From 9f8f366eeabee6273abf3628bb75ecd3b4b57f22 Mon Sep 17 00:00:00 2001
From: Nikita Popov <nikita.ppv@gmail.com>
Date: Sun, 6 May 2018 16:55:04 +0200
Subject: [PATCH] Use ManuallyDrop instead of Option in Hole implementation

The Option is always Some until drop, where it becomes None. Make
this more explicit and avoid unwraps by using ManuallyDrop.

This change should be performance-neutral as LLVM already optimizes
the unwraps away in the inlined code.
---
 src/liballoc/binary_heap.rs | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/src/liballoc/binary_heap.rs b/src/liballoc/binary_heap.rs
index 668b61c51d8bc..fcadcb544c431 100644
--- a/src/liballoc/binary_heap.rs
+++ b/src/liballoc/binary_heap.rs
@@ -157,7 +157,7 @@
 
 use core::ops::{Deref, DerefMut};
 use core::iter::{FromIterator, FusedIterator};
-use core::mem::{swap, size_of};
+use core::mem::{swap, size_of, ManuallyDrop};
 use core::ptr;
 use core::fmt;
 
@@ -864,8 +864,7 @@ impl<T: Ord> BinaryHeap<T> {
 /// position with the value that was originally removed.
 struct Hole<'a, T: 'a> {
     data: &'a mut [T],
-    /// `elt` is always `Some` from new until drop.
-    elt: Option<T>,
+    elt: ManuallyDrop<T>,
     pos: usize,
 }
 
@@ -879,7 +878,7 @@ impl<'a, T> Hole<'a, T> {
         let elt = ptr::read(&data[pos]);
         Hole {
             data,
-            elt: Some(elt),
+            elt: ManuallyDrop::new(elt),
             pos,
         }
     }
@@ -892,7 +891,7 @@ impl<'a, T> Hole<'a, T> {
     /// Returns a reference to the element removed.
     #[inline]
     fn element(&self) -> &T {
-        self.elt.as_ref().unwrap()
+        &self.elt
     }
 
     /// Returns a reference to the element at `index`.
@@ -925,7 +924,7 @@ impl<'a, T> Drop for Hole<'a, T> {
         // fill the hole again
         unsafe {
             let pos = self.pos;
-            ptr::write(self.data.get_unchecked_mut(pos), self.elt.take().unwrap());
+            ptr::copy_nonoverlapping(&*self.elt, self.data.get_unchecked_mut(pos), 1);
         }
     }
 }