(e) = self.origin() {
            return Some(e as &dyn error::Error);
        }
        match &self.kind {
            // SyntaxError is a wrapper for pest::error::Error.
            RevsetParseErrorKind::SyntaxError => {
                self.pest_error.as_ref().map(|e| e as &dyn error::Error)
            }
            // Otherwise the kind represents this error.
            e => e.source(),
        }
    }
}

// assumes index has less than u64::MAX entries.
pub const GENERATION_RANGE_FULL: Range<u64> = 0..u64::MAX;
pub const GENERATION_RANGE_EMPTY: Range<u64> = 0..0;

/// Symbol or function to be resolved to `CommitId`s.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RevsetCommitRef {
    Symbol(String),
    VisibleHeads,
    Branches(String),
    RemoteBranches {
        branch_needle: String,
        remote_needle: String,
    },
    Tags,
    GitRefs,
    GitHead,
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum RevsetFilterPredicate {
    /// Commits with number of parents in the range.
    ParentCount(Range<u32>),
    /// Commits with description containing the needle.
    Description(String),
    /// Commits with author's name or email containing the needle.
    Author(String),
    /// Commits with committer's name or email containing the needle.
    Committer(String),
    /// Commits modifying the paths specified by the pattern.
    File(Option<Vec<RepoPath>>), // TODO: embed matcher expression?
    /// Commits with conflicts
    HasConflict,
}

#[derive(Debug, PartialEq, Eq, Clone)]
pub enum RevsetExpression {
    None,
    All,
    Commits(Vec<CommitId>),
    CommitRef(RevsetCommitRef),
    Ancestors {
        heads: Rc<RevsetExpression>,
        generation: Range<u64>,
        is_legacy: bool,
    },
    Descendants {
        roots: Rc<RevsetExpression>,
        generation: Range<u64>,
        is_legacy: bool,
    },
    // Commits that are ancestors of "heads" but not ancestors of "roots"
    Range {
        roots: Rc<RevsetExpression>,
        heads: Rc<RevsetExpression>,
        generation: Range<u64>,
    },
    // Commits that are descendants of "roots" and ancestors of "heads"
    DagRange {
        roots: Rc<RevsetExpression>,
        heads: Rc<RevsetExpression>,
        is_legacy: bool,
        // TODO: maybe add generation_from_roots/heads?
    },
    Heads(Rc<RevsetExpression>),
    Roots(Rc<RevsetExpression>),
    Latest {
        candidates: Rc<RevsetExpression>,
        count: usize,
    },
    Filter(RevsetFilterPredicate),
    /// Marker for subtree that should be intersected as filter.
    AsFilter(Rc<RevsetExpression>),
    Present(Rc<RevsetExpression>),
    NotIn(Rc<RevsetExpression>),
    Union(Rc<RevsetExpression>, Rc<RevsetExpression>),
    Intersection(Rc<RevsetExpression>, Rc<RevsetExpression>),
    Difference(Rc<RevsetExpression>, Rc<RevsetExpression>),
}

impl RevsetExpression {
    pub fn none() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::None)
    }

    pub fn all() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::All)
    }

    pub fn symbol(value: String) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::Symbol(value)))
    }

    pub fn commit(commit_id: CommitId) -> Rc<RevsetExpression> {
        RevsetExpression::commits(vec![commit_id])
    }

    pub fn commits(commit_ids: Vec<CommitId>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Commits(commit_ids))
    }

    pub fn visible_heads() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::VisibleHeads))
    }

    pub fn branches(needle: String) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::Branches(
            needle,
        )))
    }

    pub fn remote_branches(branch_needle: String, remote_needle: String) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(
            RevsetCommitRef::RemoteBranches {
                branch_needle,
                remote_needle,
            },
        ))
    }

    pub fn tags() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::Tags))
    }

    pub fn git_refs() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::GitRefs))
    }

    pub fn git_head() -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::GitHead))
    }

    pub fn latest(self: &Rc<RevsetExpression>, count: usize) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Latest {
            candidates: self.clone(),
            count,
        })
    }

    pub fn filter(predicate: RevsetFilterPredicate) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Filter(predicate))
    }

    /// Commits in `self` that don't have descendants in `self`.
    pub fn heads(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Heads(self.clone()))
    }

    /// Commits in `self` that don't have ancestors in `self`.
    pub fn roots(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Roots(self.clone()))
    }

    /// Parents of `self`.
    pub fn parents(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Ancestors {
            heads: self.clone(),
            generation: 1..2,
            is_legacy: false,
        })
    }

    /// Ancestors of `self`, including `self`.
    pub fn ancestors(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Ancestors {
            heads: self.clone(),
            generation: GENERATION_RANGE_FULL,
            is_legacy: false,
        })
    }
    fn legacy_ancestors(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Ancestors {
            heads: self.clone(),
            generation: GENERATION_RANGE_FULL,
            is_legacy: true,
        })
    }

    /// Children of `self`.
    pub fn children(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Descendants {
            roots: self.clone(),
            generation: 1..2,
            is_legacy: false,
        })
    }

    /// Descendants of `self`, including `self`.
    pub fn descendants(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Descendants {
            roots: self.clone(),
            generation: GENERATION_RANGE_FULL,
            is_legacy: false,
        })
    }
    fn legacy_descendants(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Descendants {
            roots: self.clone(),
            generation: GENERATION_RANGE_FULL,
            is_legacy: true,
        })
    }

    /// Commits that are descendants of `self` and ancestors of `heads`, both
    /// inclusive.
    pub fn dag_range_to(
        self: &Rc<RevsetExpression>,
        heads: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::DagRange {
            roots: self.clone(),
            heads: heads.clone(),
            is_legacy: false,
        })
    }
    pub fn legacy_dag_range_to(
        self: &Rc<RevsetExpression>,
        heads: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::DagRange {
            roots: self.clone(),
            heads: heads.clone(),
            is_legacy: true,
        })
    }

    /// Connects any ancestors and descendants in the set by adding the commits
    /// between them.
    pub fn connected(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        self.dag_range_to(self)
    }

    /// Commits reachable from `heads` but not from `self`.
    pub fn range(
        self: &Rc<RevsetExpression>,
        heads: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Range {
            roots: self.clone(),
            heads: heads.clone(),
            generation: GENERATION_RANGE_FULL,
        })
    }

    /// Commits that are not in `self`, i.e. the complement of `self`.
    pub fn negated(self: &Rc<RevsetExpression>) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::NotIn(self.clone()))
    }

    /// Commits that are in `self` or in `other` (or both).
    pub fn union(
        self: &Rc<RevsetExpression>,
        other: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Union(self.clone(), other.clone()))
    }

    /// Commits that are in any of the `expressions`.
    pub fn union_all(expressions: &[Rc<RevsetExpression>]) -> Rc<RevsetExpression> {
        match expressions {
            [] => Self::none(),
            [expression] => expression.clone(),
            _ => {
                // Build balanced tree to minimize the recursion depth.
                let (left, right) = expressions.split_at(expressions.len() / 2);
                Self::union(&Self::union_all(left), &Self::union_all(right))
            }
        }
    }

    /// Commits that are in `self` and in `other`.
    pub fn intersection(
        self: &Rc<RevsetExpression>,
        other: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Intersection(self.clone(), other.clone()))
    }

    /// Commits that are in `self` but not in `other`.
    pub fn minus(
        self: &Rc<RevsetExpression>,
        other: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        Rc::new(RevsetExpression::Difference(self.clone(), other.clone()))
    }

    pub fn resolve(
        self: Rc<Self>,
        repo: &dyn Repo,
    ) -> Result<ResolvedExpression, RevsetResolutionError> {
        let symbol_resolver = FailingSymbolResolver;
        resolve_symbols(repo, self, &symbol_resolver)
            .map(|expression| resolve_visibility(repo, &expression))
    }

    pub fn resolve_user_expression(
        self: Rc<Self>,
        repo: &dyn Repo,
        symbol_resolver: &dyn SymbolResolver,
    ) -> Result<ResolvedExpression, RevsetResolutionError> {
        resolve_symbols(repo, self, symbol_resolver)
            .map(|expression| resolve_visibility(repo, &expression))
    }
}

#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ResolvedPredicateExpression {
    /// Pure filter predicate.
    Filter(RevsetFilterPredicate),
    /// Set expression to be evaluated as filter. This is typically a subtree
    /// node of `Union` with a pure filter predicate.
    Set(Box<ResolvedExpression>),
    NotIn(Box<ResolvedPredicateExpression>),
    Union(
        Box<ResolvedPredicateExpression>,
        Box<ResolvedPredicateExpression>,
    ),
}

/// Describes evaluation plan of revset expression.
///
/// Unlike `RevsetExpression`, this doesn't contain unresolved symbols or `View`
/// properties.
///
/// Use `RevsetExpression` API to build a query programmatically.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ResolvedExpression {
    Commits(Vec<CommitId>),
    Ancestors {
        heads: Box<ResolvedExpression>,
        generation: Range<u64>,
    },
    /// Commits that are ancestors of `heads` but not ancestors of `roots`.
    Range {
        roots: Box<ResolvedExpression>,
        heads: Box<ResolvedExpression>,
        generation: Range<u64>,
    },
    /// Commits that are descendants of `roots` and ancestors of `heads`.
    DagRange {
        roots: Box<ResolvedExpression>,
        heads: Box<ResolvedExpression>,
        generation_from_roots: Range<u64>,
    },
    Heads(Box<ResolvedExpression>),
    Roots(Box<ResolvedExpression>),
    Latest {
        candidates: Box<ResolvedExpression>,
        count: usize,
    },
    Union(Box<ResolvedExpression>, Box<ResolvedExpression>),
    /// Intersects `candidates` with `predicate` by filtering.
    FilterWithin {
        candidates: Box<ResolvedExpression>,
        predicate: ResolvedPredicateExpression,
    },
    /// Intersects expressions by merging.
    Intersection(Box<ResolvedExpression>, Box<ResolvedExpression>),
    Difference(Box<ResolvedExpression>, Box<ResolvedExpression>),
}

impl ResolvedExpression {
    pub fn evaluate<'index>(
        &self,
        repo: &'index dyn Repo,
    ) -> Result<Box<dyn Revset<'index> + 'index>, RevsetEvaluationError> {
        repo.index().evaluate_revset(self, repo.store())
    }
}

#[derive(Clone, Debug, Default)]
pub struct RevsetAliasesMap {
    symbol_aliases: HashMap<String, String>,
    function_aliases: HashMap<String, (Vec<String>, String)>,
}

impl RevsetAliasesMap {
    pub fn new() -> Self {
        Self::default()
    }

    /// Adds new substitution rule `decl = defn`.
    ///
    /// Returns error if `decl` is invalid. The `defn` part isn't checked. A bad
    /// `defn` will be reported when the alias is substituted.
    pub fn insert(
        &mut self,
        decl: impl AsRef<str>,
        defn: impl Into<String>,
    ) -> Result<(), RevsetParseError> {
        match RevsetAliasDeclaration::parse(decl.as_ref())? {
            RevsetAliasDeclaration::Symbol(name) => {
                self.symbol_aliases.insert(name, defn.into());
            }
            RevsetAliasDeclaration::Function(name, params) => {
                self.function_aliases.insert(name, (params, defn.into()));
            }
        }
        Ok(())
    }

    fn get_symbol(&self, name: &str) -> Option<(RevsetAliasId, &str)> {
        self.symbol_aliases
            .get_key_value(name)
            .map(|(name, defn)| (RevsetAliasId::Symbol(name), defn.as_ref()))
    }

    fn get_function(&self, name: &str) -> Option<(RevsetAliasId, &[String], &str)> {
        self.function_aliases
            .get_key_value(name)
            .map(|(name, (params, defn))| {
                (
                    RevsetAliasId::Function(name),
                    params.as_ref(),
                    defn.as_ref(),
                )
            })
    }
}

/// Parsed declaration part of alias rule.
#[derive(Clone, Debug)]
enum RevsetAliasDeclaration {
    Symbol(String),
    Function(String, Vec<String>),
}

impl RevsetAliasDeclaration {
    fn parse(source: &str) -> Result<Self, RevsetParseError> {
        let mut pairs = RevsetParser::parse(Rule::alias_declaration, source)?;
        let first = pairs.next().unwrap();
        match first.as_rule() {
            Rule::identifier => Ok(RevsetAliasDeclaration::Symbol(first.as_str().to_owned())),
            Rule::function_name => {
                let name = first.as_str().to_owned();
                let params_pair = pairs.next().unwrap();
                let params_span = params_pair.as_span();
                let params = params_pair
                    .into_inner()
                    .map(|pair| match pair.as_rule() {
                        Rule::identifier => pair.as_str().to_owned(),
                        r => panic!("unexpected formal parameter rule {r:?}"),
                    })
                    .collect_vec();
                if params.iter().all_unique() {
                    Ok(RevsetAliasDeclaration::Function(name, params))
                } else {
                    Err(RevsetParseError::with_span(
                        RevsetParseErrorKind::RedefinedFunctionParameter,
                        params_span,
                    ))
                }
            }
            r => panic!("unexpected alias declaration rule {r:?}"),
        }
    }
}

/// Borrowed reference to identify alias expression.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum RevsetAliasId<'a> {
    Symbol(&'a str),
    Function(&'a str),
}

impl fmt::Display for RevsetAliasId<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            RevsetAliasId::Symbol(name) => write!(f, "{name}"),
            RevsetAliasId::Function(name) => write!(f, "{name}()"),
        }
    }
}

