Rollup merge of #70766 - tspiteri:forget-to-ManuallyDrop, r=Mark-Simulacrum,RalfJung

use ManuallyDrop instead of forget inside collections

This PR 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.

Author: Dylan-DPC

src/liballoc/boxed.rs

@@ -469,8 +469,8 @@ impl<T: ?Sized> Box<T> {
     #[inline]
     #[doc(hidden)]
     pub fn into_unique(b: Box<T>) -> Unique<T> {
+        let b = mem::ManuallyDrop::new(b);
         let mut unique = b.0;
-        mem::forget(b);
         // Box is kind-of a library type, but recognized as a "unique pointer" by
         // Stacked Borrows.  This function here corresponds to "reborrowing to
         // a raw pointer", but there is no actual reborrow here -- so

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()),

src/liballoc/collections/vec_deque.rs

@@ -12,7 +12,7 @@ use core::cmp::{self, Ordering};
 use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::iter::{once, repeat_with, FromIterator, FusedIterator};
-use core::mem::{self, replace};
+use core::mem::{self, replace, ManuallyDrop};
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{Index, IndexMut, RangeBounds, Try};
 use core::ptr::{self, NonNull};
@@ -2898,12 +2898,12 @@ impl<T> From<Vec<T>> for VecDeque<T> {
     /// This avoids reallocating where possible, but the conditions for that are
     /// strict, and subject to change, and so shouldn't be relied upon unless the
     /// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
-    fn from(mut other: Vec<T>) -> Self {
+    fn from(other: Vec<T>) -> Self {
         unsafe {
+            let mut other = ManuallyDrop::new(other);
             let other_buf = other.as_mut_ptr();
             let mut buf = RawVec::from_raw_parts(other_buf, other.capacity());
             let len = other.len();
-            mem::forget(other);
 
             // We need to extend the buf if it's not a power of two, too small
             // or doesn't have at least one free space
@@ -2955,16 +2955,15 @@ impl<T> From<VecDeque<T>> for Vec<T> {
         other.make_contiguous();
 
         unsafe {
+            let other = ManuallyDrop::new(other);
             let buf = other.buf.ptr();
             let len = other.len();
             let cap = other.cap();
 
             if other.head != 0 {
                 ptr::copy(buf.add(other.tail), buf, len);
             }
-            let out = Vec::from_raw_parts(buf, len, cap);
-            mem::forget(other);
-            out
+            Vec::from_raw_parts(buf, len, cap)
         }
     }
 }

src/liballoc/raw_vec.rs

@@ -3,7 +3,7 @@
 
 use core::alloc::MemoryBlock;
 use core::cmp;
-use core::mem::{self, MaybeUninit};
+use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::ops::Drop;
 use core::ptr::{NonNull, Unique};
 use core::slice;
@@ -112,11 +112,10 @@ impl<T> RawVec<T, Global> {
     }
 
     /// Converts a `Box<[T]>` into a `RawVec<T>`.
-    pub fn from_box(mut slice: Box<[T]>) -> Self {
+    pub fn from_box(slice: Box<[T]>) -> Self {
         unsafe {
-            let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
-            mem::forget(slice);
-            result
+            let mut slice = ManuallyDrop::new(slice);
+            RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len())
         }
     }
 }
@@ -579,11 +578,10 @@ impl<T> RawVec<T, Global> {
             "`len` must be smaller than or equal to `self.capacity()`"
         );
 
+        let me = ManuallyDrop::new(self);
         // NOTE: not calling `capacity()` here; actually using the real `cap` field!
-        let slice = slice::from_raw_parts_mut(self.ptr() as *mut MaybeUninit<T>, len);
-        let output = Box::from_raw(slice);
-        mem::forget(self);
-        output
+        let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
+        Box::from_raw(slice)
     }
 }
 

src/liballoc/vec.rs

@@ -66,7 +66,7 @@ use core::hash::{self, Hash};
 use core::intrinsics::{arith_offset, assume};
 use core::iter::{FromIterator, FusedIterator, TrustedLen};
 use core::marker::PhantomData;
-use core::mem;
+use core::mem::{self, ManuallyDrop};
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{self, Index, IndexMut, RangeBounds};
 use core::ptr::{self, NonNull};
@@ -392,7 +392,7 @@ impl<T> Vec<T> {
     /// ```
     #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
     pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
-        let mut me = mem::ManuallyDrop::new(self);
+        let mut me = ManuallyDrop::new(self);
         (me.as_mut_ptr(), me.len(), me.capacity())
     }
 
@@ -678,9 +678,9 @@ impl<T> Vec<T> {
     pub fn into_boxed_slice(mut self) -> Box<[T]> {
         unsafe {
             self.shrink_to_fit();
-            let buf = ptr::read(&self.buf);
-            let len = self.len();
-            mem::forget(self);
+            let me = ManuallyDrop::new(self);
+            let buf = ptr::read(&me.buf);
+            let len = me.len();
             buf.into_box(len).assume_init()
         }
     }
@@ -1949,16 +1949,16 @@ impl<T> IntoIterator for Vec<T> {
     /// }
     /// ```
     #[inline]
-    fn into_iter(mut self) -> IntoIter<T> {
+    fn into_iter(self) -> IntoIter<T> {
         unsafe {
-            let begin = self.as_mut_ptr();
+            let mut me = ManuallyDrop::new(self);
+            let begin = me.as_mut_ptr();
             let end = if mem::size_of::<T>() == 0 {
-                arith_offset(begin as *const i8, self.len() as isize) as *const T
+                arith_offset(begin as *const i8, me.len() as isize) as *const T
             } else {
-                begin.add(self.len()) as *const T
+                begin.add(me.len()) as *const T
             };
-            let cap = self.buf.capacity();
-            mem::forget(self);
+            let cap = me.buf.capacity();
             IntoIter {
                 buf: NonNull::new_unchecked(begin),
                 phantom: PhantomData,
@@ -2081,9 +2081,8 @@ impl<T> SpecExtend<T, IntoIter<T>> for Vec<T> {
         // has not been advanced at all.
         if iterator.buf.as_ptr() as *const _ == iterator.ptr {
             unsafe {
-                let vec = Vec::from_raw_parts(iterator.buf.as_ptr(), iterator.len(), iterator.cap);
-                mem::forget(iterator);
-                vec
+                let it = ManuallyDrop::new(iterator);
+                Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap)
             }
         } else {
             let mut vector = Vec::new();