Tracking Issue for `const_mut_cursor`

[bjoernager]

Feature gate: #![feature(const_mut_cursor)]

This is a tracking issue for marking the get_mut and set_position methods in std::io::Cursor as const.

Public API

// std::io

impl<T> Cursor<T> {
    pub const fn get_mut(&mut self) -> &mut T;

    pub const fn set_position(&mut self, pos: u64);
}

Steps / History

• Implementation: Mark get_mut and set_position in std::io::Cursor as const. #130800
• Final comment period (FCP)
• Stabilization PR: Stabilise Cursor::{get_mut, set_position} in const scenarios. #136634

Unresolved Questions

• Can we also "constify" into_inner?

[RalfJung]

@rust-lang/libs-api these are two trivial already-stable methods that (I assume) haven't been made const-stable years ago just because they need mutable references. Now that we have mutable references in const, there's no reason to wait any longer with making them const-stable IMO.

[dtolnay]

@rfcbot fcp merge

• https://doc.rust-lang.org/nightly/std/io/struct.Cursor.html#method.get_mut
• https://doc.rust-lang.org/nightly/std/io/struct.Cursor.html#method.set_position

Their non-mut counterparts, get_ref and position, were stabilized in const in Rust 1.79 by #78812.

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.