Tracking Issue for const `IoSlice` and `IoSliceMut` methods

Feature gate: #![feature(io_slice_const)]

This is a tracking issue for making the new and advance methods on IoSlice and IoSliceMut const.

In particular, this enables constructing IoSlice and IoSliceMut in const contexts without users needing to break the platform abstraction by doing the cast themselves. This is useful for statics.

Since all current and plausible future platforms use simple slice-like structs, this should not impose a burden when adding a new platform with a different struct.

Public API

Methods new and advance are made const for IoSlice and IoSliceMut. advance_slices is more complicated and remains non-const. into_slice/as_slice are unstably const and tracked separately.

// std::io

impl<'a> IoSliceMut<'a> {
    #[rustc_const_unstable(feature = "io_slice_const", issue = "146459")]
    pub const fn new(buf: &'a mut [u8]) -> IoSliceMut<'a>;
    #[rustc_const_unstable(feature = "io_slice_const", issue = "146459")]
    pub const fn advance(&mut self, n: usize;

    // Other methods (unchanged):
    pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize);
    #[unstable(feature = "io_slice_as_bytes", issue = "132818")]
    pub const fn into_slice(self) -> &'a mut [u8];
}

impl<'a> IoSlice<'a> {
    #[rustc_const_unstable(feature = "io_slice_const", issue = "146459")]
    pub const fn new(buf: &'a [u8]) -> IoSlice<'a>;
    #[rustc_const_unstable(feature = "io_slice_const", issue = "146459")]
    pub const fn advance(&mut self, n: usize);

    // Other methods (unchanged):
    pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize);
    #[unstable(feature = "io_slice_as_bytes", issue = "132818")]
    pub const fn as_slice(self) -> &'a [u8];
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• Implementation: https://github.com/rust-lang/rust/pull/144090
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• None yet.

Start with linked implementation PR #144090 and the public API listed for IoSlice and IoSliceMut. Check the feature-gate history and stabilization process; done requires the implementation, final comment period, and stabilization PR steps to be completed.