tree: if matcher says that tree should be skipped, skip it

When using a sparse working copy (e.g. with no files at all) and
updating the working copy from the root commit to a commit with
millions of files, we shouldn't have to walk the parts of the diff
that doesn't match the sparse patterns. However, we still do the full
walk because our `Tree::diff()` currently doesn't care about what the
matcher tells us to visit, it only filters out unwanted files after
visiting them. This commit fixes that for the special (but common)
case of matching nothing in a directory.

I tried also adding special handling for when the matcher says that we
should only visit a few entries, but it wasn't clearly better in the
cases I tested it on. I'll keep that patch around and might send it if
I find some cases where it helps.

lib/src/tree.rs

@@ -309,26 +309,20 @@ impl<T> Diff<T> {
     }
 }
 
-struct TreeEntryDiffIterator<'trees, 'matcher> {
+struct TreeEntryDiffIterator<'trees> {
     it1: Peekable<TreeEntriesNonRecursiveIterator<'trees>>,
     it2: Peekable<TreeEntriesNonRecursiveIterator<'trees>>,
-    // TODO: Restrict walk according to Matcher::visit()
-    _matcher: &'matcher dyn Matcher,
 }
 
-impl<'trees, 'matcher> TreeEntryDiffIterator<'trees, 'matcher> {
-    fn new(tree1: &'trees Tree, tree2: &'trees Tree, matcher: &'matcher dyn Matcher) -> Self {
+impl<'trees> TreeEntryDiffIterator<'trees> {
+    fn new(tree1: &'trees Tree, tree2: &'trees Tree) -> Self {
         let it1 = tree1.entries_non_recursive().peekable();
         let it2 = tree2.entries_non_recursive().peekable();
-        TreeEntryDiffIterator {
-            it1,
-            it2,
-            _matcher: matcher,
-        }
+        TreeEntryDiffIterator { it1, it2 }
     }
 }
 
-impl<'trees, 'matcher> Iterator for TreeEntryDiffIterator<'trees, 'matcher> {
+impl<'trees> Iterator for TreeEntryDiffIterator<'trees> {
     type Item = (
         RepoPathComponent,
         Option<&'trees TreeValue>,
@@ -385,27 +379,39 @@ impl<'trees, 'matcher> Iterator for TreeEntryDiffIterator<'trees, 'matcher> {
     }
 }
 
-fn diff_entries<'trees, 'matcher>(
-    tree1: &'trees Tree,
-    tree2: &'trees Tree,
-    matcher: &'matcher dyn Matcher,
-) -> TreeEntryDiffIterator<'trees, 'matcher> {
-    // TODO: make TreeEntryDiffIterator an enum with one variant that iterates over
-    // the tree entries and filters by the matcher (i.e. what
-    // TreeEntryDiffIterator does now) and another variant that iterates over
-    // what the matcher says to visit
-    TreeEntryDiffIterator::new(tree1, tree2, matcher)
+fn diff_entries<'trees>(tree1: &'trees Tree, tree2: &'trees Tree) -> TreeEntryDiffIterator<'trees> {
+    TreeEntryDiffIterator::new(tree1, tree2)
 }
 
 pub fn recursive_tree_diff(root1: Tree, root2: Tree, matcher: &dyn Matcher) -> TreeDiffIterator {
     TreeDiffIterator::new(RepoPath::root(), root1, root2, matcher)
 }
 
+enum MaybeTreeEntryDiffIterator<'trees> {
+    WalkEntries(TreeEntryDiffIterator<'trees>),
+    Nothing,
+}
+
+impl<'trees> Iterator for MaybeTreeEntryDiffIterator<'trees> {
+    type Item = (
+        RepoPathComponent,
+        Option<&'trees TreeValue>,
+        Option<&'trees TreeValue>,
+    );
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self {
+            MaybeTreeEntryDiffIterator::WalkEntries(it) => it.next(),
+            MaybeTreeEntryDiffIterator::Nothing => None,
+        }
+    }
+}
+
 pub struct TreeDiffIterator<'matcher> {
     dir: RepoPath,
     matcher: &'matcher dyn Matcher,
     // Iterator over the diffs between tree1 and tree2
-    entry_iterator: TreeEntryDiffIterator<'static, 'matcher>,
+    entry_iterator: MaybeTreeEntryDiffIterator<'static>,
     // On drop, tree1 and tree2 must outlive entry_iterator
     tree1: Pin<Box<Tree>>,
     tree2: Pin<Box<Tree>>,
@@ -426,9 +432,13 @@ impl<'matcher> TreeDiffIterator<'matcher> {
     ) -> TreeDiffIterator {
         let tree1 = Box::pin(tree1);
         let tree2 = Box::pin(tree2);
-        let root_entry_iterator: TreeEntryDiffIterator = diff_entries(&tree1, &tree2, matcher);
-        let root_entry_iterator: TreeEntryDiffIterator<'static, 'matcher> =
-            unsafe { std::mem::transmute(root_entry_iterator) };
+        let root_entry_iterator = if matcher.visit(&dir).is_nothing() {
+            MaybeTreeEntryDiffIterator::Nothing
+        } else {
+            let iter: TreeEntryDiffIterator = diff_entries(&tree1, &tree2);
+            let iter: TreeEntryDiffIterator<'static> = unsafe { std::mem::transmute(iter) };
+            MaybeTreeEntryDiffIterator::WalkEntries(iter)
+        };
         Self {
             dir,
             matcher,
@@ -536,9 +546,7 @@ pub fn merge_trees(
     // Start with a tree identical to side 1 and modify based on changes from base
     // to side 2.
     let mut new_tree = side1_tree.data().clone();
-    for (basename, maybe_base, maybe_side2) in
-        diff_entries(base_tree, side2_tree, &EverythingMatcher)
-    {
+    for (basename, maybe_base, maybe_side2) in diff_entries(base_tree, side2_tree) {
         let maybe_side1 = side1_tree.value(&basename);
         if maybe_side1 == maybe_base {
             // side 1 is unchanged: use the value from side 2