#[derive(Clone, Copy, Debug)]
struct ParseState<'a> {
    aliases_map: &'a RevsetAliasesMap,
    aliases_expanding: &'a [RevsetAliasId<'a>],
    locals: &'a HashMap<&'a str, Rc<RevsetExpression>>,
    workspace_ctx: Option<&'a RevsetWorkspaceContext<'a>>,
}

impl ParseState<'_> {
    fn with_alias_expanding<T>(
        self,
        id: RevsetAliasId<'_>,
        locals: &HashMap<&str, Rc<RevsetExpression>>,
        span: pest::Span<'_>,
        f: impl FnOnce(ParseState<'_>) -> Result<T, RevsetParseError>,
    ) -> Result<T, RevsetParseError> {
        // The stack should be short, so let's simply do linear search and duplicate.
        if self.aliases_expanding.contains(&id) {
            return Err(RevsetParseError::with_span(
                RevsetParseErrorKind::RecursiveAlias(id.to_string()),
                span,
            ));
        }
        let mut aliases_expanding = self.aliases_expanding.to_vec();
        aliases_expanding.push(id);
        let expanding_state = ParseState {
            aliases_map: self.aliases_map,
            aliases_expanding: &aliases_expanding,
            locals,
            workspace_ctx: self.workspace_ctx,
        };
        f(expanding_state).map_err(|e| {
            RevsetParseError::with_span_and_origin(
                RevsetParseErrorKind::BadAliasExpansion(id.to_string()),
                span,
                e,
            )
        })
    }
}

fn parse_program(
    revset_str: &str,
    state: ParseState,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    let mut pairs = RevsetParser::parse(Rule::program, revset_str)?;
    let first = pairs.next().unwrap();
    parse_expression_rule(first.into_inner(), state)
}

fn parse_expression_rule(
    pairs: Pairs<Rule>,
    state: ParseState,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    fn not_postfix_op(
        op: &Pair<Rule>,
        similar_op: impl Into<String>,
        description: impl Into<String>,
    ) -> RevsetParseError {
        RevsetParseError::with_span(
            RevsetParseErrorKind::NotPostfixOperator {
                op: op.as_str().to_owned(),
                similar_op: similar_op.into(),
                description: description.into(),
            },
            op.as_span(),
        )
    }

    fn not_infix_op(
        op: &Pair<Rule>,
        similar_op: impl Into<String>,
        description: impl Into<String>,
    ) -> RevsetParseError {
        RevsetParseError::with_span(
            RevsetParseErrorKind::NotInfixOperator {
                op: op.as_str().to_owned(),
                similar_op: similar_op.into(),
                description: description.into(),
            },
            op.as_span(),
        )
    }

    static PRATT: Lazy<PrattParser<Rule>> = Lazy::new(|| {
        PrattParser::new()
            .op(Op::infix(Rule::union_op, Assoc::Left)
                | Op::infix(Rule::compat_add_op, Assoc::Left))
            .op(Op::infix(Rule::intersection_op, Assoc::Left)
                | Op::infix(Rule::difference_op, Assoc::Left)
                | Op::infix(Rule::compat_sub_op, Assoc::Left))
            .op(Op::prefix(Rule::negate_op))
            // Ranges can't be nested without parentheses. Associativity doesn't matter.
            .op(Op::infix(Rule::dag_range_op, Assoc::Left)
                | Op::infix(Rule::legacy_dag_range_op, Assoc::Left)
                | Op::infix(Rule::range_op, Assoc::Left))
            .op(Op::prefix(Rule::dag_range_pre_op)
                | Op::prefix(Rule::legacy_dag_range_pre_op)
                | Op::prefix(Rule::range_pre_op))
            .op(Op::postfix(Rule::dag_range_post_op)
                | Op::postfix(Rule::legacy_dag_range_post_op)
                | Op::postfix(Rule::range_post_op))
            // Neighbors
            .op(Op::postfix(Rule::parents_op)
                | Op::postfix(Rule::children_op)
                | Op::postfix(Rule::compat_parents_op))
    });
    PRATT
        .map_primary(|primary| parse_primary_rule(primary, state))
        .map_prefix(|op, rhs| match op.as_rule() {
            Rule::negate_op => Ok(rhs?.negated()),
            Rule::dag_range_pre_op | Rule::range_pre_op => Ok(rhs?.ancestors()),
            Rule::legacy_dag_range_pre_op => Ok(rhs?.legacy_ancestors()),
            r => panic!("unexpected prefix operator rule {r:?}"),
        })
        .map_postfix(|lhs, op| match op.as_rule() {
            Rule::dag_range_post_op => Ok(lhs?.descendants()),
            Rule::legacy_dag_range_post_op => Ok(lhs?.legacy_descendants()),
            Rule::range_post_op => Ok(lhs?.range(&RevsetExpression::visible_heads())),
            Rule::parents_op => Ok(lhs?.parents()),
            Rule::children_op => Ok(lhs?.children()),
            Rule::compat_parents_op => Err(not_postfix_op(&op, "-", "parents")),
            r => panic!("unexpected postfix operator rule {r:?}"),
        })
        .map_infix(|lhs, op, rhs| match op.as_rule() {
            Rule::union_op => Ok(lhs?.union(&rhs?)),
            Rule::compat_add_op => Err(not_infix_op(&op, "|", "union")),
            Rule::intersection_op => Ok(lhs?.intersection(&rhs?)),
            Rule::difference_op => Ok(lhs?.minus(&rhs?)),
            Rule::compat_sub_op => Err(not_infix_op(&op, "~", "difference")),
            Rule::dag_range_op => Ok(lhs?.dag_range_to(&rhs?)),
            Rule::legacy_dag_range_op => Ok(lhs?.legacy_dag_range_to(&rhs?)),
            Rule::range_op => Ok(lhs?.range(&rhs?)),
            r => panic!("unexpected infix operator rule {r:?}"),
        })
        .parse(pairs)
}

fn parse_primary_rule(
    pair: Pair<Rule>,
    state: ParseState,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    let span = pair.as_span();
    let mut pairs = pair.into_inner();
    let first = pairs.next().unwrap();
    match first.as_rule() {
        Rule::expression => parse_expression_rule(first.into_inner(), state),
        Rule::function_name => {
            let arguments_pair = pairs.next().unwrap();
            parse_function_expression(first, arguments_pair, state, span)
        }
        Rule::symbol => parse_symbol_rule(first.into_inner(), state),
        _ => {
            panic!("unexpected revset parse rule: {:?}", first.as_str());
        }
    }
}

fn parse_symbol_rule(
    mut pairs: Pairs<Rule>,
    state: ParseState,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    let first = pairs.next().unwrap();
    match first.as_rule() {
        Rule::identifier => {
            let name = first.as_str();
            if let Some(expr) = state.locals.get(name) {
                Ok(expr.clone())
            } else if let Some((id, defn)) = state.aliases_map.get_symbol(name) {
                let locals = HashMap::new(); // Don't spill out the current scope
                state.with_alias_expanding(id, &locals, first.as_span(), |state| {
                    parse_program(defn, state)
                })
            } else {
                Ok(RevsetExpression::symbol(name.to_owned()))
            }
        }
        Rule::literal_string => {
            return Ok(RevsetExpression::symbol(
                first
                    .as_str()
                    .strip_prefix('"')
                    .unwrap()
                    .strip_suffix('"')
                    .unwrap()
                    .to_owned(),
            ));
        }
        _ => {
            panic!("unexpected symbol parse rule: {:?}", first.as_str());
        }
    }
}

fn parse_function_expression(
    name_pair: Pair<Rule>,
    arguments_pair: Pair<Rule>,
    state: ParseState,
    primary_span: pest::Span<'_>,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    let name = name_pair.as_str();
    if let Some((id, params, defn)) = state.aliases_map.get_function(name) {
        // Resolve arguments in the current scope, and pass them in to the alias
        // expansion scope.
        let (required, optional) =
            expect_named_arguments_vec(name, &[], arguments_pair, params.len(), params.len())?;
        assert!(optional.is_empty());
        let args: Vec<_> = required
            .into_iter()
            .map(|arg| parse_expression_rule(arg.into_inner(), state))
            .try_collect()?;
        let locals = params.iter().map(|s| s.as_str()).zip(args).collect();
        state.with_alias_expanding(id, &locals, primary_span, |state| {
            parse_program(defn, state)
        })
    } else if let Some(func) = BUILTIN_FUNCTION_MAP.get(name) {
        func(name, arguments_pair, state)
    } else {
        Err(RevsetParseError::with_span(
            RevsetParseErrorKind::NoSuchFunction {
                name: name.to_owned(),
                candidates: collect_similar(name, &collect_function_names(state.aliases_map)),
            },
            name_pair.as_span(),
        ))
    }
}

fn collect_function_names(aliases_map: &RevsetAliasesMap) -> Vec<String> {
    let mut names = BUILTIN_FUNCTION_MAP
        .keys()
        .map(|&n| n.to_owned())
        .collect_vec();
    names.extend(aliases_map.function_aliases.keys().map(|n| n.to_owned()));
    names.sort_unstable();
    names.dedup();
    names
}

fn collect_similar(name: &str, candidates: &[impl AsRef<str>]) -> Vec<String> {
    candidates
        .iter()
        .filter_map(|cand| {
            // The parameter is borrowed from clap f5540d26
            (strsim::jaro(name, cand.as_ref()) > 0.7).then_some(cand)
        })
        .map(|s| s.as_ref().to_owned())
        .collect_vec()
}

type RevsetFunction =
    fn(&str, Pair<Rule>, ParseState) -> Result<Rc<RevsetExpression>, RevsetParseError>;

static BUILTIN_FUNCTION_MAP: Lazy<HashMap<&'static str, RevsetFunction>> = Lazy::new(|| {
    // Not using maplit::hashmap!{} or custom declarative macro here because
    // code completion inside macro is quite restricted.
    let mut map: HashMap<&'static str, RevsetFunction> = HashMap::new();
    map.insert("parents", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let expression = parse_expression_rule(arg.into_inner(), state)?;
        Ok(expression.parents())
    });
    map.insert("children", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let expression = parse_expression_rule(arg.into_inner(), state)?;
        Ok(expression.children())
    });
    map.insert("ancestors", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let expression = parse_expression_rule(arg.into_inner(), state)?;
        Ok(expression.ancestors())
    });
    map.insert("descendants", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let expression = parse_expression_rule(arg.into_inner(), state)?;
        Ok(expression.descendants())
    });
    map.insert("connected", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let candidates = parse_expression_rule(arg.into_inner(), state)?;
        Ok(candidates.connected())
    });
    map.insert("none", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::none())
    });
    map.insert("all", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::all())
    });
    map.insert("heads", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let candidates = parse_expression_rule(arg.into_inner(), state)?;
        Ok(candidates.heads())
    });
    map.insert("roots", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let candidates = parse_expression_rule(arg.into_inner(), state)?;
        Ok(candidates.roots())
    });
    map.insert("visible_heads", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::visible_heads())
    });
    map.insert("branches", |name, arguments_pair, state| {
        let ([], [opt_arg]) = expect_arguments(name, arguments_pair)?;
        let needle = if let Some(arg) = opt_arg {
            parse_function_argument_to_string(name, arg, state)?
        } else {
            "".to_owned()
        };
        Ok(RevsetExpression::branches(needle))
    });
    map.insert("remote_branches", |name, arguments_pair, state| {
        let ([], [branch_opt_arg, remote_opt_arg]) =
            expect_named_arguments(name, &["", "remote"], arguments_pair)?;
        let branch_needle = if let Some(branch_arg) = branch_opt_arg {
            parse_function_argument_to_string(name, branch_arg, state)?
        } else {
            "".to_owned()
        };
        let remote_needle = if let Some(remote_arg) = remote_opt_arg {
            parse_function_argument_to_string(name, remote_arg, state)?
        } else {
            "".to_owned()
        };
        Ok(RevsetExpression::remote_branches(
            branch_needle,
            remote_needle,
        ))
    });
    map.insert("tags", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::tags())
    });
    map.insert("git_refs", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::git_refs())
    });
    map.insert("git_head", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::git_head())
    });
    map.insert("latest", |name, arguments_pair, state| {
        let ([candidates_arg], [count_opt_arg]) = expect_arguments(name, arguments_pair)?;
        let candidates = parse_expression_rule(candidates_arg.into_inner(), state)?;
        let count = if let Some(count_arg) = count_opt_arg {
            parse_function_argument_as_literal("integer", name, count_arg, state)?
        } else {
            1
        };
        Ok(candidates.latest(count))
    });
    map.insert("merges", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::filter(
            RevsetFilterPredicate::ParentCount(2..u32::MAX),
        ))
    });
    map.insert("description", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let needle = parse_function_argument_to_string(name, arg, state)?;
        Ok(RevsetExpression::filter(
            RevsetFilterPredicate::Description(needle),
        ))
    });
    map.insert("author", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let needle = parse_function_argument_to_string(name, arg, state)?;
        Ok(RevsetExpression::filter(RevsetFilterPredicate::Author(
            needle,
        )))
    });
    map.insert("committer", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let needle = parse_function_argument_to_string(name, arg, state)?;
        Ok(RevsetExpression::filter(RevsetFilterPredicate::Committer(
            needle,
        )))
    });
    map.insert("empty", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::filter(RevsetFilterPredicate::File(None)).negated())
    });
    map.insert("file", |name, arguments_pair, state| {
        if let Some(ctx) = state.workspace_ctx {
            let arguments_span = arguments_pair.as_span();
            let paths: Vec<_> = arguments_pair
                .into_inner()
                .map(|arg| -> Result<_, RevsetParseError> {
                    let span = arg.as_span();
                    let needle = parse_function_argument_to_string(name, arg, state)?;
                    let path = RepoPath::parse_fs_path(ctx.cwd, ctx.workspace_root, needle)
                        .map_err(|e| {
                            RevsetParseError::with_span(
                                RevsetParseErrorKind::FsPathParseError(e),
                                span,
                            )
                        })?;
                    Ok(path)
                })
                .try_collect()?;
            if paths.is_empty() {
                Err(RevsetParseError::with_span(
                    RevsetParseErrorKind::InvalidFunctionArguments {
                        name: name.to_owned(),
                        message: "Expected at least 1 argument".to_string(),
                    },
                    arguments_span,
                ))
            } else {
                Ok(RevsetExpression::filter(RevsetFilterPredicate::File(Some(
                    paths,
                ))))
            }
        } else {
            Err(RevsetParseError::new(
                RevsetParseErrorKind::FsPathWithoutWorkspace,
            ))
        }
    });
    map.insert("conflict", |name, arguments_pair, _state| {
        expect_no_arguments(name, arguments_pair)?;
        Ok(RevsetExpression::filter(RevsetFilterPredicate::HasConflict))
    });
    map.insert("present", |name, arguments_pair, state| {
        let arg = expect_one_argument(name, arguments_pair)?;
        let expression = parse_expression_rule(arg.into_inner(), state)?;
        Ok(Rc::new(RevsetExpression::Present(expression)))
    });
    map
});

type OptionalArg<'i> = Option<Pair<'i, Rule>>;

fn expect_no_arguments(
    function_name: &str,
    arguments_pair: Pair<Rule>,
) -> Result<(), RevsetParseError> {
    let ([], []) = expect_arguments(function_name, arguments_pair)?;
    Ok(())
}

fn expect_one_argument<'i>(
    function_name: &str,
    arguments_pair: Pair<'i, Rule>,
) -> Result<Pair<'i, Rule>, RevsetParseError> {
    let ([arg], []) = expect_arguments(function_name, arguments_pair)?;
    Ok(arg)
}

fn expect_arguments<'i, const N: usize, const M: usize>(
    function_name: &str,
    arguments_pair: Pair<'i, Rule>,
) -> Result<([Pair<'i, Rule>; N], [OptionalArg<'i>; M]), RevsetParseError> {
    expect_named_arguments(function_name, &[], arguments_pair)
}

