Tracking Issue for const_swap_nonoverlapping · Issue #133668 · rust-lang/rust (original) (raw)

Feature gate: #![feature(const_swap_nonoverlapping)]

This is a tracking issue for making swap_nonoverlapping a const fn.

Public API

mod ptr { pub const unsafe fn swap_nonoverlapping(x: *mut T, y: *mut T, count: usize); }

Steps / History

Blocking Issues

ptr::swap_nonoverlapping has a limitation currently where it can fail when the data-to-swap contains pointers that cross the "element boundary" of such a swap (i.e., count > 1 and the pointer straddles the boundary between two T). Here's an example of code that unexpectedly fails:

const {
    let mut ptr1 = &1;
    let mut ptr2 = &666;

    // Swap ptr1 and ptr2, bytewise.
    unsafe {
        ptr::swap_nonoverlapping(
            ptr::from_mut(&mut ptr1).cast::<u8>(),
            ptr::from_mut(&mut ptr2).cast::<u8>(),
            mem::size_of::<&i32>(),
        );
    }

    // Make sure they still work.
    assert!(*ptr1 == 666);
    assert!(*ptr2 == 1);
};

The proper way to fix this is to implement rust-lang/const-eval#72.

  1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