tree 3593d163d3fe61721992c89a7be062ea844e511f
parent 07f98a8b71ea961854ae061ffe6ae1ba2458254f
author Shawn Pearce <spearce@spearce.org> 1500439071 -0700
committer Shawn Pearce <spearce@spearce.org> 1500439071 -0700

dfs: Use special ForReverseIndex DfsStreamKey wrapper instead of derive

While implementing a custom subclass of DfsStreamKey it became obvious
the required derive(String) was making it impossible to construct an
efficient key in all cases.

Instead, use a special wrapper type ForReverseIndex around the INDEX's
own DfsStreamKey to denote the reverse index stream in the
DfsBlockCache.  This adds a smaller layer of boxing, but eliminates
weird issues for DFS implementors using specialized DfsStreamKey
implementations for space efficiency reasons.

Now that DfsStreamKey is reasonably light-weight, avoid allocating the
index and reverse index keys until necessary.  DfsPackFile mostly
holds the DfsBlockCache.Ref handle to the object, and only needs the
DfsStreamKey when its looking up the handle.

Change-Id: Icea78e8f7f1514087b94ef5f525d9573ea2913f2