/// Extracts N required arguments and M optional arguments.
///
/// `argument_names` is a list of argument names. Unnamed positional arguments
/// should be padded with `""`.
fn expect_named_arguments<'i, const N: usize, const M: usize>(
    function_name: &str,
    argument_names: &[&str],
    arguments_pair: Pair<'i, Rule>,
) -> Result<([Pair<'i, Rule>; N], [OptionalArg<'i>; M]), RevsetParseError> {
    let (required, optional) =
        expect_named_arguments_vec(function_name, argument_names, arguments_pair, N, N + M)?;
    Ok((required.try_into().unwrap(), optional.try_into().unwrap()))
}

fn expect_named_arguments_vec<'i>(
    function_name: &str,
    argument_names: &[&str],
    arguments_pair: Pair<'i, Rule>,
    min_arg_count: usize,
    max_arg_count: usize,
) -> Result<(Vec<Pair<'i, Rule>>, Vec<OptionalArg<'i>>), RevsetParseError> {
    assert!(argument_names.len() <= max_arg_count);
    let arguments_span = arguments_pair.as_span();
    let make_error = |message, span| {
        RevsetParseError::with_span(
            RevsetParseErrorKind::InvalidFunctionArguments {
                name: function_name.to_owned(),
                message,
            },
            span,
        )
    };
    let make_count_error = || {
        let message = if min_arg_count == max_arg_count {
            format!("Expected {min_arg_count} arguments")
        } else {
            format!("Expected {min_arg_count} to {max_arg_count} arguments")
        };
        make_error(message, arguments_span)
    };

    let mut pos_iter = Some(0..max_arg_count);
    let mut extracted_pairs = vec![None; max_arg_count];
    for pair in arguments_pair.into_inner() {
        let span = pair.as_span();
        match pair.as_rule() {
            Rule::expression => {
                let pos = pos_iter
                    .as_mut()
                    .ok_or_else(|| {
                        make_error(
                            "Positional argument follows keyword argument".to_owned(),
                            span,
                        )
                    })?
                    .next()
                    .ok_or_else(make_count_error)?;
                assert!(extracted_pairs[pos].is_none());
                extracted_pairs[pos] = Some(pair);
            }
            Rule::keyword_argument => {
                pos_iter = None; // No more positional arguments
                let mut pairs = pair.into_inner();
                let name = pairs.next().unwrap();
                let expr = pairs.next().unwrap();
                assert_eq!(name.as_rule(), Rule::identifier);
                assert_eq!(expr.as_rule(), Rule::expression);
                let pos = argument_names
                    .iter()
                    .position(|&n| n == name.as_str())
                    .ok_or_else(|| {
                        make_error(
                            format!(r#"Unexpected keyword argument "{}""#, name.as_str()),
                            span,
                        )
                    })?;
                if extracted_pairs[pos].is_some() {
                    return Err(make_error(
                        format!(r#"Got multiple values for keyword "{}""#, name.as_str()),
                        span,
                    ));
                }
                extracted_pairs[pos] = Some(expr);
            }
            r => panic!("unexpected argument rule {r:?}"),
        }
    }

    assert_eq!(extracted_pairs.len(), max_arg_count);
    let optional = extracted_pairs.split_off(min_arg_count);
    let required = extracted_pairs.into_iter().flatten().collect_vec();
    if required.len() != min_arg_count {
        return Err(make_count_error());
    }
    Ok((required, optional))
}

fn parse_function_argument_to_string(
    name: &str,
    pair: Pair<Rule>,
    state: ParseState,
) -> Result<String, RevsetParseError> {
    parse_function_argument_as_literal("string", name, pair, state)
}

fn parse_function_argument_as_literal<T: FromStr>(
    type_name: &str,
    name: &str,
    pair: Pair<Rule>,
    state: ParseState,
) -> Result<T, RevsetParseError> {
    let span = pair.as_span();
    let make_error = || {
        RevsetParseError::with_span(
            RevsetParseErrorKind::InvalidFunctionArguments {
                name: name.to_string(),
                message: format!("Expected function argument of type {type_name}"),
            },
            span,
        )
    };
    let expression = parse_expression_rule(pair.into_inner(), state)?;
    match expression.as_ref() {
        RevsetExpression::CommitRef(RevsetCommitRef::Symbol(symbol)) => {
            symbol.parse().map_err(|_| make_error())
        }
        _ => Err(make_error()),
    }
}

pub fn parse(
    revset_str: &str,
    aliases_map: &RevsetAliasesMap,
    workspace_ctx: Option<&RevsetWorkspaceContext>,
) -> Result<Rc<RevsetExpression>, RevsetParseError> {
    let state = ParseState {
        aliases_map,
        aliases_expanding: &[],
        locals: &HashMap::new(),
        workspace_ctx,
    };
    parse_program(revset_str, state)
}

/// `Some` for rewritten expression, or `None` to reuse the original expression.
type TransformedExpression = Option<Rc<RevsetExpression>>;

/// Walks `expression` tree and applies `f` recursively from leaf nodes.
fn transform_expression_bottom_up(
    expression: &Rc<RevsetExpression>,
    mut f: impl FnMut(&Rc<RevsetExpression>) -> TransformedExpression,
) -> TransformedExpression {
    try_transform_expression::<Infallible>(expression, |_| Ok(None), |expression| Ok(f(expression)))
        .unwrap()
}

/// Walks `expression` tree and applies transformation recursively.
///
/// `pre` is the callback to rewrite subtree including children. It is
/// invoked before visiting the child nodes. If returned `Some`, children
/// won't be visited.
///
/// `post` is the callback to rewrite from leaf nodes. If returned `None`,
/// the original expression node will be reused.
///
/// If no nodes rewritten, this function returns `None`.
/// `std::iter::successors()` could be used if the transformation needs to be
/// applied repeatedly until converged.
fn try_transform_expression<E>(
    expression: &Rc<RevsetExpression>,
    mut pre: impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
    mut post: impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
) -> Result<TransformedExpression, E> {
    fn transform_child_rec<E>(
        expression: &Rc<RevsetExpression>,
        pre: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
        post: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
    ) -> Result<TransformedExpression, E> {
        Ok(match expression.as_ref() {
            RevsetExpression::None => None,
            RevsetExpression::All => None,
            RevsetExpression::Commits(_) => None,
            RevsetExpression::CommitRef(_) => None,
            RevsetExpression::Ancestors {
                heads, generation, ..
            } => transform_rec(heads, pre, post)?.map(|heads| RevsetExpression::Ancestors {
                heads,
                generation: generation.clone(),
                is_legacy: false,
            }),
            RevsetExpression::Descendants {
                roots, generation, ..
            } => transform_rec(roots, pre, post)?.map(|roots| RevsetExpression::Descendants {
                roots,
                generation: generation.clone(),
                is_legacy: false,
            }),
            RevsetExpression::Range {
                roots,
                heads,
                generation,
            } => transform_rec_pair((roots, heads), pre, post)?.map(|(roots, heads)| {
                RevsetExpression::Range {
                    roots,
                    heads,
                    generation: generation.clone(),
                }
            }),
            RevsetExpression::DagRange { roots, heads, .. } => {
                transform_rec_pair((roots, heads), pre, post)?.map(|(roots, heads)| {
                    RevsetExpression::DagRange {
                        roots,
                        heads,
                        is_legacy: false,
                    }
                })
            }
            RevsetExpression::Heads(candidates) => {
                transform_rec(candidates, pre, post)?.map(RevsetExpression::Heads)
            }
            RevsetExpression::Roots(candidates) => {
                transform_rec(candidates, pre, post)?.map(RevsetExpression::Roots)
            }
            RevsetExpression::Latest { candidates, count } => transform_rec(candidates, pre, post)?
                .map(|candidates| RevsetExpression::Latest {
                    candidates,
                    count: *count,
                }),
            RevsetExpression::Filter(_) => None,
            RevsetExpression::AsFilter(candidates) => {
                transform_rec(candidates, pre, post)?.map(RevsetExpression::AsFilter)
            }
            RevsetExpression::Present(candidates) => {
                transform_rec(candidates, pre, post)?.map(RevsetExpression::Present)
            }
            RevsetExpression::NotIn(complement) => {
                transform_rec(complement, pre, post)?.map(RevsetExpression::NotIn)
            }
            RevsetExpression::Union(expression1, expression2) => {
                transform_rec_pair((expression1, expression2), pre, post)?.map(
                    |(expression1, expression2)| RevsetExpression::Union(expression1, expression2),
                )
            }
            RevsetExpression::Intersection(expression1, expression2) => {
                transform_rec_pair((expression1, expression2), pre, post)?.map(
                    |(expression1, expression2)| {
                        RevsetExpression::Intersection(expression1, expression2)
                    },
                )
            }
            RevsetExpression::Difference(expression1, expression2) => {
                transform_rec_pair((expression1, expression2), pre, post)?.map(
                    |(expression1, expression2)| {
                        RevsetExpression::Difference(expression1, expression2)
                    },
                )
            }
        }
        .map(Rc::new))
    }

    #[allow(clippy::type_complexity)]
    fn transform_rec_pair<E>(
        (expression1, expression2): (&Rc<RevsetExpression>, &Rc<RevsetExpression>),
        pre: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
        post: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
    ) -> Result<Option<(Rc<RevsetExpression>, Rc<RevsetExpression>)>, E> {
        match (
            transform_rec(expression1, pre, post)?,
            transform_rec(expression2, pre, post)?,
        ) {
            (Some(new_expression1), Some(new_expression2)) => {
                Ok(Some((new_expression1, new_expression2)))
            }
            (Some(new_expression1), None) => Ok(Some((new_expression1, expression2.clone()))),
            (None, Some(new_expression2)) => Ok(Some((expression1.clone(), new_expression2))),
            (None, None) => Ok(None),
        }
    }

    fn transform_rec<E>(
        expression: &Rc<RevsetExpression>,
        pre: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
        post: &mut impl FnMut(&Rc<RevsetExpression>) -> Result<TransformedExpression, E>,
    ) -> Result<TransformedExpression, E> {
        if let Some(new_expression) = pre(expression)? {
            return Ok(Some(new_expression));
        }
        if let Some(new_expression) = transform_child_rec(expression, pre, post)? {
            // must propagate new expression tree
            Ok(Some(post(&new_expression)?.unwrap_or(new_expression)))
        } else {
            post(expression)
        }
    }

    transform_rec(expression, &mut pre, &mut post)
}

/// Transforms filter expressions, by applying the following rules.
///
/// a. Moves as many sets to left of filter intersection as possible, to
///    minimize the filter inputs.
/// b. TODO: Rewrites set operations to and/or/not of predicates, to
///    help further optimization (e.g. combine `file(_)` matchers.)
/// c. Wraps union of filter and set (e.g. `author(_) | heads()`), to
///    ensure inner filter wouldn't need to evaluate all the input sets.
fn internalize_filter(expression: &Rc<RevsetExpression>) -> TransformedExpression {
    fn is_filter(expression: &RevsetExpression) -> bool {
        matches!(
            expression,
            RevsetExpression::Filter(_) | RevsetExpression::AsFilter(_)
        )
    }

    fn is_filter_tree(expression: &RevsetExpression) -> bool {
        is_filter(expression) || as_filter_intersection(expression).is_some()
    }

    // Extracts 'c & f' from intersect_down()-ed node.
    fn as_filter_intersection(
        expression: &RevsetExpression,
    ) -> Option<(&Rc<RevsetExpression>, &Rc<RevsetExpression>)> {
        if let RevsetExpression::Intersection(expression1, expression2) = expression {
            is_filter(expression2).then_some((expression1, expression2))
        } else {
            None
        }
    }

    // Since both sides must have already been intersect_down()-ed, we don't need to
    // apply the whole bottom-up pass to new intersection node. Instead, just push
    // new 'c & (d & g)' down-left to '(c & d) & g' while either side is
    // an intersection of filter node.
    fn intersect_down(
        expression1: &Rc<RevsetExpression>,
        expression2: &Rc<RevsetExpression>,
    ) -> TransformedExpression {
        let recurse = |e1, e2| intersect_down(e1, e2).unwrap_or_else(|| e1.intersection(e2));
        match (expression1.as_ref(), expression2.as_ref()) {
            // Don't reorder 'f1 & f2'
            (_, e2) if is_filter(e2) => None,
            // f1 & e2 -> e2 & f1
            (e1, _) if is_filter(e1) => Some(expression2.intersection(expression1)),
            (e1, e2) => match (as_filter_intersection(e1), as_filter_intersection(e2)) {
                // e1 & (c2 & f2) -> (e1 & c2) & f2
                // (c1 & f1) & (c2 & f2) -> ((c1 & f1) & c2) & f2 -> ((c1 & c2) & f1) & f2
                (_, Some((c2, f2))) => Some(recurse(expression1, c2).intersection(f2)),
                // (c1 & f1) & e2 -> (c1 & e2) & f1
                // ((c1 & f1) & g1) & e2 -> ((c1 & f1) & e2) & g1 -> ((c1 & e2) & f1) & g1
                (Some((c1, f1)), _) => Some(recurse(c1, expression2).intersection(f1)),
                (None, None) => None,
            },
        }
    }

    // Bottom-up pass pulls up-right filter node from leaf '(c & f) & e' ->
    // '(c & e) & f', so that an intersection of filter node can be found as
    // a direct child of another intersection node. However, the rewritten
    // intersection node 'c & e' can also be a rewrite target if 'e' contains
    // a filter node. That's why intersect_down() is also recursive.
    transform_expression_bottom_up(expression, |expression| match expression.as_ref() {
        RevsetExpression::Present(e) => {
            is_filter_tree(e).then(|| Rc::new(RevsetExpression::AsFilter(expression.clone())))
        }
        RevsetExpression::NotIn(e) => {
            is_filter_tree(e).then(|| Rc::new(RevsetExpression::AsFilter(expression.clone())))
        }
        RevsetExpression::Union(e1, e2) => (is_filter_tree(e1) || is_filter_tree(e2))
            .then(|| Rc::new(RevsetExpression::AsFilter(expression.clone()))),
        RevsetExpression::Intersection(expression1, expression2) => {
            intersect_down(expression1, expression2)
        }
        // Difference(e1, e2) should have been unfolded to Intersection(e1, NotIn(e2)).
        _ => None,
    })
}

