index: introduce newtype that represents segment-local position

I'm thinking of changing some IndexSegment methods to return LocalPosition
instead of global IndexPosition, and using u32 there would be a source of bugs.

lib/src/default_index/composite.rs

@@ -21,7 +21,9 @@ use std::sync::Arc;
 
 use itertools::Itertools;
 
-use super::entry::{IndexEntry, IndexPosition, IndexPositionByGeneration, SmallIndexPositionsVec};
+use super::entry::{
+    IndexEntry, IndexPosition, IndexPositionByGeneration, LocalPosition, SmallIndexPositionsVec,
+};
 use super::readonly::ReadonlyIndexSegment;
 use super::rev_walk::RevWalk;
 use crate::backend::{ChangeId, CommitId, ObjectId};
@@ -50,15 +52,15 @@ pub(super) trait IndexSegment: Send + Sync {
 
     fn segment_resolve_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<CommitId>;
 
-    fn segment_generation_number(&self, local_pos: u32) -> u32;
+    fn segment_generation_number(&self, local_pos: LocalPosition) -> u32;
 
-    fn segment_commit_id(&self, local_pos: u32) -> CommitId;
+    fn segment_commit_id(&self, local_pos: LocalPosition) -> CommitId;
 
-    fn segment_change_id(&self, local_pos: u32) -> ChangeId;
+    fn segment_change_id(&self, local_pos: LocalPosition) -> ChangeId;
 
-    fn segment_num_parents(&self, local_pos: u32) -> u32;
+    fn segment_num_parents(&self, local_pos: LocalPosition) -> u32;
 
-    fn segment_parent_positions(&self, local_pos: u32) -> SmallIndexPositionsVec;
+    fn segment_parent_positions(&self, local_pos: LocalPosition) -> SmallIndexPositionsVec;
 }
 
 /// Abstraction over owned and borrowed types that can be cheaply converted to
@@ -151,7 +153,7 @@ impl<'a> CompositeIndex<'a> {
         self.ancestor_index_segments()
             .find_map(|segment| {
                 u32::checked_sub(pos.0, segment.segment_num_parent_commits())
-                    .map(|local_pos| IndexEntry::new(segment, pos, local_pos))
+                    .map(|local_pos| IndexEntry::new(segment, pos, LocalPosition(local_pos)))
             })
             .unwrap()
     }

lib/src/default_index/entry.rs

@@ -22,6 +22,7 @@ use smallvec::SmallVec;
 use super::composite::{CompositeIndex, IndexSegment};
 use crate::backend::{ChangeId, CommitId, ObjectId};
 
+/// Global index position.
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
 pub struct IndexPosition(pub(super) u32);
 
@@ -29,6 +30,10 @@ impl IndexPosition {
     pub const MAX: Self = IndexPosition(u32::MAX);
 }
 
+/// Local position within an index segment.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
+pub(super) struct LocalPosition(pub(super) u32);
+
 // SmallVec reuses two pointer-size fields as inline area, which meas we can
 // inline up to 16 bytes (on 64-bit platform) for free.
 pub(super) type SmallIndexPositionsVec = SmallVec<[IndexPosition; 4]>;
@@ -37,8 +42,8 @@ pub(super) type SmallIndexPositionsVec = SmallVec<[IndexPosition; 4]>;
 pub struct IndexEntry<'a> {
     source: &'a dyn IndexSegment,
     pos: IndexPosition,
-    // Position within the source segment
-    local_pos: u32,
+    /// Position within the source segment
+    local_pos: LocalPosition,
 }
 
 impl Debug for IndexEntry<'_> {
@@ -66,7 +71,11 @@ impl Hash for IndexEntry<'_> {
 }
 
 impl<'a> IndexEntry<'a> {
-    pub(super) fn new(source: &'a dyn IndexSegment, pos: IndexPosition, local_pos: u32) -> Self {
+    pub(super) fn new(
+        source: &'a dyn IndexSegment,
+        pos: IndexPosition,
+        local_pos: LocalPosition,
+    ) -> Self {
         IndexEntry {
             source,
             pos,

lib/src/default_index/mutable.rs

@@ -30,7 +30,7 @@ use smallvec::SmallVec;
 use tempfile::NamedTempFile;
 
 use super::composite::{AsCompositeIndex, CompositeIndex, IndexSegment};
-use super::entry::{IndexPosition, SmallIndexPositionsVec};
+use super::entry::{IndexPosition, LocalPosition, SmallIndexPositionsVec};
 use super::readonly::{DefaultReadonlyIndex, ReadonlyIndexSegment};
 use crate::backend::{ChangeId, CommitId, ObjectId};
 use crate::commit::Commit;
@@ -351,28 +351,28 @@ impl IndexSegment for MutableIndexSegment {
         }
     }
 
