From 2b718e8d9ca6efdccf4d72647cb3d3101e15b7fa Mon Sep 17 00:00:00 2001
From: Trevor Spiteri <tspiteri@ieee.org>
Date: Sat, 4 Apr 2020 14:24:26 +0200
Subject: use ManuallyDrop instead of forget inside collections

This commit changes some usage of mem::forget into mem::ManuallyDrop
in some Vec, VecDeque, BTreeMap and Box methods.

Before the commit, the generated IR for some of the methods was
longer, and even after optimization, some unwinding artifacts were
still present.
---
 src/liballoc/collections/btree/map.rs | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

(limited to 'src/liballoc/collections/btree')

diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index bbeced1751d..36770ee416c 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -4,9 +4,10 @@ use core::fmt::Debug;
 use core::hash::{Hash, Hasher};
 use core::iter::{FromIterator, FusedIterator, Peekable};
 use core::marker::PhantomData;
+use core::mem::{self, ManuallyDrop};
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{Index, RangeBounds};
-use core::{fmt, mem, ptr};
+use core::{fmt, ptr};
 
 use super::node::{self, marker, ForceResult::*, Handle, InsertResult::*, NodeRef};
 use super::search::{self, SearchResult::*};
@@ -190,9 +191,9 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                             // We can't destructure subtree directly
                             // because BTreeMap implements Drop
                             let (subroot, sublength) = unsafe {
+                                let subtree = ManuallyDrop::new(subtree);
                                 let root = ptr::read(&subtree.root);
                                 let length = subtree.length;
-                                mem::forget(subtree);
                                 (root, length)
                             };
 
@@ -1515,15 +1516,14 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
     type IntoIter = IntoIter<K, V>;
 
     fn into_iter(self) -> IntoIter<K, V> {
-        if self.root.is_none() {
-            mem::forget(self);
+        let me = ManuallyDrop::new(self);
+        if me.root.is_none() {
             return IntoIter { front: None, back: None, length: 0 };
         }
 
-        let root1 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
-        let root2 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
-        let len = self.length;
-        mem::forget(self);
+        let root1 = unsafe { unwrap_unchecked(ptr::read(&me.root)).into_ref() };
+        let root2 = unsafe { unwrap_unchecked(ptr::read(&me.root)).into_ref() };
+        let len = me.length;
 
         IntoIter {
             front: Some(root1.first_leaf_edge()),
-- 
cgit 1.4.1-3-g733a5