/// Eliminates redundant nodes like `x & all()`, `~~x`.
///
/// This does not rewrite 'x & none()' to 'none()' because 'x' may be an invalid
/// symbol.
fn fold_redundant_expression(expression: &Rc<RevsetExpression>) -> TransformedExpression {
    transform_expression_bottom_up(expression, |expression| match expression.as_ref() {
        RevsetExpression::NotIn(outer) => match outer.as_ref() {
            RevsetExpression::NotIn(inner) => Some(inner.clone()),
            _ => None,
        },
        RevsetExpression::Intersection(expression1, expression2) => {
            match (expression1.as_ref(), expression2.as_ref()) {
                (_, RevsetExpression::All) => Some(expression1.clone()),
                (RevsetExpression::All, _) => Some(expression2.clone()),
                _ => None,
            }
        }
        _ => None,
    })
}

/// Transforms negative intersection to difference. Redundant intersections like
/// `all() & e` should have been removed.
fn fold_difference(expression: &Rc<RevsetExpression>) -> TransformedExpression {
    fn to_difference(
        expression: &Rc<RevsetExpression>,
        complement: &Rc<RevsetExpression>,
    ) -> Rc<RevsetExpression> {
        match (expression.as_ref(), complement.as_ref()) {
            // :heads & ~(:roots) -> roots..heads
            (
                RevsetExpression::Ancestors {
                    heads, generation, ..
                },
                RevsetExpression::Ancestors {
                    heads: roots,
                    generation: GENERATION_RANGE_FULL,
                    ..
                },
            ) => Rc::new(RevsetExpression::Range {
                roots: roots.clone(),
                heads: heads.clone(),
                generation: generation.clone(),
            }),
            _ => expression.minus(complement),
        }
    }

    transform_expression_bottom_up(expression, |expression| match expression.as_ref() {
        RevsetExpression::Intersection(expression1, expression2) => {
            match (expression1.as_ref(), expression2.as_ref()) {
                // For '~x & f', don't move filter node 'f' left
                (_, RevsetExpression::Filter(_) | RevsetExpression::AsFilter(_)) => None,
                (_, RevsetExpression::NotIn(complement)) => {
                    Some(to_difference(expression1, complement))
                }
                (RevsetExpression::NotIn(complement), _) => {
                    Some(to_difference(expression2, complement))
                }
                _ => None,
            }
        }
        _ => None,
    })
}

/// Transforms binary difference to more primitive negative intersection.
///
/// For example, `all() ~ e` will become `all() & ~e`, which can be simplified
/// further by `fold_redundant_expression()`.
fn unfold_difference(expression: &Rc<RevsetExpression>) -> TransformedExpression {
    transform_expression_bottom_up(expression, |expression| match expression.as_ref() {
        // roots..heads -> :heads & ~(:roots)
        RevsetExpression::Range {
            roots,
            heads,
            generation,
        } => {
            let heads_ancestors = Rc::new(RevsetExpression::Ancestors {
                heads: heads.clone(),
                generation: generation.clone(),
                is_legacy: false,
            });
            Some(heads_ancestors.intersection(&roots.ancestors().negated()))
        }
        RevsetExpression::Difference(expression1, expression2) => {
            Some(expression1.intersection(&expression2.negated()))
        }
        _ => None,
    })
}

/// Transforms nested `ancestors()`/`parents()`/`descendants()`/`children()`
/// like `h---`/`r+++`.
fn fold_generation(expression: &Rc<RevsetExpression>) -> TransformedExpression {
    fn add_generation(generation1: &Range<u64>, generation2: &Range<u64>) -> Range<u64> {
        // For any (g1, g2) in (generation1, generation2), g1 + g2.
        if generation1.is_empty() || generation2.is_empty() {
            GENERATION_RANGE_EMPTY
        } else {
            let start = u64::saturating_add(generation1.start, generation2.start);
            let end = u64::saturating_add(generation1.end, generation2.end - 1);
            start..end
        }
    }

    transform_expression_bottom_up(expression, |expression| match expression.as_ref() {
        RevsetExpression::Ancestors {
            heads,
            generation: generation1,
            ..
        } => {
            match heads.as_ref() {
                // (h-)- -> ancestors(ancestors(h, 1), 1) -> ancestors(h, 2)
                // :(h-) -> ancestors(ancestors(h, 1), ..) -> ancestors(h, 1..)
                // (:h)- -> ancestors(ancestors(h, ..), 1) -> ancestors(h, 1..)
                RevsetExpression::Ancestors {
                    heads,
                    generation: generation2,
                    ..
                } => Some(Rc::new(RevsetExpression::Ancestors {
                    heads: heads.clone(),
                    generation: add_generation(generation1, generation2),
                    is_legacy: false,
                })),
                _ => None,
            }
        }
        RevsetExpression::Descendants {
            roots,
            generation: generation1,
            ..
        } => {
            match roots.as_ref() {
                // (r+)+ -> descendants(descendants(r, 1), 1) -> descendants(r, 2)
                // (r+): -> descendants(descendants(r, 1), ..) -> descendants(r, 1..)
                // (r:)+ -> descendants(descendants(r, ..), 1) -> descendants(r, 1..)
                RevsetExpression::Descendants {
                    roots,
                    generation: generation2,
                    ..
                } => Some(Rc::new(RevsetExpression::Descendants {
                    roots: roots.clone(),
                    generation: add_generation(generation1, generation2),
                    is_legacy: false,
                })),
                _ => None,
            }
        }
        // Range should have been unfolded to intersection of Ancestors.
        _ => None,
    })
}

/// Rewrites the given `expression` tree to reduce evaluation cost. Returns new
/// tree.
pub fn optimize(expression: Rc<RevsetExpression>) -> Rc<RevsetExpression> {
    let expression = unfold_difference(&expression).unwrap_or(expression);
    let expression = fold_redundant_expression(&expression).unwrap_or(expression);
    let expression = fold_generation(&expression).unwrap_or(expression);
    let expression = internalize_filter(&expression).unwrap_or(expression);
    fold_difference(&expression).unwrap_or(expression)
}

// TODO: find better place to host this function (or add compile-time revset
// parsing and resolution like
// `revset!("{unwanted}..{wanted}").evaluate(repo)`?)
pub fn walk_revs<'index>(
    repo: &'index dyn Repo,
    wanted: &[CommitId],
    unwanted: &[CommitId],
) -> Result<Box<dyn Revset<'index> + 'index>, RevsetEvaluationError> {
    RevsetExpression::commits(unwanted.to_vec())
        .range(&RevsetExpression::commits(wanted.to_vec()))
        .resolve(repo)
        .unwrap()
        .evaluate(repo)
}

fn resolve_git_ref(repo: &dyn Repo, symbol: &str) -> Option<Vec<CommitId>> {
    let view = repo.view();
    // TODO: We should remove `refs/remotes` from this list once we have a better
    // way to address local git repo's remote-tracking branches.
    for git_ref_prefix in &["", "refs/", "refs/tags/", "refs/remotes/"] {
        let target = view.get_git_ref(&(git_ref_prefix.to_string() + symbol));
        if target.is_present() {
            return Some(target.added_ids().cloned().collect());
        }
    }
    None
}

fn resolve_branch(repo: &dyn Repo, symbol: &str) -> Option<Vec<CommitId>> {
    let view = repo.view();
    let target = view.get_local_branch(symbol);
    if target.is_present() {
        return Some(target.added_ids().cloned().collect());
    }
    if let Some((name, remote_name)) = symbol.split_once('@') {
        let target = view.get_remote_branch(name, remote_name);
        if target.is_present() {
            return Some(target.added_ids().cloned().collect());
        }
        // A remote with name "git" will shadow local-git tracking branches
        if remote_name == "git" {
            let target = match name {
                "HEAD" => view.git_head(),
                _ => get_local_git_tracking_branch(view, name),
            };
            if target.is_present() {
                return Some(target.added_ids().cloned().collect());
            }
        }
    }
    None
}

fn collect_branch_symbols(repo: &dyn Repo, include_synced_remotes: bool) -> Vec<String> {
    let view = repo.view();
    let (all_branches, _) = git::build_unified_branches_map(view);
    all_branches
        .iter()
        .flat_map(|(name, branch_target)| {
            let local_target = &branch_target.local_target;
            let local_symbol = local_target.is_present().then(|| name.to_owned());
            let remote_symbols = branch_target
                .remote_targets
                .iter()
                .filter(move |&(_, target)| include_synced_remotes || target != local_target)
                .map(move |(remote_name, _)| format!("{name}@{remote_name}"));
            local_symbol.into_iter().chain(remote_symbols)
        })
        .chain(view.git_head().is_present().then(|| "HEAD@git".to_owned()))
        .collect()
}

fn resolve_full_commit_id(
    repo: &dyn Repo,
    symbol: &str,
) -> Result<Option<Vec<CommitId>>, RevsetResolutionError> {
    if let Ok(binary_commit_id) = hex::decode(symbol) {
        if repo.store().commit_id_length() != binary_commit_id.len() {
            return Ok(None);
        }
        let commit_id = CommitId::new(binary_commit_id);
        match repo.store().get_commit(&commit_id) {
            // Only recognize a commit if we have indexed it
            Ok(_) if repo.index().has_id(&commit_id) => Ok(Some(vec![commit_id])),
            Ok(_) | Err(BackendError::ObjectNotFound { .. }) => Ok(None),
            Err(err) => Err(RevsetResolutionError::StoreError(err)),
        }
    } else {
        Ok(None)
    }
}

pub trait SymbolResolver {
    fn resolve_symbol(&self, symbol: &str) -> Result<Vec<CommitId>, RevsetResolutionError>;
}

/// Fails on any attempt to resolve a symbol.
pub struct FailingSymbolResolver;

impl SymbolResolver for FailingSymbolResolver {
    fn resolve_symbol(&self, symbol: &str) -> Result<Vec<CommitId>, RevsetResolutionError> {
        Err(RevsetResolutionError::NoSuchRevision {
            name: format!(
                "Won't resolve symbol {symbol:?}. When creating revsets programmatically, avoid \
                 using RevsetExpression::symbol(); use RevsetExpression::commits() instead."
            ),
            candidates: Default::default(),
        })
    }
}

pub type PrefixResolver<'a, T> = Box<dyn Fn(&dyn Repo, &HexPrefix) -> PrefixResolution<T> + 'a>;

/// Resolves the "root" and "@" symbols, branches, remote branches, tags, git
/// refs, and full and abbreviated commit and change ids.
pub struct DefaultSymbolResolver<'a> {
    repo: &'a dyn Repo,
    workspace_id: Option<&'a WorkspaceId>,
    commit_id_resolver: PrefixResolver<'a, CommitId>,
    change_id_resolver: PrefixResolver<'a, Vec<CommitId>>,
}

impl<'a> DefaultSymbolResolver<'a> {
    pub fn new(repo: &'a dyn Repo, workspace_id: Option<&'a WorkspaceId>) -> Self {
        DefaultSymbolResolver {
            repo,
            workspace_id,
            commit_id_resolver: Box::new(|repo, prefix| repo.index().resolve_prefix(prefix)),
            change_id_resolver: Box::new(|repo, prefix| repo.resolve_change_id_prefix(prefix)),
        }
    }

    pub fn with_commit_id_resolver(
        mut self,
        commit_id_resolver: PrefixResolver<'a, CommitId>,
    ) -> Self {
        self.commit_id_resolver = commit_id_resolver;
        self
    }

    pub fn with_change_id_resolver(
        mut self,
        change_id_resolver: PrefixResolver<'a, Vec<CommitId>>,
    ) -> Self {
        self.change_id_resolver = change_id_resolver;
        self
    }
}

impl SymbolResolver for DefaultSymbolResolver<'_> {
    fn resolve_symbol(&self, symbol: &str) -> Result<Vec<CommitId>, RevsetResolutionError> {
        if symbol.ends_with('@') {
            let target_workspace = if symbol == "@" {
                if let Some(workspace_id) = self.workspace_id {
                    workspace_id.clone()
                } else {
                    return Err(RevsetResolutionError::NoSuchRevision {
                        name: symbol.to_owned(),
                        candidates: Default::default(),
                    });
                }
            } else {
                WorkspaceId::new(symbol.strip_suffix('@').unwrap().to_string())
            };
            if let Some(commit_id) = self.repo.view().get_wc_commit_id(&target_workspace) {
                Ok(vec![commit_id.clone()])
            } else {
                Err(RevsetResolutionError::NoSuchRevision {
                    name: symbol.to_owned(),
                    candidates: Default::default(),
                })
            }
        } else if symbol == "root" {
            Ok(vec![self.repo.store().root_commit_id().clone()])
        } else if symbol.is_empty() {
            Err(RevsetResolutionError::EmptyString)
        } else {
            // Try to resolve as a tag
            let target = self.repo.view().get_tag(symbol);
            if target.is_present() {
                return Ok(target.added_ids().cloned().collect());
            }

            // Try to resolve as a branch
            if let Some(ids) = resolve_branch(self.repo, symbol) {
                return Ok(ids);
            }

            // Try to resolve as a git ref
            if let Some(ids) = resolve_git_ref(self.repo, symbol) {
                return Ok(ids);
            }

            // Try to resolve as a full commit id.
            if let Some(ids) = resolve_full_commit_id(self.repo, symbol)? {
                return Ok(ids);
            }

            // Try to resolve as a commit id.
            if let Some(prefix) = HexPrefix::new(symbol) {
                match (self.commit_id_resolver)(self.repo, &prefix) {
                    PrefixResolution::AmbiguousMatch => {
                        return Err(RevsetResolutionError::AmbiguousCommitIdPrefix(
                            symbol.to_owned(),
                        ));
                    }
                    PrefixResolution::SingleMatch(id) => {
                        return Ok(vec![id]);
                    }
                    PrefixResolution::NoMatch => {
                        // Fall through
                    }
                }
            }

            // Try to resolve as a change id.
            if let Some(prefix) = to_forward_hex(symbol).as_deref().and_then(HexPrefix::new) {
                match (self.change_id_resolver)(self.repo, &prefix) {
                    PrefixResolution::AmbiguousMatch => {
                        return Err(RevsetResolutionError::AmbiguousChangeIdPrefix(
                            symbol.to_owned(),
                        ));
                    }
                    PrefixResolution::SingleMatch(ids) => {
                        return Ok(ids);
                    }
                    PrefixResolution::NoMatch => {
                        // Fall through
                    }
                }
            }

