Allow reverse iteration of lowercase'd/uppercase'd chars

[spektom]

The PR implements DoubleEndedIterator trait for ToLowercase and ToUppercase.

This enables reverse iteration of lowercase/uppercase variants of character sequences.
One of use cases: determining whether a char sequence is a suffix of another one.

Example:

fn endswith_ignore_case(s1: &str, s2: &str) -> bool {
    for eob in s1
        .chars()
        .flat_map(|c| c.to_lowercase())
        .rev()
        .zip_longest(s2.chars().flat_map(|c| c.to_lowercase()).rev())
    {
        match eob {
            EitherOrBoth::Both(c1, c2) => {
                if c1 != c2 {
                    return false;
                }
            }
            EitherOrBoth::Left(_) => return true,
            EitherOrBoth::Right(_) => return false,
        }
    }
    true
}

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @scottmcm (or someone else) soon.

Please see the contribution instructions for more information.

[scottmcm]

Thanks for the PR, @spektom! This seems reasonable to me.

As trait impls are insta-stable, though, this needs an FCP from the libs team, so...

r? @yaahc

(It also needs the versions in the #[stable] attributes changed, but that can wait until we know when it could go in.)

[dtolnay]

@rfcbot fcp merge

Reverse iteration for https://doc.rust-lang.org/core/char/struct.ToLowercase.html and https://doc.rust-lang.org/core/char/struct.ToUppercase.html.

[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
• @yaahc

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!

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 417b6f3 has been approved by dtolnay

[bors]

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.