add fast path on Path::eq for exact equality

Author: the8472

library/std/src/path.rs

@@ -979,6 +979,25 @@ impl FusedIterator for Components<'_> {}
 impl<'a> cmp::PartialEq for Components<'a> {
     #[inline]
     fn eq(&self, other: &Components<'a>) -> bool {
+        let Components { path: _, front: _, back: _, has_physical_root: _, prefix: _ } = self;
+
+        // Fast path for exact matches, e.g. for hashmap lookups.
+        // Don't explicitly compare the prefix or has_physical_root fields since they'll
+        // either be covered by the `path` buffer or are only relevant for `prefix_verbatim()`.
+        if self.path.len() == other.path.len()
+            && self.front == other.front
+            && self.back == State::Body
+            && other.back == State::Body
+            && self.prefix_verbatim() == other.prefix_verbatim()
+        {
+            // possible future improvement: this could bail out earlier if there were a
+            // reverse memcmp/bcmp comparing back to front
+            if self.path == other.path {
+                return true;
+            }
+        }
+
+        // compare back to front since absolute paths often share long prefixes
         Iterator::eq(self.clone().rev(), other.clone().rev())
     }
 }
@@ -1013,7 +1032,7 @@ fn compare_components(mut left: Components<'_>, mut right: Components<'_>) -> cm
     // The fast path isn't taken for paths with a PrefixComponent to avoid backtracking into
     // the middle of one
     if left.prefix.is_none() && right.prefix.is_none() && left.front == right.front {
-        // this might benefit from a [u8]::first_mismatch simd implementation, if it existed
+        // possible future improvement: a [u8]::first_mismatch simd implementation
         let first_difference =
             match left.path.iter().zip(right.path.iter()).position(|(&a, &b)| a != b) {
                 None if left.path.len() == right.path.len() => return cmp::Ordering::Equal,