            Err(RevsetResolutionError::NoSuchRevision {
                name: symbol.to_owned(),
                candidates: {
                    // TODO: include tags?
                    let mut branch_names = collect_branch_symbols(self.repo, symbol.contains('@'));
                    branch_names.sort_unstable();
                    collect_similar(symbol, &branch_names)
                },
            })
        }
    }
}

fn resolve_commit_ref(
    repo: &dyn Repo,
    commit_ref: &RevsetCommitRef,
    symbol_resolver: &dyn SymbolResolver,
) -> Result<Vec<CommitId>, RevsetResolutionError> {
    match commit_ref {
        RevsetCommitRef::Symbol(symbol) => symbol_resolver.resolve_symbol(symbol),
        RevsetCommitRef::VisibleHeads => Ok(repo.view().heads().iter().cloned().collect_vec()),
        RevsetCommitRef::Branches(needle) => {
            let mut commit_ids = vec![];
            for (branch_name, branch_target) in repo.view().branches() {
                if !branch_name.contains(needle) {
                    continue;
                }
                commit_ids.extend(branch_target.local_target.added_ids().cloned());
            }
            Ok(commit_ids)
        }
        RevsetCommitRef::RemoteBranches {
            branch_needle,
            remote_needle,
        } => {
            let mut commit_ids = vec![];
            for (branch_name, branch_target) in repo.view().branches() {
                if !branch_name.contains(branch_needle) {
                    continue;
                }
                for (remote_name, remote_target) in branch_target.remote_targets.iter() {
                    if remote_name.contains(remote_needle) {
                        commit_ids.extend(remote_target.added_ids().cloned());
                    }
                }
            }
            Ok(commit_ids)
        }
        RevsetCommitRef::Tags => {
            let mut commit_ids = vec![];
            for ref_target in repo.view().tags().values() {
                commit_ids.extend(ref_target.added_ids().cloned());
            }
            Ok(commit_ids)
        }
        RevsetCommitRef::GitRefs => {
            let mut commit_ids = vec![];
            for ref_target in repo.view().git_refs().values() {
                commit_ids.extend(ref_target.added_ids().cloned());
            }
            Ok(commit_ids)
        }
        RevsetCommitRef::GitHead => Ok(repo.view().git_head().added_ids().cloned().collect()),
    }
}

fn resolve_symbols(
    repo: &dyn Repo,
    expression: Rc<RevsetExpression>,
    symbol_resolver: &dyn SymbolResolver,
) -> Result<Rc<RevsetExpression>, RevsetResolutionError> {
    Ok(try_transform_expression(
        &expression,
        |expression| match expression.as_ref() {
            // 'present(x)' opens new symbol resolution scope to map error to 'none()'.
            RevsetExpression::Present(candidates) => {
                resolve_symbols(repo, candidates.clone(), symbol_resolver)
                    .or_else(|err| match err {
                        RevsetResolutionError::NoSuchRevision { .. } => {
                            Ok(RevsetExpression::none())
                        }
                        RevsetResolutionError::EmptyString
                        | RevsetResolutionError::AmbiguousCommitIdPrefix(_)
                        | RevsetResolutionError::AmbiguousChangeIdPrefix(_)
                        | RevsetResolutionError::StoreError(_) => Err(err),
                    })
                    .map(Some) // Always rewrite subtree
            }
            // Otherwise resolve symbols recursively.
            _ => Ok(None),
        },
        |expression| match expression.as_ref() {
            RevsetExpression::CommitRef(commit_ref) => {
                let commit_ids = resolve_commit_ref(repo, commit_ref, symbol_resolver)?;
                Ok(Some(RevsetExpression::commits(commit_ids)))
            }
            _ => Ok(None),
        },
    )?
    .unwrap_or(expression))
}

/// Inserts implicit `all()` and `visible_heads()` nodes to the `expression`.
///
/// Symbols and commit refs in the `expression` should have been resolved.
///
/// This is a separate step because a symbol-resolved `expression` could be
/// transformed further to e.g. combine OR-ed `Commits(_)`, or to collect
/// commit ids to make `all()` include hidden-but-specified commits. The
/// return type `ResolvedExpression` is stricter than `RevsetExpression`,
/// and isn't designed for such transformation.
fn resolve_visibility(repo: &dyn Repo, expression: &RevsetExpression) -> ResolvedExpression {
    // If we add "operation" scope (#1283), visible_heads might be translated to
    // `RevsetExpression::WithinOperation(visible_heads, expression)` node to
    // evaluate filter predicates and "all()" against that scope.
    let context = VisibilityResolutionContext {
        visible_heads: &repo.view().heads().iter().cloned().collect_vec(),
    };
    context.resolve(expression)
}

#[derive(Clone, Debug)]
struct VisibilityResolutionContext<'a> {
    visible_heads: &'a [CommitId],
}

impl VisibilityResolutionContext<'_> {
    /// Resolves expression tree as set.
    fn resolve(&self, expression: &RevsetExpression) -> ResolvedExpression {
        match expression {
            RevsetExpression::None => ResolvedExpression::Commits(vec![]),
            RevsetExpression::All => self.resolve_all(),
            RevsetExpression::Commits(commit_ids) => {
                ResolvedExpression::Commits(commit_ids.clone())
            }
            RevsetExpression::CommitRef(_) => {
                panic!("Expression '{expression:?}' should have been resolved by caller");
            }
            RevsetExpression::Ancestors {
                heads, generation, ..
            } => ResolvedExpression::Ancestors {
                heads: self.resolve(heads).into(),
                generation: generation.clone(),
            },
            RevsetExpression::Descendants {
                roots, generation, ..
            } => ResolvedExpression::DagRange {
                roots: self.resolve(roots).into(),
                heads: self.resolve_visible_heads().into(),
                generation_from_roots: generation.clone(),
            },
            RevsetExpression::Range {
                roots,
                heads,
                generation,
            } => ResolvedExpression::Range {
                roots: self.resolve(roots).into(),
                heads: self.resolve(heads).into(),
                generation: generation.clone(),
            },
            RevsetExpression::DagRange { roots, heads, .. } => ResolvedExpression::DagRange {
                roots: self.resolve(roots).into(),
                heads: self.resolve(heads).into(),
                generation_from_roots: GENERATION_RANGE_FULL,
            },
            RevsetExpression::Heads(candidates) => {
                ResolvedExpression::Heads(self.resolve(candidates).into())
            }
            RevsetExpression::Roots(candidates) => {
                ResolvedExpression::Roots(self.resolve(candidates).into())
            }
            RevsetExpression::Latest { candidates, count } => ResolvedExpression::Latest {
                candidates: self.resolve(candidates).into(),
                count: *count,
            },
            RevsetExpression::Filter(_) | RevsetExpression::AsFilter(_) => {
                // Top-level filter without intersection: e.g. "~author(_)" is represented as
                // `AsFilter(NotIn(Filter(Author(_))))`.
                ResolvedExpression::FilterWithin {
                    candidates: self.resolve_all().into(),
                    predicate: self.resolve_predicate(expression),
                }
            }
            RevsetExpression::Present(_) => {
                panic!("Expression '{expression:?}' should have been resolved by caller");
            }
            RevsetExpression::NotIn(complement) => ResolvedExpression::Difference(
                self.resolve_all().into(),
                self.resolve(complement).into(),
            ),
            RevsetExpression::Union(expression1, expression2) => ResolvedExpression::Union(
                self.resolve(expression1).into(),
                self.resolve(expression2).into(),
            ),
            RevsetExpression::Intersection(expression1, expression2) => {
                match expression2.as_ref() {
                    RevsetExpression::Filter(_) | RevsetExpression::AsFilter(_) => {
                        ResolvedExpression::FilterWithin {
                            candidates: self.resolve(expression1).into(),
                            predicate: self.resolve_predicate(expression2),
                        }
                    }
                    _ => ResolvedExpression::Intersection(
                        self.resolve(expression1).into(),
                        self.resolve(expression2).into(),
                    ),
                }
            }
            RevsetExpression::Difference(expression1, expression2) => {
                ResolvedExpression::Difference(
                    self.resolve(expression1).into(),
                    self.resolve(expression2).into(),
                )
            }
        }
    }

    fn resolve_all(&self) -> ResolvedExpression {
        // Since `all()` does not include hidden commits, some of the logical
        // transformation rules may subtly change the evaluated set. For example,
        // `all() & x` is not `x` if `x` is hidden. This wouldn't matter in practice,
        // but if it does, the heads set could be extended to include the commits
        // (and `remote_branches()`) specified in the revset expression. Alternatively,
        // some optimization rules could be removed, but that means `author(_) & x`
        // would have to test `:visible_heads() & x`.
        ResolvedExpression::Ancestors {
            heads: self.resolve_visible_heads().into(),
            generation: GENERATION_RANGE_FULL,
        }
    }

    fn resolve_visible_heads(&self) -> ResolvedExpression {
        ResolvedExpression::Commits(self.visible_heads.to_owned())
    }

    /// Resolves expression tree as filter predicate.
    ///
    /// For filter expression, this never inserts a hidden `all()` since a
    /// filter predicate doesn't need to produce revisions to walk.
    fn resolve_predicate(&self, expression: &RevsetExpression) -> ResolvedPredicateExpression {
        match expression {
            RevsetExpression::None
            | RevsetExpression::All
            | RevsetExpression::Commits(_)
            | RevsetExpression::CommitRef(_)
            | RevsetExpression::Ancestors { .. }
            | RevsetExpression::Descendants { .. }
            | RevsetExpression::Range { .. }
            | RevsetExpression::DagRange { .. }
            | RevsetExpression::Heads(_)
            | RevsetExpression::Roots(_)
            | RevsetExpression::Latest { .. } => {
                ResolvedPredicateExpression::Set(self.resolve(expression).into())
            }
            RevsetExpression::Filter(predicate) => {
                ResolvedPredicateExpression::Filter(predicate.clone())
            }
            RevsetExpression::AsFilter(candidates) => self.resolve_predicate(candidates),
            RevsetExpression::Present(_) => {
                panic!("Expression '{expression:?}' should have been resolved by caller")
            }
            RevsetExpression::NotIn(complement) => {
                ResolvedPredicateExpression::NotIn(self.resolve_predicate(complement).into())
            }
            RevsetExpression::Union(expression1, expression2) => {
                let predicate1 = self.resolve_predicate(expression1);
                let predicate2 = self.resolve_predicate(expression2);
                ResolvedPredicateExpression::Union(predicate1.into(), predicate2.into())
            }
            // Intersection of filters should have been substituted by optimize().
            // If it weren't, just fall back to the set evaluation path.
            RevsetExpression::Intersection(..) | RevsetExpression::Difference(..) => {
                ResolvedPredicateExpression::Set(self.resolve(expression).into())
            }
        }
    }
}

pub trait Revset<'index>: fmt::Debug {
    /// Iterate in topological order with children before parents.
    fn iter(&self) -> Box<dyn Iterator<Item = CommitId> + '_>;

    /// Iterates commit/change id pairs in topological order.
    fn commit_change_ids(&self) -> Box<dyn Iterator<Item = (CommitId, ChangeId)> + '_>;

    fn iter_graph(&self) -> Box<dyn Iterator<Item = (CommitId, Vec<RevsetGraphEdge>)> + '_>;

    fn change_id_index(&self) -> Box<dyn ChangeIdIndex + 'index>;

    fn is_empty(&self) -> bool;

    fn count(&self) -> usize;
}

pub trait ChangeIdIndex: Send + Sync {
    /// Resolve an unambiguous change ID prefix to the commit IDs in the revset.
    fn resolve_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<Vec<CommitId>>;

    /// This function returns the shortest length of a prefix of `key` that
    /// disambiguates it from every other key in the index.
    ///
    /// The length to be returned is a number of hexadecimal digits.
    ///
    /// This has some properties that we do not currently make much use of:
    ///
    /// - The algorithm works even if `key` itself is not in the index.
    ///
    /// - In the special case when there are keys in the trie for which our
    ///   `key` is an exact prefix, returns `key.len() + 1`. Conceptually, in
    ///   order to disambiguate, you need every letter of the key *and* the
    ///   additional fact that it's the entire key). This case is extremely
    ///   unlikely for hashes with 12+ hexadecimal characters.
    fn shortest_unique_prefix_len(&self, change_id: &ChangeId) -> usize;
}

pub trait RevsetIteratorExt<'index, I> {
    fn commits(self, store: &Arc<Store>) -> RevsetCommitIterator<I>;
    fn reversed(self) -> ReverseRevsetIterator;
}

impl<'index, I: Iterator<Item = CommitId>> RevsetIteratorExt<'index, I> for I {
    fn commits(self, store: &Arc<Store>) -> RevsetCommitIterator<I> {
        RevsetCommitIterator {
            iter: self,
            store: store.clone(),
        }
    }

    fn reversed(self) -> ReverseRevsetIterator {
        ReverseRevsetIterator {
            entries: self.into_iter().collect_vec(),
        }
    }
}

pub struct RevsetCommitIterator<I> {
    store: Arc<Store>,
    iter: I,
}

impl<I: Iterator<Item = CommitId>> Iterator for RevsetCommitIterator<I> {
    type Item = BackendResult<Commit>;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter
            .next()
            .map(|commit_id| self.store.get_commit(&commit_id))
    }
}

pub struct ReverseRevsetIterator {
    entries: Vec<CommitId>,
}

impl Iterator for ReverseRevsetIterator {
    type Item = CommitId;

    fn next(&mut self) -> Option<Self::Item> {
        self.entries.pop()
    }
}

/// Workspace information needed to evaluate revset expression.
#[derive(Clone, Debug)]
pub struct RevsetWorkspaceContext<'a> {
    pub cwd: &'a Path,
    pub workspace_id: &'a WorkspaceId,
    pub workspace_root: &'a Path,
}

#[cfg(test)]
mod tests {
    use super::*;

    fn parse(revset_str: &str) -> Result<Rc<RevsetExpression>, RevsetParseErrorKind> {
        parse_with_aliases(revset_str, [] as [(&str, &str); 0])
    }

    fn parse_with_aliases(
        revset_str: &str,
        aliases: impl IntoIterator<Item = (impl AsRef<str>, impl Into<String>)>,
    ) -> Result<Rc<RevsetExpression>, RevsetParseErrorKind> {
        let mut aliases_map = RevsetAliasesMap::new();
        for (decl, defn) in aliases {
            aliases_map.insert(decl, defn).unwrap();
        }
        // Set up pseudo context to resolve file(path)
        let workspace_ctx = RevsetWorkspaceContext {
            cwd: Path::new("/"),
            workspace_id: &WorkspaceId::default(),
            workspace_root: Path::new("/"),
        };
        // Map error to comparable object
        super::parse(revset_str, &aliases_map, Some(&workspace_ctx)).map_err(|e| e.kind)
    }

