Tracking Issue for const `[u8]::is_ascii` (`const_slice_is_ascii`)

[scottmcm]

Feature gate: #![feature(const_slice_is_ascii)]

This is a tracking issue for [u8]::is_ascii being a const fn.

UTF-8 checking in const fn stabilized back in 1.63 (#97367), but apparently somehow simpler ASCII checking was never const-ified.

Public API

// core::slice

impl [u8] {
    const fn is_ascii(&self) -> bool;
}

// core::str

impl str {
    const fn is_ascii(&self) -> bool;
}

Steps / History

• Implementation: Constify [u8]::is_ascii (unstably) #111222
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

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

[scottmcm]

Hello libs-api! This hasn't been around that long, but I'd like to nominate it for stabilization.

str::from_utf8 has been const-stable since 1.63 (a year ago), so I'd like to also be able to use the substantially-simpler [u8]::is_ascii in const as well.

It can be implemented using only stable const fn support (https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dd32ebaa5103731de9fab0ab4057f8bc), so this doesn't require exposing any new capabilities.

[m-ou-se]

@rfcbot merge

[rfcbot]

Team member @m-ou-se 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.