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<T>(x: *mut T, y: *mut T, count: usize);
}

Steps / History

• Split out of Tracking Issue for const_swap #83163
• Resolve blocking issues
• Final comment period (FCP)¹
• Stabilization PR

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.