Auto merge of #39062 - martinhath:placement-in-binaryheap, r=nagisa

Implement placement-in protocol for `BinaryHeap`

Related to #30172, and loosley based on #38551.

At the moment, this PR is in a pretty rough state, but I wanted to get some feedback to see if I'm going in the right direction.

I hope the Mentor label of #30172 is still applicable, even though it's a year old 😄

Author: bors

src/libcollections/binary_heap.rs

@@ -151,7 +151,7 @@
 #![allow(missing_docs)]
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use core::ops::{Deref, DerefMut};
+use core::ops::{Deref, DerefMut, Place, Placer, InPlace};
 use core::iter::{FromIterator, FusedIterator};
 use core::mem::{swap, size_of};
 use core::ptr;
@@ -673,7 +673,7 @@ impl<T: Ord> BinaryHeap<T> {
     // the hole is filled back at the end of its scope, even on panic.
     // Using a hole reduces the constant factor compared to using swaps,
     // which involves twice as many moves.
-    fn sift_up(&mut self, start: usize, pos: usize) {
+    fn sift_up(&mut self, start: usize, pos: usize) -> usize {
         unsafe {
             // Take out the value at `pos` and create a hole.
             let mut hole = Hole::new(&mut self.data, pos);
@@ -685,6 +685,7 @@ impl<T: Ord> BinaryHeap<T> {
                 }
                 hole.move_to(parent);
             }
+            hole.pos()
         }
     }
 
@@ -1189,3 +1190,56 @@ impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
         self.extend(iter.into_iter().cloned());
     }
 }
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+pub struct BinaryHeapPlace<'a, T: 'a>
+where T: Clone + Ord {
+    heap: *mut BinaryHeap<T>,
+    place: vec::PlaceBack<'a, T>,
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T: 'a> Placer<T> for &'a mut BinaryHeap<T>
+where T: Clone + Ord {
+    type Place = BinaryHeapPlace<'a, T>;
+
+    fn make_place(self) -> Self::Place {
+        let ptr = self as *mut BinaryHeap<T>;
+        let place = Placer::make_place(self.data.place_back());
+        BinaryHeapPlace {
+            heap: ptr,
+            place: place,
+        }
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> Place<T> for BinaryHeapPlace<'a, T>
+where T: Clone + Ord {
+    fn pointer(&mut self) -> *mut T {
+        self.place.pointer()
+    }
+}
+
+#[unstable(feature = "collection_placement",
+           reason = "placement protocol is subject to change",
+           issue = "30172")]
+impl<'a, T> InPlace<T> for BinaryHeapPlace<'a, T>
+where T: Clone + Ord {
+    type Owner = &'a T;
+
+    unsafe fn finalize(self) -> &'a T {
+        self.place.finalize();
+
+        let heap: &mut BinaryHeap<T> = &mut *self.heap;
+        let len = heap.len();
+        let i = heap.sift_up(0, len - 1);
+        heap.data.get_unchecked(i)
+    }
+}

src/libcollectionstest/binary_heap.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::panic;
 use std::collections::BinaryHeap;
 use std::collections::binary_heap::{Drain, PeekMut};
 
@@ -310,6 +311,26 @@ fn test_extend_specialization() {
     assert_eq!(a.into_sorted_vec(), [-20, -10, 1, 2, 3, 3, 5, 43]);
 }
 
+#[test]
+fn test_placement() {
+    let mut a = BinaryHeap::new();
+    &mut a <- 2;
+    &mut a <- 4;
+    &mut a <- 3;
+    assert_eq!(a.peek(), Some(&4));
+    assert_eq!(a.len(), 3);
+    &mut a <- 1;
+    assert_eq!(a.into_sorted_vec(), vec![1, 2, 3, 4]);
+}
+
+#[test]
+fn test_placement_panic() {
+    let mut heap = BinaryHeap::from(vec![1, 2, 3]);
+    fn mkpanic() -> usize { panic!() }
+    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| { &mut heap <- mkpanic(); }));
+    assert_eq!(heap.len(), 3);
+}
+
 #[allow(dead_code)]
 fn assert_covariance() {
     fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> {