index: specialize evaluate_revset_static() to change_id_index_static()

I'd like to move `change_id_index()` from `Revset` to `Index` (and
make it take the set of visible heads as argument). We currently use
`evaluate_revset_static()` only to get a `ChangeIdIndex`, so a good
place to start is to convert that into `change_id_index_static()`.

lib/src/default_index/composite.rs

@@ -396,6 +396,23 @@ pub(super) struct ChangeIdIndexImpl<I> {
     pub pos_by_change: IdIndex<ChangeId, IndexPosition, 4>,
 }
 
+impl<I: AsCompositeIndex> ChangeIdIndexImpl<I> {
+    pub fn new(index: I, heads: &mut dyn Iterator<Item = &CommitId>) -> ChangeIdIndexImpl<I> {
+        let mut pos_by_change = IdIndex::builder();
+        let composite = index.as_composite();
+        let head_positions = heads
+            .map(|id| composite.commit_id_to_pos(id).unwrap())
+            .collect_vec();
+        for entry in composite.walk_revs(&head_positions, &[]) {
+            pos_by_change.insert(&entry.change_id(), entry.position());
+        }
+        Self {
+            index,
+            pos_by_change: pos_by_change.build(),
+        }
+    }
+}
+
 impl<I: AsCompositeIndex + Send + Sync> ChangeIdIndex for ChangeIdIndexImpl<I> {
     fn resolve_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<Vec<CommitId>> {
         self.pos_by_change

lib/src/default_index/readonly.rs

@@ -26,12 +26,11 @@ use std::sync::Arc;
 use smallvec::SmallVec;
 use thiserror::Error;
 
-use super::composite::{AsCompositeIndex, CompositeIndex, IndexSegment};
+use super::composite::{AsCompositeIndex, ChangeIdIndexImpl, CompositeIndex, IndexSegment};
 use super::entry::{IndexPosition, LocalPosition, SmallIndexPositionsVec};
 use super::mutable::DefaultMutableIndex;
-use super::revset_engine;
 use crate::backend::{ChangeId, CommitId};
-use crate::index::{Index, MutableIndex, ReadonlyIndex};
+use crate::index::{ChangeIdIndex, Index, MutableIndex, ReadonlyIndex};
 use crate::object_id::{HexPrefix, ObjectId, PrefixResolution};
 use crate::revset::{ResolvedExpression, Revset, RevsetEvaluationError};
 use crate::store::Store;
@@ -528,13 +527,11 @@ impl ReadonlyIndex for DefaultReadonlyIndex {
         self
     }
 
-    fn evaluate_revset_static(
+    fn change_id_index_static(
         &self,
-        expression: &ResolvedExpression,
-        store: &Arc<Store>,
-    ) -> Result<Box<dyn Revset<'static>>, RevsetEvaluationError> {
-        let revset_impl = revset_engine::evaluate(expression, store, self.clone())?;
-        Ok(Box::new(revset_impl))
+        heads: &mut dyn Iterator<Item = &CommitId>,
+    ) -> Box<dyn ChangeIdIndex> {
+        Box::new(ChangeIdIndexImpl::new(self.clone(), heads))
     }
 
     fn start_modification(&self) -> Box<dyn MutableIndex> {

lib/src/index.rs

@@ -84,13 +84,12 @@ pub trait ReadonlyIndex: Send + Sync {
 
     fn as_index(&self) -> &dyn Index;
 
-    // TODO: might be better to split Index::evaluate_revset() to
-    // Readonly/MutableIndex::evaluate_static().
-    fn evaluate_revset_static(
+    // TODO: might be better to split Index::change_id_index() to
+    // Readonly/MutableIndex::change_id_index_static().
+    fn change_id_index_static(
         &self,
-        expression: &ResolvedExpression,
-        store: &Arc<Store>,
-    ) -> Result<Box<dyn Revset<'static>>, RevsetEvaluationError>;
+        heads: &mut dyn Iterator<Item = &CommitId>,
+    ) -> Box<dyn ChangeIdIndex>;
 
     fn start_modification(&self) -> Box<dyn MutableIndex>;
 }

lib/src/repo.rs

@@ -268,12 +268,8 @@ impl ReadonlyRepo {
             .get_or_init(|| {
                 // TODO: maybe add abstraction over 'static/'index revset
                 // evaluation, and use it.
-                let expression = RevsetExpression::all().resolve_programmatic(self);
-                let revset = self
-                    .readonly_index()
-                    .evaluate_revset_static(&expression, self.store())
-                    .unwrap();
-                revset.change_id_index()
+                self.readonly_index()
+                    .change_id_index_static(&mut self.view().heads().iter())
             })
             .as_ref()
     }