revset: add origin field to RevsetParseError to chain alias errors

This could be embedded in a variant of RevsetParseErrorKind, but I want to
keep the enum comparable.

lib/src/revset.rs

@@ -198,6 +198,7 @@ pub struct RevsetParser;
 pub struct RevsetParseError {
     kind: RevsetParseErrorKind,
     pest_error: Option<Box<pest::error::Error<Rule>>>,
+    origin: Option<Box<RevsetParseError>>,
 }
 
 #[derive(Debug, Error, PartialEq, Eq)]
@@ -219,6 +220,7 @@ impl RevsetParseError {
         RevsetParseError {
             kind,
             pest_error: None,
+            origin: None,
         }
     }
 
@@ -232,12 +234,36 @@ impl RevsetParseError {
         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 {
@@ -245,6 +271,7 @@ impl From<pest::error::Error<Rule>> for RevsetParseError {
         RevsetParseError {
             kind: RevsetParseErrorKind::SyntaxError,
             pest_error: Some(Box::new(err)),
+            origin: None,
         }
     }
 }
@@ -261,6 +288,9 @@ impl fmt::Display for RevsetParseError {
 
 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 => {

src/cli_util.rs

@@ -16,6 +16,7 @@ use std::collections::{HashSet, VecDeque};
 use std::env::ArgsOs;
 use std::ffi::OsString;
 use std::fmt::Debug;
+use std::iter;
 use std::path::PathBuf;
 use std::rc::Rc;
 use std::sync::Arc;
@@ -175,7 +176,8 @@ impl From<GitExportError> for CommandError {
 
 impl From<RevsetParseError> for CommandError {
     fn from(err: RevsetParseError) -> Self {
-        user_error(format!("Failed to parse revset: {err}"))
+        let message = iter::successors(Some(&err), |e| e.origin()).join("\n");
+        user_error(format!("Failed to parse revset: {message}"))
     }
 }