Improve E0277 diagnostics for conditionally implemented traits by raushan728 · Pull Request #158494 · rust-lang/rust

raushan728 · 2026-06-27T16:48:43Z

The diagnostic previously reported that a trait was "implemented" even when the matching impl could not apply because its where-clause requirements were not satisfied, which was misleading.

This change:

reports that the trait is "conditionally implemented" when a matching impl is rejected due to unsatisfied where-clause requirements;
highlights the unsatisfied requirement in the impl and includes the corresponding predicate in the diagnostic;
preserves the existing diagnostic flow while making it clearer why the candidate impl was rejected.

rustbot · 2026-06-27T16:48:48Z

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mu001999

Could you try to add the label on unsatisfied bounds, as desired in the issue? I think we can achieve this with ObligationCtxt::new_with_diagnostics.

And for the logic here, perhaps you could check the implementation of impl_may_apply. Maybe the logic could even be shared between them.

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rustbot · 2026-06-28T13:22:48Z

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

raushan728 · 2026-06-29T11:57:49Z

Could you try to add the label on unsatisfied bounds, as desired in the issue? I think we can achieve this with ObligationCtxt::new_with_diagnostics.

And for the logic here, perhaps you could check the implementation of impl_may_apply. Maybe the logic could even be shared between them.

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Thanks! I updated this to use ObligationCtxt::new_with_diagnostics, so the diagnostic now highlights the unsatisfied trait bound. I also looked into impl_may_apply, but kept the logic local since it serves a different purpose.

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rustbot · 2026-06-30T06:47:47Z

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.

raushan728 · 2026-06-30T06:53:26Z

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mu001999 · 2026-07-02T14:36:08Z

Thanks!

@bors r+

rust-bors · 2026-07-02T14:36:12Z

📌 Commit 4613bf9 has been approved by mu001999

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

Improve E0277 diagnostics for conditionally implemented traits Fixes rust-lang#158423. The diagnostic previously reported that a trait was "implemented" even when the matching impl could not apply because its where-clause requirements were not satisfied, which was misleading. This change: - reports that the trait is "conditionally implemented" when a matching impl is rejected due to unsatisfied where-clause requirements; - highlights the unsatisfied requirement in the impl and includes the corresponding predicate in the diagnostic; - preserves the existing diagnostic flow while making it clearer why the candidate impl was rejected.

…uwer Rollup of 22 pull requests Successful merges: - #155429 (Support `u128`/`i128` c-variadic arguments) - #158100 (Emit retags in codegen to support BorrowSanitizer (part 4)) - #158494 (Improve E0277 diagnostics for conditionally implemented traits) - #158606 (use ProjectionPredicate instead of AliasRelate) - #158627 (Simplify option-iterator flattening in the compiler) - #158658 (Update LLVM submodule) - #158665 (Revert "Remove redundant dyn-compatibility check.") - #158021 (Remove old MinGW workaround) - #158473 (Add `riscv32imfc-unknown-none-elf` bare-metal target) - #158549 (process::exec: using appropriate exit code on vxworks.) - #158585 (Improve diagnostic for too many super keywords) - #158637 (hir_ty_lowering: avoid self type lookup for inherent aliases) - #158651 (ptr doc: reduce use of unsafe block to where needed) - #158669 (Remove `src/tools/test-float-parse/Cargo.lock`) - #158674 (library: Polish transmute's `split_at_stdlib` example) - #158677 (Add extra splat tests) - #158680 (Avoid ICE for `NonZero<char>` in improper_ctypes) - #158681 (Remove unnecessary `Hash` derives from MIR types) - #158682 (Avoid delayed bug for disabled on_type_error arguments) - #158684 (Add missing generic test coverage for ```#[splat]```) - #158687 (Streamline `MacEager`) - #158688 (Cleanup attribute docs and add links to other mentioned attributes)

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test(traits): add baseline test for conditionally implemented traits

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raushan728 force-pushed the issues/158423 branch from 9f2d7b1 to 28e57e4 Compare June 30, 2026 06:47

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Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs Outdated

Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs Outdated

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fix(traits): clarify conditional impl diagnostics

4613bf9

raushan728 force-pushed the issues/158423 branch from 607962a to 4613bf9 Compare July 1, 2026 05:47

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JonathanBrouwer mentioned this pull request Jul 2, 2026

Rollup of 22 pull requests #158700

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