Tracking Issue for const_swap

[usbalbin]

Feature gate: #![feature(const_swap)]

This is a tracking issue for making the functions mem::swap and ptr::swap[_nonoverlapping] and some other swap related functions const fn.

Public API

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

mod mem {
    pub const fn swap<T>(x: &mut T, y: &mut T);
}

impl<T> [T] {
    pub const fn swap(&mut self, a: usize, b: usize);
    pub const unsafe fn swap_unchecked(&mut self, a: usize, b: usize);
}

impl<T: ?Sized> *mut T {
    pub const unsafe fn swap(self, with: *mut T);
}

Steps / History

• Implementation (mem and ptr): Constify copy related functions #83091
• Implementation ([T] and *mut T ) Extend the const swap feature #90644
• Blocked on const_mut_refs
• Final commenting period (FCP)
• Stabilization PR

Unresolved Questions

• What do we do with the bytewise pointer swap situation?

[RalfJung]

This got reverted again by #86003, but hopefully we can re-land it soon.

[RalfJung]

@pnkfelix I don't quite understand why you un-constified swap... it seems write is the underlying source of the problem. Does it not work to add the const stability attribute here?

[usbalbin]

I have reverted the constness reverts in #86295

[est31]

I've filed #90644 to extend the const_swap feature to three more functions. Some of them can panic, but const_panic is stable as of #89508. Edit: seems that std library functions need #90687 on top of const_panic stabilization. I expect that PR gets merged quite soon.

[jhpratt]

This should be blocked on const mut refs, I believe.

[RalfJung]

The following code used to work with swap_nonoverlapping, but doesn't any more:

#![feature(const_swap)]
#![feature(const_mut_refs)]
use std::{
    mem::{self, MaybeUninit},
    ptr,
};

const X: () = {
    let mut ptr1 = &1;
    let mut ptr2 = &2;

    // Swap them, bytewise.
    unsafe {
        ptr::swap_nonoverlapping(
            &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
            &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
            mem::size_of::<&i32>(),
        );
    }
    
    // Make sure they still work.
    assert!(*ptr1 == 2);
    assert!(*ptr2 == 1);
};

That is a regression introduced by the new implementation of swap_nonoverlapping in #94212. Though arguably it is a mere coincidence that the previous implementation worked: this is fundamentally caused by a limitation of compile-time evaluation. We have no good way to represent a 'part' of a pointer, so we cannot work on pointers bytewise. Changing the above code to copy a single &i32 rather than 8 MaybeUninit<u8> fixes it.

Not being able to work bytewise on pointer is a general limitation of CTFE, so we could document this as such. However, note that copy and copy_nonoverlapping actually work here; this is because they are implemented via intrinsics which then do copy the entire memory range at once, thus not having to deal with pointers bytewise.