Implement AsMut<T> and AsRef<T> for !.

[kpreid]

View all comments

This will allow e.g. &[!] to satisfy &[T] where T: AsRef<str>. It follows the recommendation from the never documentation:

When writing your own traits, ! should have an impl whenever there is an obvious impl which doesn’t panic!.

— https://doc.rust-lang.org/1.97.1/std/primitive.never.html#-and-traits

The test tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs had to be updated because it depended on this impl not existing. I’ve confirmed that the modified test still functions as a regression test by compiling it in nightly-2022-08-28 and seeing it ICE.

Tracking issue for never: #35121
(This change has no ACP or tracking issue of its own; I assume it is simple enough that T-libs-api can just accept or reject this PR, and the implementation will be stable when ! is stable.)

@rustbot label -T-libs +T-libs-api +F-never_type

[QuineDot]

As Infallible will become !, this conflicts with implementations that can exist today.

use std::convert::Infallible;
struct S;
impl AsRef<S> for Infallible {
    fn as_ref(&self) -> &S {
        match *self {}
    }
}

[kpreid]

@QuineDot Thanks for pointing that out. That means this PR must either be rejected or get a crater run. I think the possible benefit is significant, so I won’t just close it immediately.

…unrelatedly, I just noticed that rustbot hasn’t assigned a reviewer at all.

r? libs-api

[theemathas]

Not sure how to run crater on this one. Do we wait until after the never type stabilization PR is merged?

[JonathanBrouwer]

You could run a crater with a AsRef<Infallible> and AsMut<Infallible> implementation, that should be representative.
That said, the stabilization PR looks likely to merge in the next few days so you could also just have some patience

[clarfonthey]

@kpreid FYI, libs-api isn't a thing any more, so, please r? libs in the future

ditto for the T-libs-api tag

[steffahn]

You could run a crater with a AsRef<Infallible> and AsMut<Infallible> implementation, that should be representative.

That would be AsRef<T> for Infallible / AsMut<T> for Infallible (clarifying just to avoid confusion).

[kpreid]

I’ll rebase this PR after the stabilization PR merges.

[JonathanBrouwer]

(oops, thanks)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[kpreid]

Rebased, and the implementations are now marked #[stable].

[theemathas]

Preparing for crater run
@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 65c7d39 (65c7d39f9c2b9d5c316725cb48fa3d360c64c663)
Base parent: e776960 (e7769602aca3770e8d8ea55716becb22e839a579)

[kpreid]

Updated tests/ui/impl-trait/generic-with-implicit-hrtb-without-dyn.rs to not depend on this impl not existing. I’ve confirmed that the modified test still functions as a regression test by compiling it with cargo +nightly-2022-08-28 check and seeing an ICE.

[theemathas]

@craterbot check

[craterbot]

👌 Experiment pr-161253 created and queued.
🤖 Automatically detected try build 65c7d39
⚠️ Try build based on commit e7ce9a3, but latest commit is e028a86. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[JohnTitor]

r? libs

[theemathas]

@craterbot cancel

See #162233

[craterbot]

🗑️ Experiment pr-161253 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more