Make obvious that we steal data from the free list (#293)

Thanks @joshtriplett for the review and suggestions!

Author: Byron

git-repository/src/easy/handle.rs

@@ -74,6 +74,10 @@ impl easy::Handle {
         self.bufs.borrow_mut().pop().unwrap_or_default()
     }
 
+    /// This method is commonly called from the destructor of objects that previously claimed an entry
+    /// in the free-list with `free_buf()`.
+    /// They are welcome to take out the data themselves, for instance when the object is detached, to avoid
+    /// it to be reclaimed.
     #[inline]
     pub(crate) fn reuse_buffer(&self, data: &mut Vec<u8>) {
         if data.capacity() > 0 {

git-repository/src/easy/object/impls.rs

@@ -29,16 +29,13 @@ impl AsRef<[u8]> for DetachedObject {
 impl<'repo> TryFrom<Object<'repo>> for Commit<'repo> {
     type Error = Object<'repo>;
 
-    fn try_from(value: Object<'repo>) -> Result<Self, Self::Error> {
+    fn try_from(mut value: Object<'repo>) -> Result<Self, Self::Error> {
         let handle = value.handle;
         match value.kind {
             object::Kind::Commit => Ok(Commit {
                 id: value.id,
                 handle,
-                data: {
-                    drop(value);
-                    handle.free_buf()
-                },
+                data: steal(&mut value.data),
             }),
             _ => Err(value),
         }
@@ -48,18 +45,21 @@ impl<'repo> TryFrom<Object<'repo>> for Commit<'repo> {
 impl<'repo> TryFrom<Object<'repo>> for Tree<'repo> {
     type Error = Object<'repo>;
 
-    fn try_from(value: Object<'repo>) -> Result<Self, Self::Error> {
+    fn try_from(mut value: Object<'repo>) -> Result<Self, Self::Error> {
         let handle = value.handle;
         match value.kind {
             object::Kind::Tree => Ok(Tree {
                 id: value.id,
                 handle,
-                data: {
-                    drop(value);
-                    handle.free_buf()
-                },
+                data: steal(&mut value.data),
             }),
             _ => Err(value),
         }
     }
 }
+
+/// In conjunction with the handles free list, leaving an empty Vec in place of the original causes it to not be
+/// returned to the free list.
+fn steal(data: &mut Vec<u8>) -> Vec<u8> {
+    std::mem::take(data)
+}