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use raw-ptr-addr-of for slice::swap #81160

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merged 1 commit into from
Jan 22, 2021
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use raw-ptr-addr-of for slice::swap #81160

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1 change: 1 addition & 0 deletions library/core/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,7 @@
#![feature(auto_traits)]
#![feature(or_patterns)]
#![feature(prelude_import)]
#![feature(raw_ref_macros)]
#![feature(repr_simd, platform_intrinsics)]
#![feature(rustc_attrs)]
#![feature(simd_ffi)]
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7 changes: 3 additions & 4 deletions library/core/src/slice/mod.rs
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Expand Up @@ -542,10 +542,9 @@ impl<T> [T] {
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
pub fn swap(&mut self, a: usize, b: usize) {
// Can't take two mutable loans from one vector, so instead just cast
// them to their raw pointers to do the swap.
let pa: *mut T = &mut self[a];
let pb: *mut T = &mut self[b];
// Can't take two mutable loans from one vector, so instead use raw pointers.
let pa = ptr::raw_mut!(self[a]);
let pb = ptr::raw_mut!(self[b]);
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I'm just curious - the reason that this works, is that we use intrinsic indexing of the slice? If we would go through IndexMut, the problem wouldn't be solved (?)

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Yes, indexing a slice directly translates to a MIR instruction without any function calls. This is the same as what we do for arithmetic operators for primitive integers. The trait impls only exist for the type system and generics.

// SAFETY: `pa` and `pb` have been created from safe mutable references and refer
// to elements in the slice and therefore are guaranteed to be valid and aligned.
// Note that accessing the elements behind `a` and `b` is checked and will
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