-    fn segment_generation_number(&self, local_pos: u32) -> u32 {
-        self.graph[local_pos as usize].generation_number
+    fn segment_generation_number(&self, local_pos: LocalPosition) -> u32 {
+        self.graph[local_pos.0 as usize].generation_number
     }
 
-    fn segment_commit_id(&self, local_pos: u32) -> CommitId {
-        self.graph[local_pos as usize].commit_id.clone()
+    fn segment_commit_id(&self, local_pos: LocalPosition) -> CommitId {
+        self.graph[local_pos.0 as usize].commit_id.clone()
     }
 
-    fn segment_change_id(&self, local_pos: u32) -> ChangeId {
-        self.graph[local_pos as usize].change_id.clone()
+    fn segment_change_id(&self, local_pos: LocalPosition) -> ChangeId {
+        self.graph[local_pos.0 as usize].change_id.clone()
     }
 
-    fn segment_num_parents(&self, local_pos: u32) -> u32 {
-        self.graph[local_pos as usize]
+    fn segment_num_parents(&self, local_pos: LocalPosition) -> u32 {
+        self.graph[local_pos.0 as usize]
             .parent_positions
             .len()
             .try_into()
             .unwrap()
     }
 
-    fn segment_parent_positions(&self, local_pos: u32) -> SmallIndexPositionsVec {
-        self.graph[local_pos as usize].parent_positions.clone()
+    fn segment_parent_positions(&self, local_pos: LocalPosition) -> SmallIndexPositionsVec {
+        self.graph[local_pos.0 as usize].parent_positions.clone()
     }
 }
 

lib/src/default_index/readonly.rs

@@ -27,7 +27,7 @@ use smallvec::SmallVec;
 use thiserror::Error;
 
 use super::composite::{AsCompositeIndex, CompositeIndex, IndexSegment};
-use super::entry::{IndexPosition, SmallIndexPositionsVec};
+use super::entry::{IndexPosition, LocalPosition, SmallIndexPositionsVec};
 use super::mutable::DefaultMutableIndex;
 use crate::backend::{ChangeId, CommitId, ObjectId};
 use crate::default_revset_engine;
@@ -310,9 +310,9 @@ impl ReadonlyIndexSegment {
         self.change_id_length
     }
 
-    fn graph_entry(&self, local_pos: u32) -> CommitGraphEntry {
-        assert!(local_pos < self.num_local_commits);
-        let offset = (local_pos as usize) * self.commit_graph_entry_size;
+    fn graph_entry(&self, local_pos: LocalPosition) -> CommitGraphEntry {
+        assert!(local_pos.0 < self.num_local_commits);
+        let offset = (local_pos.0 as usize) * self.commit_graph_entry_size;
         CommitGraphEntry {
             data: &self.data[offset..][..self.commit_graph_entry_size],
             commit_id_length: self.commit_id_length,
@@ -426,23 +426,23 @@ impl IndexSegment for ReadonlyIndexSegment {
         }
     }
 
-    fn segment_generation_number(&self, local_pos: u32) -> u32 {
+    fn segment_generation_number(&self, local_pos: LocalPosition) -> u32 {
         self.graph_entry(local_pos).generation_number()
     }
 
-    fn segment_commit_id(&self, local_pos: u32) -> CommitId {
+    fn segment_commit_id(&self, local_pos: LocalPosition) -> CommitId {
         self.graph_entry(local_pos).commit_id()
     }
 
-    fn segment_change_id(&self, local_pos: u32) -> ChangeId {
+    fn segment_change_id(&self, local_pos: LocalPosition) -> ChangeId {
         self.graph_entry(local_pos).change_id()
     }
 
-    fn segment_num_parents(&self, local_pos: u32) -> u32 {
+    fn segment_num_parents(&self, local_pos: LocalPosition) -> u32 {
         self.graph_entry(local_pos).num_parents()
     }
 
-    fn segment_parent_positions(&self, local_pos: u32) -> SmallIndexPositionsVec {
+    fn segment_parent_positions(&self, local_pos: LocalPosition) -> SmallIndexPositionsVec {
         let graph_entry = self.graph_entry(local_pos);
         let mut parent_entries = SmallVec::with_capacity(graph_entry.num_parents() as usize);
         if graph_entry.num_parents() >= 1 {