ACP: impl From<&mut {slice}> for Rc<{slice}> and Arc<{slice}>

[eduardosm]

Proposal

Problem statement

Sometimes, I need to build reference counted slice (e.g., Rc<str>, Rc<[u8]>, Rc<Path>... or their Arc counterparts) from a borrowed sliced. If it is an immutable borrow (e.g., &str), I can use From<&str> for Rc<str>. If it is a mutable borrow (e.g., &mut str), there is not a From<&mut str> for Rc<str>.

Motivating examples or use cases

let chr: char = ...;
let s: Rc<str> = chr.encode_utf8(&mut [0; 4]).into();

Since char::encode_utf8 returns a &mut str, it fails to compile with "the trait bound Rc<str>: From<&mut str> is not satisfied"

Solution sketch

The proposal is to add the following impls:

impl From<&mut CStr> for Rc<CStr>
impl From<&mut OsStr> for Rc<OsStr>
impl From<&mut Path> for Rc<Path>
impl From<&mut str> for Rc<str>

And their Arc counterparts.

An easy implementation would consist on coercing the mutable slice to an immutable one and leverage the existing impls, which take immutable slices, to perform the conversion.

Alternatives

The alternative right now is to manually coerce the &mut _ to &_ before creating the Rc.

let chr: char = ...;
let mut buf = [0; 4];
let s: &str = chr.encode_utf8(&mut buf);
let s: Rc<str> = s.into();

Links and related work

rust-lang/rust#128885

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[pitaj]

Can't you just reborrow?

let s: Rc<str> = (&*chr.encode_utf8(&mut [0; 4])).into();

[eduardosm]

That should work too.

[Amanieu]

We discussed this in the libs-api meeting and are happy to add these. This should also include the implementation for &mut [T] which seems to be missing from the ACP.

Considering we've had recent breakage from new trait implementations, the PR for this should probably go through a crater run.