Implement AsMut<String> for String

[nwtnni]

Related to #68741 and #68742.

There's a parallel implementation of AsMut<[T]> for Vec<T> and AsMut<Vec<T>> for Vec<T>, so I thought we might want to do the same for String?

EDIT: actually, would it make more sense to just write the following global implementations?

impl<T: ?Sized> AsRef<T> for T {
    fn as_ref(&self) -> &T {
        self
    }
}

impl<T: ?Sized> AsMut<T> for T {
    fn as_mut(&mut self) -> &mut T {
        self
    }
}

Similarly to what the Borrow and BorrowMut traits do? This is my first time contributing, so I'm not sure if this would require an issue or RFC--please let me know if there's a more appropriate place to suggest this.

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[shepmaster]

There's a parallel implementation of AsMut<[T]> for Vec<T> and AsMut<Vec<T>> for Vec<T>,

Because we already have the equivalent for Vec<T>, this change makes sense to me. It would be good to review how deliberate that existing implementation was, ensuring that it wasn't an accident we want to not propagate.

However, this modifies the public API, so I'm going to kick it to the libs team...

r? @LukasKalbertodt

would it make more sense to just write the following global implementations

Perhaps, but the lack of a blanket implementation seems deliberate:

AsRef has the same signature as Borrow, but Borrow is different in few aspects:

Unlike AsRef, Borrow has a blanket impl for any T, and can be used to accept either a reference or a value.

[nwtnni]

Perhaps, but the lack of a blanket implementation seems deliberate:

AsRef has the same signature as Borrow, but Borrow is different in few aspects:

Unlike AsRef, Borrow has a blanket impl for any T, and can be used to accept either a reference or a value.

You're right, I do see that in the AsRef docs. The implementations of AsRef<Vec<T>> and AsMut<Vec<T>> for Vec<T> seem to be the odd ones out, looking at the other implementations, so I don't mind closing this PR.

[JohnCSimon]

Ping from triage:
@nwtnni if no more work is going to happen with this PR can you please close it? Thanks.

[nwtnni]

Ping from triage:
@nwtnni if no more work is going to happen with this PR can you please close it? Thanks.

@LukasKalbertodt, can I get a quick confirmation before I close?

[LukasKalbertodt]

@nwtnni I wanted to think about it a bit more, but had no time for it yet. Sorry! Will do in the coming day and then comment here. You can keep this open until then.

[LukasKalbertodt]

I still haven't completely understood the design decisions behind Borrow and AsRef, but yes: the existing Vec impl seems to be a special case. So let's close this for now, as already suggested.