    #[test]
    #[allow(clippy::redundant_clone)] // allow symbol.clone()
    fn test_revset_expression_building() {
        let wc_symbol = RevsetExpression::symbol("@".to_string());
        let foo_symbol = RevsetExpression::symbol("foo".to_string());
        let bar_symbol = RevsetExpression::symbol("bar".to_string());
        let baz_symbol = RevsetExpression::symbol("baz".to_string());
        assert_eq!(
            wc_symbol,
            Rc::new(RevsetExpression::CommitRef(RevsetCommitRef::Symbol(
                "@".to_string()
            ))),
        );
        assert_eq!(
            wc_symbol.heads(),
            Rc::new(RevsetExpression::Heads(wc_symbol.clone()))
        );
        assert_eq!(
            wc_symbol.roots(),
            Rc::new(RevsetExpression::Roots(wc_symbol.clone()))
        );
        assert_eq!(
            wc_symbol.parents(),
            Rc::new(RevsetExpression::Ancestors {
                heads: wc_symbol.clone(),
                generation: 1..2,
                is_legacy: false,
            })
        );
        assert_eq!(
            wc_symbol.ancestors(),
            Rc::new(RevsetExpression::Ancestors {
                heads: wc_symbol.clone(),
                generation: GENERATION_RANGE_FULL,
                is_legacy: false,
            })
        );
        assert_eq!(
            foo_symbol.children(),
            Rc::new(RevsetExpression::Descendants {
                roots: foo_symbol.clone(),
                generation: 1..2,
                is_legacy: false,
            }),
        );
        assert_eq!(
            foo_symbol.descendants(),
            Rc::new(RevsetExpression::Descendants {
                roots: foo_symbol.clone(),
                generation: GENERATION_RANGE_FULL,
                is_legacy: false,
            })
        );
        assert_eq!(
            foo_symbol.dag_range_to(&wc_symbol),
            Rc::new(RevsetExpression::DagRange {
                roots: foo_symbol.clone(),
                heads: wc_symbol.clone(),
                is_legacy: false,
            })
        );
        assert_eq!(
            foo_symbol.connected(),
            Rc::new(RevsetExpression::DagRange {
                roots: foo_symbol.clone(),
                heads: foo_symbol.clone(),
                is_legacy: false,
            })
        );
        assert_eq!(
            foo_symbol.range(&wc_symbol),
            Rc::new(RevsetExpression::Range {
                roots: foo_symbol.clone(),
                heads: wc_symbol.clone(),
                generation: GENERATION_RANGE_FULL,
            })
        );
        assert_eq!(
            foo_symbol.negated(),
            Rc::new(RevsetExpression::NotIn(foo_symbol.clone()))
        );
        assert_eq!(
            foo_symbol.union(&wc_symbol),
            Rc::new(RevsetExpression::Union(
                foo_symbol.clone(),
                wc_symbol.clone()
            ))
        );
        assert_eq!(
            RevsetExpression::union_all(&[]),
            Rc::new(RevsetExpression::None)
        );
        assert_eq!(RevsetExpression::union_all(&[wc_symbol.clone()]), wc_symbol);
        assert_eq!(
            RevsetExpression::union_all(&[wc_symbol.clone(), foo_symbol.clone()]),
            Rc::new(RevsetExpression::Union(
                wc_symbol.clone(),
                foo_symbol.clone(),
            ))
        );
        assert_eq!(
            RevsetExpression::union_all(&[
                wc_symbol.clone(),
                foo_symbol.clone(),
                bar_symbol.clone(),
            ]),
            Rc::new(RevsetExpression::Union(
                wc_symbol.clone(),
                Rc::new(RevsetExpression::Union(
                    foo_symbol.clone(),
                    bar_symbol.clone(),
                ))
            ))
        );
        assert_eq!(
            RevsetExpression::union_all(&[
                wc_symbol.clone(),
                foo_symbol.clone(),
                bar_symbol.clone(),
                baz_symbol.clone(),
            ]),
            Rc::new(RevsetExpression::Union(
                Rc::new(RevsetExpression::Union(
                    wc_symbol.clone(),
                    foo_symbol.clone(),
                )),
                Rc::new(RevsetExpression::Union(
                    bar_symbol.clone(),
                    baz_symbol.clone(),
                ))
            ))
        );
        assert_eq!(
            foo_symbol.intersection(&wc_symbol),
            Rc::new(RevsetExpression::Intersection(
                foo_symbol.clone(),
                wc_symbol.clone()
            ))
        );
        assert_eq!(
            foo_symbol.minus(&wc_symbol),
            Rc::new(RevsetExpression::Difference(foo_symbol, wc_symbol.clone()))
        );
    }

    #[test]
    fn test_parse_revset() {
        let wc_symbol = RevsetExpression::symbol("@".to_string());
        let foo_symbol = RevsetExpression::symbol("foo".to_string());
        let bar_symbol = RevsetExpression::symbol("bar".to_string());
        // Parse a single symbol (specifically the "checkout" symbol)
        assert_eq!(parse("@"), Ok(wc_symbol.clone()));
        // Parse a single symbol
        assert_eq!(parse("foo"), Ok(foo_symbol.clone()));
        // Internal '.', '-', and '+' are allowed
        assert_eq!(
            parse("foo.bar-v1+7"),
            Ok(RevsetExpression::symbol("foo.bar-v1+7".to_string()))
        );
        assert_eq!(
            parse("foo.bar-v1+7-"),
            Ok(RevsetExpression::symbol("foo.bar-v1+7".to_string()).parents())
        );
        // Default arguments for *branches() are all ""
        assert_eq!(parse("branches()"), parse(r#"branches("")"#));
        assert_eq!(parse("remote_branches()"), parse(r#"remote_branches("")"#));
        assert_eq!(
            parse("remote_branches()"),
            parse(r#"remote_branches("", "")"#)
        );
        // '.' is not allowed at the beginning or end
        assert_eq!(parse(".foo"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo."), Err(RevsetParseErrorKind::SyntaxError));
        // Multiple '.', '-', '+' are not allowed
        assert_eq!(parse("foo.+bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo--bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo+-bar"), Err(RevsetParseErrorKind::SyntaxError));
        // Parse a parenthesized symbol
        assert_eq!(parse("(foo)"), Ok(foo_symbol.clone()));
        // Parse a quoted symbol
        assert_eq!(parse("\"foo\""), Ok(foo_symbol.clone()));
        // Parse the "parents" operator
        assert_eq!(parse("@-"), Ok(wc_symbol.parents()));
        // Parse the "children" operator
        assert_eq!(parse("@+"), Ok(wc_symbol.children()));
        // Parse the "ancestors" operator
        assert_eq!(parse("::@"), Ok(wc_symbol.ancestors()));
        // Parse the "descendants" operator
        assert_eq!(parse("@::"), Ok(wc_symbol.descendants()));
        // Parse the "dag range" operator
        assert_eq!(parse("foo::bar"), Ok(foo_symbol.dag_range_to(&bar_symbol)));
        // Parse the "range" prefix operator
        assert_eq!(parse("..@"), Ok(wc_symbol.ancestors()));
        assert_eq!(
            parse("@.."),
            Ok(wc_symbol.range(&RevsetExpression::visible_heads()))
        );
        assert_eq!(parse("foo..bar"), Ok(foo_symbol.range(&bar_symbol)));
        // Parse the "negate" operator
        assert_eq!(parse("~ foo"), Ok(foo_symbol.negated()));
        assert_eq!(
            parse("~ ~~ foo"),
            Ok(foo_symbol.negated().negated().negated())
        );
        // Parse the "intersection" operator
        assert_eq!(parse("foo & bar"), Ok(foo_symbol.intersection(&bar_symbol)));
        // Parse the "union" operator
        assert_eq!(parse("foo | bar"), Ok(foo_symbol.union(&bar_symbol)));
        // Parse the "difference" operator
        assert_eq!(parse("foo ~ bar"), Ok(foo_symbol.minus(&bar_symbol)));
        // Parentheses are allowed before suffix operators
        assert_eq!(parse("(@)-"), Ok(wc_symbol.parents()));
        // Space is allowed around expressions
        assert_eq!(parse(" ::@ "), Ok(wc_symbol.ancestors()));
        assert_eq!(parse("( ::@ )"), Ok(wc_symbol.ancestors()));
        // Space is not allowed around prefix operators
        assert_eq!(parse(" :: @ "), Err(RevsetParseErrorKind::SyntaxError));
        // Incomplete parse
        assert_eq!(parse("foo | -"), Err(RevsetParseErrorKind::SyntaxError));
        // Space is allowed around infix operators and function arguments
        assert_eq!(
            parse("   description(  arg1 ) ~    file(  arg1 ,   arg2 )  ~ visible_heads(  )  "),
            Ok(
                RevsetExpression::filter(RevsetFilterPredicate::Description("arg1".to_string()))
                    .minus(&RevsetExpression::filter(RevsetFilterPredicate::File(
                        Some(vec![
                            RepoPath::from_internal_string("arg1"),
                            RepoPath::from_internal_string("arg2"),
                        ])
                    )))
                    .minus(&RevsetExpression::visible_heads())
            )
        );
        // Space is allowed around keyword arguments
        assert_eq!(
            parse("remote_branches( remote  =   foo  )").unwrap(),
            parse("remote_branches(remote=foo)").unwrap(),
        );

        // Trailing comma isn't allowed for empty argument
        assert!(parse("branches(,)").is_err());
        // Trailing comma is allowed for the last argument
        assert!(parse("branches(a,)").is_ok());
        assert!(parse("branches(a ,  )").is_ok());
        assert!(parse("branches(,a)").is_err());
        assert!(parse("branches(a,,)").is_err());
        assert!(parse("branches(a  , , )").is_err());
        assert!(parse("file(a,b,)").is_ok());
        assert!(parse("file(a,,b)").is_err());
        assert!(parse("remote_branches(a,remote=b  , )").is_ok());
        assert!(parse("remote_branches(a,,remote=b)").is_err());
    }

    #[test]
    fn test_parse_whitespace() {
        let ascii_whitespaces: String = ('\x00'..='\x7f')
            .filter(char::is_ascii_whitespace)
            .collect();
        assert_eq!(
            parse(&format!("{ascii_whitespaces}all()")).unwrap(),
            parse("all()").unwrap(),
        );
    }

    #[test]
    fn test_parse_revset_alias_formal_parameter() {
        let mut aliases_map = RevsetAliasesMap::new();
        // Trailing comma isn't allowed for empty parameter
        assert!(aliases_map.insert("f(,)", "none()").is_err());
        // Trailing comma is allowed for the last parameter
        assert!(aliases_map.insert("g(a,)", "none()").is_ok());
        assert!(aliases_map.insert("h(a ,  )", "none()").is_ok());
        assert!(aliases_map.insert("i(,a)", "none()").is_err());
        assert!(aliases_map.insert("j(a,,)", "none()").is_err());
        assert!(aliases_map.insert("k(a  , , )", "none()").is_err());
        assert!(aliases_map.insert("l(a,b,)", "none()").is_ok());
        assert!(aliases_map.insert("m(a,,b)", "none()").is_err());
    }

    #[test]
    fn test_parse_revset_compat_operator() {
        assert_eq!(
            parse("foo^"),
            Err(RevsetParseErrorKind::NotPostfixOperator {
                op: "^".to_owned(),
                similar_op: "-".to_owned(),
                description: "parents".to_owned(),
            })
        );
        assert_eq!(
            parse("foo + bar"),
            Err(RevsetParseErrorKind::NotInfixOperator {
                op: "+".to_owned(),
                similar_op: "|".to_owned(),
                description: "union".to_owned(),
            })
        );
        assert_eq!(
            parse("foo - bar"),
            Err(RevsetParseErrorKind::NotInfixOperator {
                op: "-".to_owned(),
                similar_op: "~".to_owned(),
                description: "difference".to_owned(),
            })
        );
    }

    #[test]
    fn test_parse_revset_operator_combinations() {
        let foo_symbol = RevsetExpression::symbol("foo".to_string());
        // Parse repeated "parents" operator
        assert_eq!(
            parse("foo---"),
            Ok(foo_symbol.parents().parents().parents())
        );
        // Parse repeated "children" operator
        assert_eq!(
            parse("foo+++"),
            Ok(foo_symbol.children().children().children())
        );
        // Set operator associativity/precedence
        assert_eq!(parse("~x|y").unwrap(), parse("(~x)|y").unwrap());
        assert_eq!(parse("x&~y").unwrap(), parse("x&(~y)").unwrap());
        assert_eq!(parse("x~~y").unwrap(), parse("x~(~y)").unwrap());
        assert_eq!(parse("x~~~y").unwrap(), parse("x~(~(~y))").unwrap());
        assert_eq!(parse("~x:y").unwrap(), parse("~(x:y)").unwrap());
        assert_eq!(parse("x|y|z").unwrap(), parse("(x|y)|z").unwrap());
        assert_eq!(parse("x&y|z").unwrap(), parse("(x&y)|z").unwrap());
        assert_eq!(parse("x|y&z").unwrap(), parse("x|(y&z)").unwrap());
        assert_eq!(parse("x|y~z").unwrap(), parse("x|(y~z)").unwrap());
        // Parse repeated "ancestors"/"descendants"/"dag range"/"range" operators
        assert_eq!(parse("::foo::"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse(":::foo"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("::::foo"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo:::"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo::::"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo:::bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo::::bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("::foo::bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo::bar::"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("....foo"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo...."), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo.....bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("..foo..bar"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(parse("foo..bar.."), Err(RevsetParseErrorKind::SyntaxError));
        // Parse combinations of "parents"/"children" operators and the range operators.
        // The former bind more strongly.
        assert_eq!(parse("foo-+"), Ok(foo_symbol.parents().children()));
        assert_eq!(parse("foo-::"), Ok(foo_symbol.parents().descendants()));
        assert_eq!(parse("::foo+"), Ok(foo_symbol.children().ancestors()));
    }

