index: add wrapper functions to DefaultMutableIndex to remove pub(super) field

into_segment() could be added instead of save_in(), but I decided to wrap
save_in(). save_in() may squash ancestor files, so it could be considered an
index-level operation.

lib/src/default_index/mod.rs

@@ -33,7 +33,6 @@ use smallvec::SmallVec;
 
 pub use self::composite::{CompositeIndex, IndexLevelStats, IndexStats};
 pub use self::mutable::DefaultMutableIndex;
-use self::mutable::MutableIndexSegment;
 pub use self::rev_walk::{
     RevWalk, RevWalkDescendants, RevWalkDescendantsGenerationRange, RevWalkGenerationRange,
 };
@@ -171,8 +170,7 @@ impl ReadonlyIndex for DefaultReadonlyIndex {
     }
 
     fn start_modification(&self) -> Box<dyn MutableIndex> {
-        let mutable_segment = MutableIndexSegment::incremental(self.0.clone());
-        Box::new(DefaultMutableIndex(mutable_segment))
+        Box::new(DefaultMutableIndex::incremental(self.0.clone()))
     }
 }
 
@@ -602,6 +600,7 @@ mod tests {
     use smallvec::smallvec_inline;
     use test_case::test_case;
 
+    use super::mutable::MutableIndexSegment;
     use super::*;
     use crate::backend::{ChangeId, CommitId, ObjectId};
     use crate::index::Index;

lib/src/default_index/mutable.rs

@@ -395,7 +395,7 @@ impl IndexSegment for MutableIndexSegment {
 }
 
 /// In-memory mutable records for the on-disk commit index backend.
-pub struct DefaultMutableIndex(pub(super) MutableIndexSegment);
+pub struct DefaultMutableIndex(MutableIndexSegment);
 
 impl DefaultMutableIndex {
     #[cfg(test)]
@@ -404,6 +404,11 @@ impl DefaultMutableIndex {
         DefaultMutableIndex(mutable_segment)
     }
 
+    pub(super) fn incremental(parent_file: Arc<ReadonlyIndexSegment>) -> Self {
+        let mutable_segment = MutableIndexSegment::incremental(parent_file);
+        DefaultMutableIndex(mutable_segment)
+    }
+
     pub fn as_composite(&self) -> CompositeIndex {
         self.0.as_composite()
     }
@@ -417,6 +422,10 @@ impl DefaultMutableIndex {
     ) {
         self.0.add_commit_data(commit_id, change_id, parent_ids);
     }
+
+    pub(super) fn save_in(self, dir: PathBuf) -> io::Result<Arc<ReadonlyIndexSegment>> {
+        self.0.save_in(dir)
+    }
 }
 
 impl Index for DefaultMutableIndex {

lib/src/default_index/store.rs

@@ -248,7 +248,7 @@ impl IndexStore for DefaultIndexStore {
             .into_any()
             .downcast::<DefaultMutableIndex>()
             .expect("index to merge in must be a DefaultMutableIndex");
-        let index_segment = index.0.save_in(self.dir.clone()).map_err(|err| {
+        let index_segment = index.save_in(self.dir.clone()).map_err(|err| {
             IndexWriteError::Other(format!("Failed to write commit index file: {err}"))
         })?;
         self.associate_file_with_operation(&index_segment, op_id)