Tracking Issue for `casefold`

Feature gate: #![feature(casefold)]

This is a tracking issue for Unicode-aware case folding APIs in the standard library. These APIs do not perform NFC/NFKC normalization.

Public API

// `core`

impl char {
    pub fn to_casefold_unnormalized(self) -> ToCasefold;
}

mod char {
    #[derive(Debug, Clone)]
    pub struct ToCasefold { … };
    impl Iterator for ToCasefold {
        type Item = char;
        …
    }
    impl DoubleEndedIterator for ToCasefold { … }
    impl FusedIterator for ToCasefold {}
    impl ExactSizeIterator for ToCasefold {}
    impl fmt::Display for ToCasefold { … }
}

impl str {
    pub fn eq_ignore_case_unnormalized(&self, other: &str) -> bool;
}

// `alloc`

impl str {
    pub fn to_casefold_unnormalized(&self) -> String;
}

Steps / History

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

• Implementation: #154742
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Is the lack of normalization a deal-breaker? What is the best way to deal with that? https://github.com/rust-lang/rust/pull/154742#issuecomment-4479220168

@rustbot label A-Unicode

Start by reviewing implementation PR #154742 and the listed public APIs behind the #![feature(casefold)] gate. Resolve the open question about normalization, then follow the FCP and stabilization steps; done means the unresolved concerns are addressed and a stabilization PR is completed.