    #[test]
    fn test_parse_revset_function() {
        let wc_symbol = RevsetExpression::symbol("@".to_string());
        assert_eq!(parse("parents(@)"), Ok(wc_symbol.parents()));
        assert_eq!(parse("parents((@))"), Ok(wc_symbol.parents()));
        assert_eq!(parse("parents(\"@\")"), Ok(wc_symbol.parents()));
        assert_eq!(
            parse("ancestors(parents(@))"),
            Ok(wc_symbol.parents().ancestors())
        );
        assert_eq!(parse("parents(@"), Err(RevsetParseErrorKind::SyntaxError));
        assert_eq!(
            parse("parents(@,@)"),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "parents".to_string(),
                message: "Expected 1 arguments".to_string()
            })
        );
        assert_eq!(
            parse(r#"description("")"#),
            Ok(RevsetExpression::filter(
                RevsetFilterPredicate::Description("".to_string())
            ))
        );
        assert_eq!(
            parse("description(foo)"),
            Ok(RevsetExpression::filter(
                RevsetFilterPredicate::Description("foo".to_string())
            ))
        );
        assert_eq!(
            parse("description(visible_heads())"),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "description".to_string(),
                message: "Expected function argument of type string".to_string()
            })
        );
        assert_eq!(
            parse("description((foo))"),
            Ok(RevsetExpression::filter(
                RevsetFilterPredicate::Description("foo".to_string())
            ))
        );
        assert_eq!(
            parse("description(\"(foo)\")"),
            Ok(RevsetExpression::filter(
                RevsetFilterPredicate::Description("(foo)".to_string())
            ))
        );
        assert_eq!(
            parse("empty()"),
            Ok(RevsetExpression::filter(RevsetFilterPredicate::File(None)).negated())
        );
        assert!(parse("empty(foo)").is_err());
        assert!(parse("file()").is_err());
        assert_eq!(
            parse("file(foo)"),
            Ok(RevsetExpression::filter(RevsetFilterPredicate::File(Some(
                vec![RepoPath::from_internal_string("foo")]
            ))))
        );
        assert_eq!(
            parse("file(foo, bar, baz)"),
            Ok(RevsetExpression::filter(RevsetFilterPredicate::File(Some(
                vec![
                    RepoPath::from_internal_string("foo"),
                    RepoPath::from_internal_string("bar"),
                    RepoPath::from_internal_string("baz"),
                ]
            ))))
        );
    }

    #[test]
    fn test_parse_revset_keyword_arguments() {
        assert_eq!(
            parse("remote_branches(remote=foo)").unwrap(),
            parse(r#"remote_branches("", foo)"#).unwrap(),
        );
        assert_eq!(
            parse("remote_branches(foo, remote=bar)").unwrap(),
            parse(r#"remote_branches(foo, bar)"#).unwrap(),
        );
        insta::assert_debug_snapshot!(
            parse(r#"remote_branches(remote=foo, bar)"#).unwrap_err(),
            @r###"
        InvalidFunctionArguments {
            name: "remote_branches",
            message: "Positional argument follows keyword argument",
        }
        "###);
        insta::assert_debug_snapshot!(
            parse(r#"remote_branches("", foo, remote=bar)"#).unwrap_err(),
            @r###"
        InvalidFunctionArguments {
            name: "remote_branches",
            message: "Got multiple values for keyword \"remote\"",
        }
        "###);
        insta::assert_debug_snapshot!(
            parse(r#"remote_branches(remote=bar, remote=bar)"#).unwrap_err(),
            @r###"
        InvalidFunctionArguments {
            name: "remote_branches",
            message: "Got multiple values for keyword \"remote\"",
        }
        "###);
        insta::assert_debug_snapshot!(
            parse(r#"remote_branches(unknown=bar)"#).unwrap_err(),
            @r###"
        InvalidFunctionArguments {
            name: "remote_branches",
            message: "Unexpected keyword argument \"unknown\"",
        }
        "###);
    }

    #[test]
    fn test_expand_symbol_alias() {
        assert_eq!(
            parse_with_aliases("AB|c", [("AB", "a|b")]).unwrap(),
            parse("(a|b)|c").unwrap()
        );
        assert_eq!(
            parse_with_aliases("AB:heads(AB)", [("AB", "a|b")]).unwrap(),
            parse("(a|b):heads(a|b)").unwrap()
        );

        // Not string substitution 'a&b|c', but tree substitution.
        assert_eq!(
            parse_with_aliases("a&BC", [("BC", "b|c")]).unwrap(),
            parse("a&(b|c)").unwrap()
        );

        // String literal should not be substituted with alias.
        assert_eq!(
            parse_with_aliases(r#"A|"A""#, [("A", "a")]).unwrap(),
            parse("a|A").unwrap()
        );

        // Alias can be substituted to string literal.
        assert_eq!(
            parse_with_aliases("author(A)", [("A", "a")]).unwrap(),
            parse("author(a)").unwrap()
        );

        // Multi-level substitution.
        assert_eq!(
            parse_with_aliases("A", [("A", "BC"), ("BC", "b|C"), ("C", "c")]).unwrap(),
            parse("b|c").unwrap()
        );

        // Infinite recursion, where the top-level error isn't of RecursiveAlias kind.
        assert_eq!(
            parse_with_aliases("A", [("A", "A")]),
            Err(RevsetParseErrorKind::BadAliasExpansion("A".to_owned()))
        );
        assert_eq!(
            parse_with_aliases("A", [("A", "B"), ("B", "b|C"), ("C", "c|B")]),
            Err(RevsetParseErrorKind::BadAliasExpansion("A".to_owned()))
        );

        // Error in alias definition.
        assert_eq!(
            parse_with_aliases("A", [("A", "a(")]),
            Err(RevsetParseErrorKind::BadAliasExpansion("A".to_owned()))
        );
    }

    #[test]
    fn test_expand_function_alias() {
        assert_eq!(
            parse_with_aliases("F()", [("F(  )", "a")]).unwrap(),
            parse("a").unwrap()
        );
        assert_eq!(
            parse_with_aliases("F(a)", [("F( x  )", "x")]).unwrap(),
            parse("a").unwrap()
        );
        assert_eq!(
            parse_with_aliases("F(a, b)", [("F( x,  y )", "x|y")]).unwrap(),
            parse("a|b").unwrap()
        );

        // Arguments should be resolved in the current scope.
        assert_eq!(
            parse_with_aliases("F(a:y,b:x)", [("F(x,y)", "x|y")]).unwrap(),
            parse("(a:y)|(b:x)").unwrap()
        );
        // F(a) -> G(a)&y -> (x|a)&y
        assert_eq!(
            parse_with_aliases("F(a)", [("F(x)", "G(x)&y"), ("G(y)", "x|y")]).unwrap(),
            parse("(x|a)&y").unwrap()
        );
        // F(G(a)) -> F(x|a) -> G(x|a)&y -> (x|(x|a))&y
        assert_eq!(
            parse_with_aliases("F(G(a))", [("F(x)", "G(x)&y"), ("G(y)", "x|y")]).unwrap(),
            parse("(x|(x|a))&y").unwrap()
        );

        // Function parameter should precede the symbol alias.
        assert_eq!(
            parse_with_aliases("F(a)|X", [("F(X)", "X"), ("X", "x")]).unwrap(),
            parse("a|x").unwrap()
        );

        // Function parameter shouldn't be expanded in symbol alias.
        assert_eq!(
            parse_with_aliases("F(a)", [("F(x)", "x|A"), ("A", "x")]).unwrap(),
            parse("a|x").unwrap()
        );

        // String literal should not be substituted with function parameter.
        assert_eq!(
            parse_with_aliases("F(a)", [("F(x)", r#"x|"x""#)]).unwrap(),
            parse("a|x").unwrap()
        );

        // Pass string literal as parameter.
        assert_eq!(
            parse_with_aliases("F(a)", [("F(x)", "author(x)|committer(x)")]).unwrap(),
            parse("author(a)|committer(a)").unwrap()
        );

        // Function and symbol aliases reside in separate namespaces.
        assert_eq!(
            parse_with_aliases("A()", [("A()", "A"), ("A", "a")]).unwrap(),
            parse("a").unwrap()
        );

        // Invalid number of arguments.
        assert_eq!(
            parse_with_aliases("F(a)", [("F()", "x")]),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "F".to_owned(),
                message: "Expected 0 arguments".to_owned()
            })
        );
        assert_eq!(
            parse_with_aliases("F()", [("F(x)", "x")]),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "F".to_owned(),
                message: "Expected 1 arguments".to_owned()
            })
        );
        assert_eq!(
            parse_with_aliases("F(a,b,c)", [("F(x,y)", "x|y")]),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "F".to_owned(),
                message: "Expected 2 arguments".to_owned()
            })
        );

        // Keyword argument isn't supported for now.
        assert_eq!(
            parse_with_aliases("F(x=y)", [("F(x)", "x")]),
            Err(RevsetParseErrorKind::InvalidFunctionArguments {
                name: "F".to_owned(),
                message: r#"Unexpected keyword argument "x""#.to_owned()
            })
        );

        // Infinite recursion, where the top-level error isn't of RecursiveAlias kind.
        assert_eq!(
            parse_with_aliases(
                "F(a)",
                [("F(x)", "G(x)"), ("G(x)", "H(x)"), ("H(x)", "F(x)")]
            ),
            Err(RevsetParseErrorKind::BadAliasExpansion("F()".to_owned()))
        );
    }

    #[test]
    fn test_optimize_subtree() {
        // Check that transform_expression_bottom_up() never rewrites enum variant
        // (e.g. Range -> DagRange) nor reorders arguments unintentionally.

        assert_eq!(
            optimize(parse("parents(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).parents()
        );
        assert_eq!(
            optimize(parse("children(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).children()
        );
        assert_eq!(
            optimize(parse("ancestors(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).ancestors()
        );
        assert_eq!(
            optimize(parse("descendants(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).descendants()
        );

        assert_eq!(
            optimize(parse("(branches() & all())..(all() & tags())").unwrap()),
            RevsetExpression::branches("".to_owned()).range(&RevsetExpression::tags())
        );
        assert_eq!(
            optimize(parse("(branches() & all()):(all() & tags())").unwrap()),
            RevsetExpression::branches("".to_owned()).dag_range_to(&RevsetExpression::tags())
        );

        assert_eq!(
            optimize(parse("heads(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).heads()
        );
        assert_eq!(
            optimize(parse("roots(branches() & all())").unwrap()),
            RevsetExpression::branches("".to_owned()).roots()
        );

        assert_eq!(
            optimize(parse("latest(branches() & all(), 2)").unwrap()),
            RevsetExpression::branches("".to_owned()).latest(2)
        );

        assert_eq!(
            optimize(parse("present(foo ~ bar)").unwrap()),
            Rc::new(RevsetExpression::Present(
                RevsetExpression::symbol("foo".to_owned())
                    .minus(&RevsetExpression::symbol("bar".to_owned()))
            ))
        );
        assert_eq!(
            optimize(parse("present(branches() & all())").unwrap()),
            Rc::new(RevsetExpression::Present(RevsetExpression::branches(
                "".to_owned()
            )))
        );

        assert_eq!(
            optimize(parse("~branches() & all()").unwrap()),
            RevsetExpression::branches("".to_owned()).negated()
        );
        assert_eq!(
            optimize(parse("(branches() & all()) | (all() & tags())").unwrap()),
            RevsetExpression::branches("".to_owned()).union(&RevsetExpression::tags())
        );
        assert_eq!(
            optimize(parse("(branches() & all()) & (all() & tags())").unwrap()),
            RevsetExpression::branches("".to_owned()).intersection(&RevsetExpression::tags())
        );
        assert_eq!(
            optimize(parse("(branches() & all()) ~ (all() & tags())").unwrap()),
            RevsetExpression::branches("".to_owned()).minus(&RevsetExpression::tags())
        );
    }

    #[test]
    fn test_optimize_unchanged_subtree() {
        fn unwrap_union(
            expression: &RevsetExpression,
        ) -> (&Rc<RevsetExpression>, &Rc<RevsetExpression>) {
            match expression {
                RevsetExpression::Union(left, right) => (left, right),
                _ => panic!("unexpected expression: {expression:?}"),
            }
        }

        // transform_expression_bottom_up() should not recreate tree unnecessarily.
        let parsed = parse("foo-").unwrap();
        let optimized = optimize(parsed.clone());
        assert!(Rc::ptr_eq(&parsed, &optimized));

        let parsed = parse("branches() | tags()").unwrap();
        let optimized = optimize(parsed.clone());
        assert!(Rc::ptr_eq(&parsed, &optimized));

        let parsed = parse("branches() & tags()").unwrap();
        let optimized = optimize(parsed.clone());
        assert!(Rc::ptr_eq(&parsed, &optimized));

        // Only left subtree should be rewritten.
        let parsed = parse("(branches() & all()) | tags()").unwrap();
        let optimized = optimize(parsed.clone());
        assert_eq!(
            unwrap_union(&optimized).0.as_ref(),
            &RevsetExpression::CommitRef(RevsetCommitRef::Branches("".to_owned()))
        );
        assert!(Rc::ptr_eq(
            unwrap_union(&parsed).1,
            unwrap_union(&optimized).1
        ));

        // Only right subtree should be rewritten.
        let parsed = parse("branches() | (all() & tags())").unwrap();
        let optimized = optimize(parsed.clone());
        assert!(Rc::ptr_eq(
            unwrap_union(&parsed).0,
            unwrap_union(&optimized).0
        ));
        assert_eq!(
            unwrap_union(&optimized).1.as_ref(),
            &RevsetExpression::CommitRef(RevsetCommitRef::Tags),
        );
    }

    #[test]
    fn test_optimize_difference() {
        insta::assert_debug_snapshot!(optimize(parse("foo & ~bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("~foo & bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("~foo & bar & ~baz").unwrap()), @r###"
        Difference(
            Difference(
                CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
            ),
            CommitRef(
                Symbol(
                    "baz",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("(all() & ~foo) & bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
        )
        "###);

        // Binary difference operation should go through the same optimization passes.
        insta::assert_debug_snapshot!(optimize(parse("all() ~ foo").unwrap()), @r###"
        NotIn(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo ~ bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("(all() ~ foo) & bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
        )
        "###);

        // Range expression.
        insta::assert_debug_snapshot!(optimize(parse(":foo & ~:bar").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 0..18446744073709551615,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("~:foo & :bar").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            heads: CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            generation: 0..18446744073709551615,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo..").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            heads: CommitRef(
                VisibleHeads,
            ),
            generation: 0..18446744073709551615,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo..bar").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            heads: CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            generation: 0..18446744073709551615,
        }
        "###);

        // Double/triple negates.
        insta::assert_debug_snapshot!(optimize(parse("foo & ~~bar").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo & ~~~bar").unwrap()), @r###"
        Difference(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("~(all() & ~foo) & bar").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);

        // Should be better than '(all() & ~foo) & (all() & ~bar)'.
        insta::assert_debug_snapshot!(optimize(parse("~foo & ~bar").unwrap()), @r###"
        Difference(
            NotIn(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
            ),
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
        )
        "###);
    }

    #[test]
    fn test_optimize_filter_difference() {
        // '~empty()' -> '~~file(*)' -> 'file(*)'
        insta::assert_debug_snapshot!(optimize(parse("~empty()").unwrap()), @r###"
        Filter(
            File(
                None,
            ),
        )
        "###);

