Implement missing AsMut<str> for str

[Yaulendil]

Allows &mut str to be taken by a Generic which requires T such that T: AsMut<str>. Motivating example:

impl<'i, T> From<T> for StructImmut<'i> where
    T: AsRef<str> + 'i,
{
    fn from(asref: T) -> Self {
        let string: &str = asref.as_ref();
        //  ...
    }
}

impl<'i, T> From<T> for StructMut<'i> where
    T: AsMut<str> + 'i,
{
    fn from(mut asmut: T) -> Self {
        let string: &mut str = asmut.as_mut();
        //  ...
    }
}

The Immutable form of this structure can be constructed by StructImmut::from(s) where s may be a &String or a &str, because AsRef<str> is implemented for str. However, the mutable form of the structure can be constructed in the same way only with a &mut String, and not with a &mut str.

This change does have some precedent, because as can be seen in the Implementors, AsMut<[T]> is implemented for [T] as well as for Vec<T>, but AsMut<str> is implemented only for String. This would complete the symmetry.

As a trait implementation, this should be immediately stable.

[rust-highfive]

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[Mark-Simulacrum]

r? @dtolnay for t-libs approval of stable api surface area

@Yaulendil - do you have a library or other example in the "real world" that would use this? the example you gave seems synthetic at first glance

[Yaulendil]

@Yaulendil - do you have a library or other example in the "real world" that would use this? the example you gave seems synthetic at first glance

That is the only concrete example I have come across. It is a minimalized snippet of this experimental project of mine, an iterator yielding mutable slices over a String, corresponding to the unicode characters. I noticed the missing implementation when I tried to make it generic for external types implementing AsMut<str>, and found that it broke the original constructions from &mut strs in the tests.

The primary application of this project was selective in-place capitalization without cloning. The original use case had only ASCII text, so I was able to get away with just using str::as_bytes_mut for that project, but something like this would probably be required to do the same for a dataset including multiple-byte unicode (A non-English translation, for example).

[dtolnay]

@rust-lang/libs:
@rfcbot fcp merge

impl AsMut<str> for str

Currently the only types that implement AsMut<str> are String, Box<str>, and &mut of one of the previous. This makes real-world abstractions built on AsMut<str> not work for perhaps the most obvious choice of type, &mut str, as described by @Yaulendil above.

[rfcbot]

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

• @Amanieu
• @BurntSushi
• @KodrAus
• @dtolnay
• @m-ou-se
• @sfackler
• @withoutboats

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.

[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.

The RFC will be merged soon.

[m-ou-se]

@bors r+ rollup

[bors]

📌 Commit bef4ec2 has been approved by m-ou-se