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

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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.

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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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 2, 2026

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label May 2, 2026

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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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JonathanBrouwer assigned lcnr May 3, 2026

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

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