        // '& baz' can be moved into the filter node, and form a difference node.
        insta::assert_debug_snapshot!(
            optimize(parse("(author(foo) & ~bar) & baz").unwrap()), @r###"
        Intersection(
            Difference(
                CommitRef(
                    Symbol(
                        "baz",
                    ),
                ),
                CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
            ),
            Filter(
                Author(
                    "foo",
                ),
            ),
        )
        "###);

        // '~set & filter()' shouldn't be substituted.
        insta::assert_debug_snapshot!(
            optimize(parse("~foo & author(bar)").unwrap()), @r###"
        Intersection(
            NotIn(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
            ),
            Filter(
                Author(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("~foo & (author(bar) | baz)").unwrap()), @r###"
        Intersection(
            NotIn(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
            ),
            AsFilter(
                Union(
                    Filter(
                        Author(
                            "bar",
                        ),
                    ),
                    CommitRef(
                        Symbol(
                            "baz",
                        ),
                    ),
                ),
            ),
        )
        "###);

        // Filter should be moved right of the intersection.
        insta::assert_debug_snapshot!(
            optimize(parse("author(foo) ~ bar").unwrap()), @r###"
        Intersection(
            NotIn(
                CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
            ),
            Filter(
                Author(
                    "foo",
                ),
            ),
        )
        "###);
    }

    #[test]
    fn test_optimize_filter_intersection() {
        insta::assert_debug_snapshot!(optimize(parse("author(foo)").unwrap()), @r###"
        Filter(
            Author(
                "foo",
            ),
        )
        "###);

        insta::assert_debug_snapshot!(optimize(parse("foo & description(bar)").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            Filter(
                Description(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("author(foo) & bar").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            Filter(
                Author(
                    "foo",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("author(foo) & committer(bar)").unwrap()), @r###"
        Intersection(
            Filter(
                Author(
                    "foo",
                ),
            ),
            Filter(
                Committer(
                    "bar",
                ),
            ),
        )
        "###);

        insta::assert_debug_snapshot!(
            optimize(parse("foo & description(bar) & author(baz)").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                Filter(
                    Description(
                        "bar",
                    ),
                ),
            ),
            Filter(
                Author(
                    "baz",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("committer(foo) & bar & author(baz)").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
                Filter(
                    Committer(
                        "foo",
                    ),
                ),
            ),
            Filter(
                Author(
                    "baz",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("committer(foo) & file(bar) & baz").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "baz",
                    ),
                ),
                Filter(
                    Committer(
                        "foo",
                    ),
                ),
            ),
            Filter(
                File(
                    Some(
                        [
                            "bar",
                        ],
                    ),
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("committer(foo) & file(bar) & author(baz)").unwrap()), @r###"
        Intersection(
            Intersection(
                Filter(
                    Committer(
                        "foo",
                    ),
                ),
                Filter(
                    File(
                        Some(
                            [
                                "bar",
                            ],
                        ),
                    ),
                ),
            ),
            Filter(
                Author(
                    "baz",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo & file(bar) & baz").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                CommitRef(
                    Symbol(
                        "baz",
                    ),
                ),
            ),
            Filter(
                File(
                    Some(
                        [
                            "bar",
                        ],
                    ),
                ),
            ),
        )
        "###);

        insta::assert_debug_snapshot!(
            optimize(parse("foo & description(bar) & author(baz) & qux").unwrap()), @r###"
        Intersection(
            Intersection(
                Intersection(
                    CommitRef(
                        Symbol(
                            "foo",
                        ),
                    ),
                    CommitRef(
                        Symbol(
                            "qux",
                        ),
                    ),
                ),
                Filter(
                    Description(
                        "bar",
                    ),
                ),
            ),
            Filter(
                Author(
                    "baz",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("foo & description(bar) & parents(author(baz)) & qux").unwrap()), @r###"
        Intersection(
            Intersection(
                Intersection(
                    CommitRef(
                        Symbol(
                            "foo",
                        ),
                    ),
                    Ancestors {
                        heads: Filter(
                            Author(
                                "baz",
                            ),
                        ),
                        generation: 1..2,
                        is_legacy: false,
                    },
                ),
                CommitRef(
                    Symbol(
                        "qux",
                    ),
                ),
            ),
            Filter(
                Description(
                    "bar",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("foo & description(bar) & parents(author(baz) & qux)").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                Ancestors {
                    heads: Intersection(
                        CommitRef(
                            Symbol(
                                "qux",
                            ),
                        ),
                        Filter(
                            Author(
                                "baz",
                            ),
                        ),
                    ),
                    generation: 1..2,
                    is_legacy: false,
                },
            ),
            Filter(
                Description(
                    "bar",
                ),
            ),
        )
        "###);

        // Symbols have to be pushed down to the innermost filter node.
        insta::assert_debug_snapshot!(
            optimize(parse("(a & author(A)) & (b & author(B)) & (c & author(C))").unwrap()), @r###"
        Intersection(
            Intersection(
                Intersection(
                    Intersection(
                        Intersection(
                            CommitRef(
                                Symbol(
                                    "a",
                                ),
                            ),
                            CommitRef(
                                Symbol(
                                    "b",
                                ),
                            ),
                        ),
                        CommitRef(
                            Symbol(
                                "c",
                            ),
                        ),
                    ),
                    Filter(
                        Author(
                            "A",
                        ),
                    ),
                ),
                Filter(
                    Author(
                        "B",
                    ),
                ),
            ),
            Filter(
                Author(
                    "C",
                ),
            ),
        )
        "###);
        insta::assert_debug_snapshot!(
            optimize(parse("(a & author(A)) & ((b & author(B)) & (c & author(C))) & d").unwrap()),
            @r###"
        Intersection(
            Intersection(
                Intersection(
                    Intersection(
                        Intersection(
                            CommitRef(
                                Symbol(
                                    "a",
                                ),
                            ),
                            Intersection(
                                CommitRef(
                                    Symbol(
                                        "b",
                                    ),
                                ),
                                CommitRef(
                                    Symbol(
                                        "c",
                                    ),
                                ),
                            ),
                        ),
                        CommitRef(
                            Symbol(
                                "d",
                            ),
                        ),
                    ),
                    Filter(
                        Author(
                            "A",
                        ),
                    ),
                ),
                Filter(
                    Author(
                        "B",
                    ),
                ),
            ),
            Filter(
                Author(
                    "C",
                ),
            ),
        )
        "###);

        // 'all()' moves in to 'filter()' first, so 'A & filter()' can be found.
        insta::assert_debug_snapshot!(
            optimize(parse("foo & (all() & description(bar)) & (author(baz) & all())").unwrap()),
            @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                Filter(
                    Description(
                        "bar",
                    ),
                ),
            ),
            Filter(
                Author(
                    "baz",
                ),
            ),
        )
        "###);
    }

    #[test]
    fn test_optimize_filter_subtree() {
        insta::assert_debug_snapshot!(
            optimize(parse("(author(foo) | bar) & baz").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "baz",
                ),
            ),
            AsFilter(
                Union(
                    Filter(
                        Author(
                            "foo",
                        ),
                    ),
                    CommitRef(
                        Symbol(
                            "bar",
                        ),
                    ),
                ),
            ),
        )
        "###);

        insta::assert_debug_snapshot!(
            optimize(parse("(foo | committer(bar)) & description(baz) & qux").unwrap()), @r###"
        Intersection(
            Intersection(
                CommitRef(
                    Symbol(
                        "qux",
                    ),
                ),
                AsFilter(
                    Union(
                        CommitRef(
                            Symbol(
                                "foo",
                            ),
                        ),
                        Filter(
                            Committer(
                                "bar",
                            ),
                        ),
                    ),
                ),
            ),
            Filter(
                Description(
                    "baz",
                ),
            ),
        )
        "###);

        insta::assert_debug_snapshot!(
            optimize(parse("(~present(author(foo) & bar) | baz) & qux").unwrap()), @r###"
        Intersection(
            CommitRef(
                Symbol(
                    "qux",
                ),
            ),
            AsFilter(
                Union(
                    AsFilter(
                        NotIn(
                            AsFilter(
                                Present(
                                    Intersection(
                                        CommitRef(
                                            Symbol(
                                                "bar",
                                            ),
                                        ),
                                        Filter(
                                            Author(
                                                "foo",
                                            ),
                                        ),
                                    ),
                                ),
                            ),
                        ),
                    ),
                    CommitRef(
                        Symbol(
                            "baz",
                        ),
                    ),
                ),
            ),
        )
        "###);

        // Symbols have to be pushed down to the innermost filter node.
        insta::assert_debug_snapshot!(
            optimize(parse(
                "(a & (author(A) | 0)) & (b & (author(B) | 1)) & (c & (author(C) | 2))").unwrap()),
            @r###"
        Intersection(
            Intersection(
                Intersection(
                    Intersection(
                        Intersection(
                            CommitRef(
                                Symbol(
                                    "a",
                                ),
                            ),
                            CommitRef(
                                Symbol(
                                    "b",
                                ),
                            ),
                        ),
                        CommitRef(
                            Symbol(
                                "c",
                            ),
                        ),
                    ),
                    AsFilter(
                        Union(
                            Filter(
                                Author(
                                    "A",
                                ),
                            ),
                            CommitRef(
                                Symbol(
                                    "0",
                                ),
                            ),
                        ),
                    ),
                ),
                AsFilter(
                    Union(
                        Filter(
                            Author(
                                "B",
                            ),
                        ),
                        CommitRef(
                            Symbol(
                                "1",
                            ),
                        ),
                    ),
                ),
            ),
            AsFilter(
                Union(
                    Filter(
                        Author(
                            "C",
                        ),
                    ),
                    CommitRef(
                        Symbol(
                            "2",
                        ),
                    ),
                ),
            ),
        )
        "###);
    }

    #[test]
    fn test_optimize_ancestors() {
        // Typical scenario: fold nested parents()
        insta::assert_debug_snapshot!(optimize(parse("foo--").unwrap()), @r###"
        Ancestors {
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 2..3,
            is_legacy: false,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse(":(foo---)").unwrap()), @r###"
        Ancestors {
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 3..18446744073709551615,
            is_legacy: false,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("(:foo)---").unwrap()), @r###"
        Ancestors {
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 3..18446744073709551615,
            is_legacy: false,
        }
        "###);

        // 'foo-+' is not 'foo'.
        insta::assert_debug_snapshot!(optimize(parse("foo---+").unwrap()), @r###"
        Descendants {
            roots: Ancestors {
                heads: CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                generation: 3..4,
                is_legacy: false,
            },
            generation: 1..2,
            is_legacy: false,
        }
        "###);

        // For 'roots..heads', heads can be folded.
        insta::assert_debug_snapshot!(optimize(parse("foo..(bar--)").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            heads: CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            generation: 2..18446744073709551615,
        }
        "###);
        // roots can also be folded, but range expression cannot be reconstructed.
        // No idea if this is better than the original range expression.
        insta::assert_debug_snapshot!(optimize(parse("(foo--)..(bar---)").unwrap()), @r###"
        Difference(
            Ancestors {
                heads: CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
                generation: 3..18446744073709551615,
                is_legacy: false,
            },
            Ancestors {
                heads: CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                generation: 2..18446744073709551615,
                is_legacy: false,
            },
        )
        "###);

        // If inner range is bounded by roots, it cannot be merged.
        // e.g. '..(foo..foo)' is equivalent to '..none()', not to '..foo'
        insta::assert_debug_snapshot!(optimize(parse("(foo..bar)--").unwrap()), @r###"
        Ancestors {
            heads: Range {
                roots: CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                heads: CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
                generation: 0..18446744073709551615,
            },
            generation: 2..3,
            is_legacy: false,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("foo..(bar..baz)").unwrap()), @r###"
        Range {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            heads: Range {
                roots: CommitRef(
                    Symbol(
                        "bar",
                    ),
                ),
                heads: CommitRef(
                    Symbol(
                        "baz",
                    ),
                ),
                generation: 0..18446744073709551615,
            },
            generation: 0..18446744073709551615,
        }
        "###);

        // Ancestors of empty generation range should be empty.
        // TODO: rewrite these tests if we added syntax for arbitrary generation
        // ancestors
        let empty_generation_ancestors = |heads| {
            Rc::new(RevsetExpression::Ancestors {
                heads,
                generation: GENERATION_RANGE_EMPTY,
                is_legacy: false,
            })
        };
        insta::assert_debug_snapshot!(
            optimize(empty_generation_ancestors(
                RevsetExpression::symbol("foo".to_owned()).ancestors()
            )),
            @r###"
        Ancestors {
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 0..0,
            is_legacy: false,
        }
        "###
        );
        insta::assert_debug_snapshot!(
            optimize(
                empty_generation_ancestors(RevsetExpression::symbol("foo".to_owned())).ancestors()
            ),
            @r###"
        Ancestors {
            heads: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 0..0,
            is_legacy: false,
        }
        "###
        );
    }

    #[test]
    fn test_optimize_descendants() {
        // Typical scenario: fold nested children()
        insta::assert_debug_snapshot!(optimize(parse("foo++").unwrap()), @r###"
        Descendants {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 2..3,
            is_legacy: false,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("(foo+++):").unwrap()), @r###"
        Descendants {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 3..18446744073709551615,
            is_legacy: false,
        }
        "###);
        insta::assert_debug_snapshot!(optimize(parse("(foo:)+++").unwrap()), @r###"
        Descendants {
            roots: CommitRef(
                Symbol(
                    "foo",
                ),
            ),
            generation: 3..18446744073709551615,
            is_legacy: false,
        }
        "###);

        // 'foo+-' is not 'foo'.
        insta::assert_debug_snapshot!(optimize(parse("foo+++-").unwrap()), @r###"
        Ancestors {
            heads: Descendants {
                roots: CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                generation: 3..4,
                is_legacy: false,
            },
            generation: 1..2,
            is_legacy: false,
        }
        "###);

        // TODO: Inner Descendants can be folded into DagRange. Perhaps, we can rewrite
        // 'x:y' to 'x: & :y' first, so the common substitution rule can handle both
        // 'x+:y' and 'x+ & :y'.
        insta::assert_debug_snapshot!(optimize(parse("(foo++):bar").unwrap()), @r###"
        DagRange {
            roots: Descendants {
                roots: CommitRef(
                    Symbol(
                        "foo",
                    ),
                ),
                generation: 2..3,
                is_legacy: false,
            },
            heads: CommitRef(
                Symbol(
                    "bar",
                ),
            ),
            is_legacy: false,
        }
        "###);
    }
}