Fix trait method resolution on an adjusted never type by JonathanBrouwer · Pull Request #156047 · rust-lang/rust

JonathanBrouwer · 2026-05-01T14:22:44Z

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Fixes #143349

r? @WaffleLapkin
cc @lcnr

rustbot · 2026-05-01T14:22:49Z

WaffleLapkin is not on the review rotation at the moment.
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JonathanBrouwer · 2026-05-01T14:23:14Z

Note that this PR allows more code to compile on stable, for example see this test.

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If this is a problem we could delay the problem by putting this change behind the never_type feature gate

lcnr

I think this a reasonable change, I would expect us to emit a FCW here, I don't want to support this permanently or more importantly, this may overlap with other kinds of inference fallback in the future, so we generally don't want users to rely on this

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JonathanBrouwer · 2026-05-01T15:22:00Z

This PR is now based on #156048, still need to add the FCW lint, I'll do that later.
@rustbot author

Is "Trait method call on adjusted never type" the correct terminology? I'm not super familiar with the type system yet

rustbot · 2026-05-02T16:12:44Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-05-03T10:54:15Z

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.

JonathanBrouwer · 2026-05-03T10:55:01Z

 }
+
+declare_lint! {
+    /// The `trait_method_on_coerced_never_type` lint detects situations in which a never type, which


I'm really unsure of what the correct terminology is, both for the lint definition and the diagnostic, so please give feedback on this

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IMO the proper way to say it would be "method_call_on_diverging_infer_variable" and "detects situations in which a method is called on a value resulting from a never-to-any coercion, without necessary information to infer a type for it", but I'm not sure how much we want to surface this internal language...

JonathanBrouwer · 2026-05-03T10:55:27Z

+    "detects trait method calls on an coerced never type",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: fcw!(FutureReleaseError #156047),
+        report_in_deps: true,


I decided on a lint with level Warn with report_in_deps: true. If something else is desired, let me know

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JonathanBrouwer · 2026-05-03T10:55:51Z

@rustbot ready

I rebased on #156079 and #156048 so the diff is nicely small now

WaffleLapkin

Code changes look good to me. This is blocked on T-types FCP (also the wording in the lint could be improved).

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WaffleLapkin · 2026-05-04T13:28:15Z

 }
+
+declare_lint! {
+    /// The `trait_method_on_coerced_never_type` lint detects situations in which a never type, which


IMO the proper way to say it would be "method_call_on_diverging_infer_variable" and "detects situations in which a method is called on a value resulting from a never-to-any coercion, without necessary information to infer a type for it", but I'm not sure how much we want to surface this internal language...

WaffleLapkin · 2026-05-04T13:30:17Z

+    /// Calling trait methods on a coerced `!` was previously disallowed for the never type,
+    /// but it did work for empty enums such as `Infallible` since these don't coerce.
+    /// This means that changing the definition of `Infallible` to become a type alias to `!` (a long-term goal),
+    /// would break code that called a trait method on `Infallible`, in such a way that the `!` would coerce.
+    ///
+    /// Therefore, to aid in the transition of changing `Infallible` to a type alias, this is temporarily allowed with a FCW.


Please also add a note that calling a method on ! is never useful (it's unreachable code) and propose removing the method call / using the never to any coercion.

Also mention that if people want to support MSRVs before Infallible = !, the proper fix is to match e {} as DesiredType.

lcnr · 2026-06-04T14:15:44Z

Code changes look good to me. This is blocked on T-types FCP (also the wording in the lint could be improved).

Can one of you writeup a quick proposal of what this is doing and why?

WaffleLapkin · 2026-06-09T13:04:46Z

Proposal

When trying to call methods on a type containing diverging inference variables, fallback them to ! and emit a FCW.

Motivation

#155924 describes current breakage from making Infallible = !. One of the issues (number 1 in the list there) is code that tries to convert Result<_, Infallible> to Result<_, anyhow::Error> with .map_err(|e| anyhow::anyhow!(e)).

The problem is that anyhow! does auto-ref based specialization on the error, in the form of

let error = match $err {
    error => (&error).anyhow_kind().new(error),
};

match scrutinee is a coercion point, so error happens to be a diverging inference variable and method selection currently fails.

I would like to avoid this breakage.

FCW?

I personally think that one ought to be able to call a method on a diverging inference variable regardless. Otherwise you end up with code breaking in confusing situations, see #143349 for example.

However if I recall correctly @lcnr was against having this hack permanently. So we can report an FCW and eventually remove this hack.

lcnr · 2026-06-11T12:02:35Z

@rfcbot fcp merge types

rust-rfcbot · 2026-06-11T12:02:38Z

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

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.

rustbot assigned WaffleLapkin May 1, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 1, 2026

JonathanBrouwer commented May 1, 2026

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lcnr reviewed May 1, 2026

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Comment thread compiler/rustc_hir_typeck/src/method/probe.rs Outdated

lcnr reviewed May 1, 2026

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WaffleLapkin added the F-never_type `#![feature(never_type)]` label May 1, 2026

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JonathanBrouwer force-pushed the never-ty-method branch from 75d6b9a to d72e38a Compare May 1, 2026 15:01

WaffleLapkin added needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. T-types Relevant to the types team, which will review and decide on the PR/issue. labels May 1, 2026

JonathanBrouwer mentioned this pull request May 2, 2026

Move and rename the clone-never.rs test #156079

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JonathanBrouwer force-pushed the never-ty-method branch from d72e38a to 3787326 Compare May 3, 2026 10:54

JonathanBrouwer commented May 3, 2026

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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 May 3, 2026

Fix trait method resolution on an adjusted never type

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JonathanBrouwer force-pushed the never-ty-method branch from 3787326 to a8f68d6 Compare May 3, 2026 11:10

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JonathanBrouwer force-pushed the never-ty-method branch from a8f68d6 to 2dd2577 Compare May 3, 2026 11:55

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Add FCW lint trait_method_on_coerced_never_type

388ae0f

JonathanBrouwer force-pushed the never-ty-method branch from 2dd2577 to 388ae0f Compare May 3, 2026 12:41

WaffleLapkin approved these changes May 4, 2026

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WaffleLapkin added S-waiting-on-fcp Status: PR is in FCP and is awaiting for FCP to complete. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 4, 2026

JonathanBrouwer added the I-types-nominated Nominated for discussion during a types team meeting. label May 8, 2026

JonathanBrouwer added the S-waiting-on-t-types Status: Awaiting decision from T-types label May 29, 2026

rust-rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Jun 11, 2026

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lcnr commented Jun 4, 2026

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WaffleLapkin commented Jun 9, 2026

Proposal

Motivation

FCW?

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lcnr commented Jun 11, 2026

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