Auto merge of rust-lang#108448 - ishitatsuyuki:binary-heap, r=Mark-Simulacrum

binary_heap: Optimize Extend implementation.

This PR makes the `Extend` implementation for `BinaryHeap` no longer rely on specialization, so that it always use the bulk rebuild optimization that was previously only available for the `Vec` specialization.

Author: bors

library/alloc/src/collections/binary_heap/mod.rs

@@ -154,8 +154,6 @@ use crate::collections::TryReserveError;
 use crate::slice;
 use crate::vec::{self, AsVecIntoIter, Vec};
 
-use super::SpecExtend;
-
 #[cfg(test)]
 mod tests;
 
@@ -400,6 +398,17 @@ impl<T: fmt::Debug> fmt::Debug for BinaryHeap<T> {
     }
 }
 
+struct RebuildOnDrop<'a, T: Ord> {
+    heap: &'a mut BinaryHeap<T>,
+    rebuild_from: usize,
+}
+
+impl<'a, T: Ord> Drop for RebuildOnDrop<'a, T> {
+    fn drop(&mut self) {
+        self.heap.rebuild_tail(self.rebuild_from);
+    }
+}
+
 impl<T: Ord> BinaryHeap<T> {
     /// Creates an empty `BinaryHeap` as a max-heap.
     ///
@@ -850,30 +859,19 @@ impl<T: Ord> BinaryHeap<T> {
     where
         F: FnMut(&T) -> bool,
     {
-        struct RebuildOnDrop<'a, T: Ord> {
-            heap: &'a mut BinaryHeap<T>,
-            first_removed: usize,
-        }
-
-        let mut guard = RebuildOnDrop { first_removed: self.len(), heap: self };
-
+        // rebuild_start will be updated to the first touched element below, and the rebuild will
+        // only be done for the tail.
+        let mut guard = RebuildOnDrop { rebuild_from: self.len(), heap: self };
         let mut i = 0;
+
         guard.heap.data.retain(|e| {
             let keep = f(e);
-            if !keep && i < guard.first_removed {
-                guard.first_removed = i;
+            if !keep && i < guard.rebuild_from {
+                guard.rebuild_from = i;
             }
             i += 1;
             keep
         });
-
-        impl<'a, T: Ord> Drop for RebuildOnDrop<'a, T> {
-            fn drop(&mut self) {
-                // data[..first_removed] is untouched, so we only need to
-                // rebuild the tail:
-                self.heap.rebuild_tail(self.first_removed);
-            }
-        }
     }
 }
 
@@ -1728,7 +1726,8 @@ impl<'a, T> IntoIterator for &'a BinaryHeap<T> {
 impl<T: Ord> Extend<T> for BinaryHeap<T> {
     #[inline]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        <Self as SpecExtend<I>>::spec_extend(self, iter);
+        let guard = RebuildOnDrop { rebuild_from: self.len(), heap: self };
+        guard.heap.data.extend(iter);
     }
 
     #[inline]
@@ -1742,37 +1741,6 @@ impl<T: Ord> Extend<T> for BinaryHeap<T> {
     }
 }
 
-impl<T: Ord, I: IntoIterator<Item = T>> SpecExtend<I> for BinaryHeap<T> {
-    default fn spec_extend(&mut self, iter: I) {
-        self.extend_desugared(iter.into_iter());
-    }
-}
-
-impl<T: Ord> SpecExtend<Vec<T>> for BinaryHeap<T> {
-    fn spec_extend(&mut self, ref mut other: Vec<T>) {
-        let start = self.data.len();
-        self.data.append(other);
-        self.rebuild_tail(start);
-    }
-}
-
-impl<T: Ord> SpecExtend<BinaryHeap<T>> for BinaryHeap<T> {
-    fn spec_extend(&mut self, ref mut other: BinaryHeap<T>) {
-        self.append(other);
-    }
-}
-
-impl<T: Ord> BinaryHeap<T> {
-    fn extend_desugared<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        let iterator = iter.into_iter();
-        let (lower, _) = iterator.size_hint();
-
-        self.reserve(lower);
-
-        iterator.for_each(move |elem| self.push(elem));
-    }
-}
-
 #[stable(feature = "extend_ref", since = "1.2.0")]
 impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {