index: move Index::as_any() to MutableIndex, obtain CompositeIndex from there

It might sound scary to add public .mutable_index() accessor, but I think
it's okay because immutable MutableIndex reference has no more power than
Index.

This allows us to implement Index for lifetime-bound type such as
CompositeIndex<'_>.

lib/src/default_index_store.rs

@@ -688,10 +688,6 @@ impl MutableIndexImpl {
 }
 
 impl Index for MutableIndexImpl {
-    fn as_any(&self) -> &dyn Any {
-        self
-    }
-
     fn shortest_unique_commit_id_prefix_len(&self, commit_id: &CommitId) -> usize {
         CompositeIndex(self).shortest_unique_commit_id_prefix_len(commit_id)
     }
@@ -732,6 +728,10 @@ impl Index for MutableIndexImpl {
 }
 
 impl MutableIndex for MutableIndexImpl {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+
     fn into_any(self: Box<Self>) -> Box<dyn Any> {
         Box::new(*self)
     }
@@ -1976,10 +1976,6 @@ impl ReadonlyIndexImpl {
 }
 
 impl Index for ReadonlyIndexImpl {
-    fn as_any(&self) -> &dyn Any {
-        self
-    }
-
     fn shortest_unique_commit_id_prefix_len(&self, commit_id: &CommitId) -> usize {
         CompositeIndex(self).shortest_unique_commit_id_prefix_len(commit_id)
     }

lib/src/index.rs

@@ -46,8 +46,6 @@ pub trait IndexStore: Send + Sync + Debug {
 }
 
 pub trait Index: Send + Sync {
-    fn as_any(&self) -> &dyn Any;
-
     fn shortest_unique_commit_id_prefix_len(&self, commit_id: &CommitId) -> usize;
 
     fn resolve_prefix(&self, prefix: &HexPrefix) -> PrefixResolution<CommitId>;
@@ -79,6 +77,8 @@ pub trait ReadonlyIndex: Send + Sync {
 }
 
 pub trait MutableIndex: Any {
+    fn as_any(&self) -> &dyn Any;
+
     fn into_any(self: Box<Self>) -> Box<dyn Any>;
 
     fn as_index(&self) -> &dyn Index;

lib/src/repo.rs

@@ -662,6 +662,10 @@ impl MutableRepo {
         self.view.get_mut()
     }
 
+    pub fn mutable_index(&self) -> &dyn MutableIndex {
+        self.index.as_ref()
+    }
+
     pub fn has_changes(&self) -> bool {
         !(self.abandoned_commits.is_empty()
             && self.rewritten_commits.is_empty()

lib/tests/test_index.rs

@@ -457,7 +457,7 @@ fn as_readonly_composite(repo: &Arc<ReadonlyRepo>) -> CompositeIndex<'_> {
 }
 
 fn as_mutable_impl(repo: &MutableRepo) -> &MutableIndexImpl {
-    repo.index()
+    repo.mutable_index()
         .as_any()
         .downcast_ref::<MutableIndexImpl>()
         .unwrap()