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Speed up ptr::replace #98197

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Speed up ptr::replace #98197

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bc0788b
Use mem::replace in ptr::replace instead of mem::swap
Jun 17, 2022
6bf4c57
update comment
Jun 24, 2022
d8d2412
replace mem::replace with ptr::replace if applicable
Jun 24, 2022
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5 changes: 2 additions & 3 deletions library/core/src/ptr/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1012,16 +1012,15 @@ const unsafe fn swap_nonoverlapping_simple<T>(x: *mut T, y: *mut T, count: usize
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_replace", issue = "83164")]
pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
pub const unsafe fn replace<T>(dst: *mut T, src: T) -> T {
// SAFETY: the caller must guarantee that `dst` is valid to be
// cast to a mutable reference (valid for writes, aligned, initialized),
// and cannot overlap `src` since `dst` must point to a distinct
// allocated object.
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@RalfJung RalfJung Jun 24, 2022 •
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// and cannot overlap `src` since `dst` must point to a distinct
// allocated object.
// and cannot overlap `src` since `src` is a freshly stack-allocated local variable.

Not that we even still need this part of the argument with mem::replace.

unsafe {
assert_unsafe_precondition!(is_aligned_and_not_null(dst));
mem::swap(&mut *dst, &mut src); // cannot overlap
mem::replace(&mut *dst, src)
}
src
}

/// Reads the value from `src` without moving it. This leaves the
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