Auto merge of #85157 - the8472:drain-drop-in-place, r=Mark-Simulacrum

replace vec::Drain drop loops with drop_in_place

The `Drain::drop` implementation came up in #82185 (comment) as potentially interfering with other optimization work due its widespread use somewhere in `println!`

`@rustbot` label T-libs-impl

library/alloc/src/vec/drain.rs

@@ -1,7 +1,7 @@
 use crate::alloc::{Allocator, Global};
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};
-use core::mem::{self};
+use core::mem;
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
@@ -104,16 +104,11 @@ impl<T, A: Allocator> DoubleEndedIterator for Drain<'_, T, A> {
 #[stable(feature = "drain", since = "1.6.0")]
 impl<T, A: Allocator> Drop for Drain<'_, T, A> {
  fn drop(&mut self) {
- /// Continues dropping the remaining elements in the `Drain`, then moves back the
- /// un-`Drain`ed elements to restore the original `Vec`.
+ /// Moves back the un-`Drain`ed elements to restore the original `Vec`.
  struct DropGuard<'r, 'a, T, A: Allocator>(&'r mut Drain<'a, T, A>);
 
  impl<'r, 'a, T, A: Allocator> Drop for DropGuard<'r, 'a, T, A> {
  fn drop(&mut self) {
- // Continue the same loop we have below. If the loop already finished, this does
- // nothing.
- self.0.for_each(drop);
-
  if self.0.tail_len > 0 {
  unsafe {
  let source_vec = self.0.vec.as_mut();
@@ -131,15 +126,45 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
  }
  }
 
- // exhaust self first
- while let Some(item) = self.next() {
- let guard = DropGuard(self);
- drop(item);
- mem::forget(guard);
+ let iter = mem::replace(&mut self.iter, (&mut []).iter());
+ let drop_len = iter.len();
+ let drop_ptr = iter.as_slice().as_ptr();
+
+ // forget iter so there's no aliasing reference
+ drop(iter);
+
+ let mut vec = self.vec;
+
+ if mem::size_of::<T>() == 0 {
+ // ZSTs have no identity, so we don't need to move them around, we only need to drop the correct amount.
+ // this can be achieved by manipulating the Vec length instead of moving values out from `iter`.
+ unsafe {
+ let vec = vec.as_mut();
+ let old_len = vec.len();
+ vec.set_len(old_len + drop_len + self.tail_len);
+ vec.truncate(old_len + self.tail_len);
+ }
+
+ return;
+ }
+
+ // ensure elements are moved back into their appropriate places, even when drop_in_place panics
+ let _guard = DropGuard(self);
+
+ if drop_len == 0 {
+ return;
  }
 
- // Drop a `DropGuard` to move back the non-drained tail of `self`.
- DropGuard(self);
+ unsafe {
+ // drop_ptr comes from a slice::Iter which only gives us a &[T] but for drop_in_place
+ // a pointer with mutable provenance is necessary. Therefore we must reconstruct
+ // it from the original vec but also avoid creating a &mut to the front since that could
+ // invalidate raw pointers to it which some unsafe code might rely on.
+ let vec_ptr = vec.as_mut().as_mut_ptr();
+ let drop_offset = drop_ptr.offset_from(vec_ptr) as usize;
+ let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
+ ptr::drop_in_place(to_drop);
+ }
  }
 }