Implement `AsMut<T>` and `AsRef<T>` for `!`. by kpreid · Pull Request #161253 · rust-lang/rust

kpreid · 2026-08-17T18:00:33Z

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

A class of use case for these impls is:

some code is generic over T: AsRef<SomeOtherType>,
and is still useful in the case where there are no Ts, because it takes Option<T> or &[T] or any other input that can be empty or never-called, yet still do things with other inputs,

in which case providing T = ! is slightly better than the alternative of providing an inhabited placeholder type, because it ensures that the generic code is compiled with knowledge of the fact that that there will never be any Ts, even if the specific code that cares about whether there are any Ts doesn’t get inlined into the call site that doesn’t provide any Ts.

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 · 2026-08-20T06:04:38Z

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 · 2026-08-20T14:54:21Z

@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 · 2026-08-24T12:25:10Z

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

JonathanBrouwer · 2026-08-24T12:31:22Z

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 · 2026-08-24T12:57:59Z

@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 · 2026-08-24T17:03:22Z

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 · 2026-08-24T17:16:21Z

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

JonathanBrouwer · 2026-08-24T17:48:46Z

(oops, thanks)

kpreid · 2026-08-25T00:54:03Z

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

theemathas · 2026-08-25T01:23:02Z

Preparing for crater run
@bors try

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

rust-bors · 2026-08-25T02:21:56Z

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

rust-rfcbot · 2026-10-20T23:57:36Z

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.

clarfonthey · 2026-10-21T03:23:09Z

Should be ready to merge after making the change Josh mentioned.

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> A class of use case for these impls is: * some code is generic over `T: AsRef<SomeOtherType>`, * and is still useful in the case where there are no `T`s, because it takes `Option<T>` or `&[T]` or any other input that can be empty or never-called, yet still do things with other inputs, in which case providing `T = !` is slightly better than the alternative of providing an inhabited placeholder type, because it ensures that the generic code is compiled with knowledge of the fact that that there will never be any `T`s, even if the specific code that cares about whether there are any `T`s doesn’t get inlined into the call site that doesn’t provide any `T`s. 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.

rustbot · 2026-10-21T04:31:47Z

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 · 2026-10-21T04:33:15Z

@rustbot ready

clarfonthey · 2026-10-21T13:29:21Z

@bors r+ rollup

Thank you!

rust-bors · 2026-10-21T13:29:24Z

📋 This PR cannot be approved because it currently has the following label: needs-crater.

theemathas · 2026-10-21T14:22:08Z

@bors r=clarfonthey rollup

rust-bors · 2026-10-21T14:22:14Z

📌 Commit d016eb1 has been approved by clarfonthey

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

@rustbot

Implement `AsMut<T>` and `AsRef<T>` for `!`. 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> A class of use case for these impls is: * some code is generic over `T: AsRef<SomeOtherType>`, * and is still useful in the case where there are no `T`s, because it takes `Option<T>` or `&[T]` or any other input that can be empty or never-called, yet still do things with other inputs, in which case providing `T = !` is slightly better than the alternative of providing an inhabited placeholder type, because it ensures that the generic code is compiled with knowledge of the fact that that there will never be any `T`s, even if the specific code that cares about whether there are any `T`s doesn’t get inlined into the call site that doesn’t provide any `T`s. --- 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: rust-lang#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

…uwer Rollup of 13 pull requests Successful merges: - #162460 (Feat - `#[inline(always)] + #[target_feature(enable = "....")]` #2) - #163485 (`rustc_builtin_macros` cleanup, part 8) - #164457 (Add `vec_visualizer` lldb test) - #161253 (Implement `AsMut<T>` and `AsRef<T>` for `!`.) - #162369 (Document the `recursion_limit` built-in attribute) - #162840 (tier 3 target policy: target CPU and features must be supported by upstream backend) - #164329 (use default_field_values in declare_lint) - #164330 (Skip redundant monomorphization checks) - #164527 (doc: added addtional docs for Write) - #164656 (stabilize ExitCodeExt for Windows) - #164660 (Touch up attribute docs) - #164681 (tidy: use repository-relative path for style checks) - #164688 (re-enable test_dir_remove_dir test on QNX)

@rustbot

Rollup merge of #161253 - kpreid:neverref, r=clarfonthey Implement `AsMut<T>` and `AsRef<T>` for `!`. 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> A class of use case for these impls is: * some code is generic over `T: AsRef<SomeOtherType>`, * and is still useful in the case where there are no `T`s, because it takes `Option<T>` or `&[T]` or any other input that can be empty or never-called, yet still do things with other inputs, in which case providing `T = !` is slightly better than the alternative of providing an inhabited placeholder type, because it ensures that the generic code is compiled with knowledge of the fact that that there will never be any `T`s, even if the specific code that cares about whether there are any `T`s doesn’t get inlined into the call site that doesn’t provide any `T`s. --- 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

rustbot assigned BurntSushi Aug 20, 2026

theemathas added needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. needs-crater This change needs a crater run to check for possible breakage in the ecosystem. labels Aug 24, 2026

clarfonthey added T-libs Relevant to the library team, which will review and decide on the PR/issue. and removed T-libs-api [DEPRECATED; DO NOT USE] labels Aug 24, 2026

rustbot assigned JohnTitor and unassigned BurntSushi Aug 24, 2026

kpreid force-pushed the neverref branch from 5dcfce9 to e7ce9a3 Compare August 25, 2026 00:51

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rust-bors Bot pushed a commit that referenced this pull request Aug 25, 2026

Auto merge of #161253 - kpreid:neverref, r=<try>

65c7d39

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

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kpreid force-pushed the neverref branch from e7ce9a3 to f60f751 Compare August 25, 2026 02:19

kpreid force-pushed the neverref branch from f60f751 to e028a86 Compare August 25, 2026 02:28

rust-rfcbot added the to-announce Announce this issue on triage meeting label Oct 20, 2026

rustbot mentioned this pull request Oct 20, 2026

Tracking issue for release notes of #161253: Implement AsMut<T> and AsRef<T> for !. #164661

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clarfonthey added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 21, 2026

kpreid force-pushed the neverref branch from 584c26f to 2ba6c5c Compare October 21, 2026 04:30

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kpreid force-pushed the neverref branch from 2ba6c5c to d016eb1 Compare October 21, 2026 04:31

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 21, 2026

theemathas removed the needs-crater This change needs a crater run to check for possible breakage in the ecosystem. label Oct 21, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 21, 2026

This was referenced Oct 21, 2026

Rollup of 14 pull requests #164697

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Rollup of 13 pull requests #164699

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rust-bors Bot merged commit 59d9a8f into rust-lang:main Oct 21, 2026
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rustbot added this to the 1.101.0 milestone Oct 21, 2026

github-actions Bot mentioned this pull request Oct 22, 2026

Toolchain upgrade to nightly-2026-10-22 flux-rs/flux#1847

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kpreid deleted the neverref branch October 22, 2026 02:48

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