Simplify `type_of_opaque`. by nnethercote · Pull Request #153581 · rust-lang/rust

nnethercote · 2026-03-09T02:33:26Z

There is a bunch of complexity supporting the "cannot check whether the hidden type of opaque type satisfies auto traits" error that shows up in tests/ui/impl-trait/auto-trait-leak.rs. This is an obscure error that shows up in a single test. If we are willing to downgrade that error message to a cycle error, we can do the following.

Simplify the type_of_opaque return value.
Remove the cycle_stash query modifier.
Remove the CyclePlaceholder type.
Remove the SelectionError::OpaqueTypeAutoTraitLeakageUnknown variant.
Remove a FromCycleError impl.
Remove report_opaque_type_auto_trait_leakage.
Remove the StashKey::Cycle variant.
Remove the CycleErrorHandling::Stash variant.

That's a lot! I think this is a worthwhile trade-off.

r? @oli-obk

rustbot · 2026-03-09T02:33:31Z

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

nnethercote · 2026-03-09T02:35:06Z

cc @Zoxc @Zalathar @zetanumbers

This overlaps with #153247, and implements the suggestion in this comment. Zoxc gave me the go ahead to take this over via Zulip DM.

zetanumbers · 2026-03-10T12:59:57Z

tests/ui/impl-trait/auto-trait-leakage/auto-trait-leak.stderr

This new error message is a mouthful. As a C++ refuge I don't like this length of a diagnostic. Is there no way to keep it more tight?

That's a general cycle error problem. We should def tackle it, but lots of ppl have different opinions here so we need to likely cook up some PRs of variants and then start a discussion

Very well then

I think it makes sense to replace multiple steps in the cycle error with single step that better represents the cycle in the source code.

The formatting of the error could be improved too, it should be a list, not notes.

oli-obk · 2026-03-11T08:21:53Z

@bors r+

rust-bors · 2026-03-11T08:21:56Z

📌 Commit 45a010c has been approved by oli-obk

It is now in the queue for this repository.

oli-obk · 2026-03-11T08:22:45Z

compiler/rustc_hir_analysis/src/collect/type_of.rs

-    tcx: TyCtxt<'_>,
-    def_id: DefId,
-) -> Result<ty::EarlyBinder<'_, Ty<'_>>, CyclePlaceholder> {
+pub(super) fn type_of_opaque(tcx: TyCtxt<'_>, def_id: DefId) -> ty::EarlyBinder<'_, Ty<'_>> {


I wonder if it makes sense to move it back into type_of now, but that can be experimented with in a separate PR

zetanumbers · 2026-03-11T09:04:06Z

This PR basically just removes no longer used parts of query cycle handling, so it should be fine. Although I very briefly looked at the trait selection changes.

There is a bunch of complexity supporting the "cannot check whether the hidden type of opaque type satisfies auto traits" error that shows up in `tests/ui/impl-trait/auto-trait-leak.rs`. This is an obscure error that shows up in a single test. If we are willing to downgrade that error message to a cycle error, we can do the following. - Simplify the `type_of_opaque` return value. - Remove the `cycle_stash` query modifier. - Remove the `CyclePlaceholder` type. - Remove the `SelectionError::OpaqueTypeAutoTraitLeakageUnknown` variant. - Remove a `FromCycleError` impl. - Remove `report_opaque_type_auto_trait_leakage`. - Remove the `StashKey::Cycle` variant. - Remove the `CycleErrorHandling::Stash` variant. That's a lot! I think this is a worthwhile trade-off.

rustbot · 2026-03-11T10:57:03Z

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.

nnethercote · 2026-03-11T10:57:23Z

I rebased.

@bors r=oli-obk

rust-bors · 2026-03-11T10:57:27Z

📌 Commit 3399ed3 has been approved by oli-obk

It is now in the queue for this repository.

@oli-obk

Simplify `type_of_opaque`. There is a bunch of complexity supporting the "cannot check whether the hidden type of opaque type satisfies auto traits" error that shows up in `tests/ui/impl-trait/auto-trait-leak.rs`. This is an obscure error that shows up in a single test. If we are willing to downgrade that error message to a cycle error, we can do the following. - Simplify the `type_of_opaque` return value. - Remove the `cycle_stash` query modifier. - Remove the `CyclePlaceholder` type. - Remove the `SelectionError::OpaqueTypeAutoTraitLeakageUnknown` variant. - Remove a `FromCycleError` impl. - Remove `report_opaque_type_auto_trait_leakage`. - Remove the `StashKey::Cycle` variant. - Remove the `CycleErrorHandling::Stash` variant. That's a lot! I think this is a worthwhile trade-off. r? @oli-obk

…uwer Rollup of 4 pull requests Successful merges: - #153571 (Avoid ICE when an EII declaration conflicts with a constructor) - #153581 (Simplify `type_of_opaque`.) - #153685 (Introduce `for_each_query_vtable!` to move more code out of query macros) - #153710 (remove `.ftl` checks from tidy)

@oli-obk

Simplify `type_of_opaque`. There is a bunch of complexity supporting the "cannot check whether the hidden type of opaque type satisfies auto traits" error that shows up in `tests/ui/impl-trait/auto-trait-leak.rs`. This is an obscure error that shows up in a single test. If we are willing to downgrade that error message to a cycle error, we can do the following. - Simplify the `type_of_opaque` return value. - Remove the `cycle_stash` query modifier. - Remove the `CyclePlaceholder` type. - Remove the `SelectionError::OpaqueTypeAutoTraitLeakageUnknown` variant. - Remove a `FromCycleError` impl. - Remove `report_opaque_type_auto_trait_leakage`. - Remove the `StashKey::Cycle` variant. - Remove the `CycleErrorHandling::Stash` variant. That's a lot! I think this is a worthwhile trade-off. r? @oli-obk

…uwer Rollup of 9 pull requests Successful merges: - #153571 (Avoid ICE when an EII declaration conflicts with a constructor) - #153581 (Simplify `type_of_opaque`.) - #153611 (interpret: go back to regular string interpolation for error messages) - #153635 (Unify same-span labels in move error diagnostics) - #153660 (mir-opt: Drop invalid debuginfos after SingleUseConsts.) - #153685 (Introduce `for_each_query_vtable!` to move more code out of query macros) - #153671 (Make Enzyme has dependent on LLVM hash) - #153710 (remove `.ftl` checks from tidy) - #153720 (doc/rustc: clarify how to contact arm-maintainers)

…uwer Rollup of 12 pull requests Successful merges: - #152569 (Stop using rustc_layout_scalar_valid_range_* in rustc) - #153421 (Fix ICE in fn_delegation when child segment resolves to a trait) - #153571 (Avoid ICE when an EII declaration conflicts with a constructor) - #153581 (Simplify `type_of_opaque`.) - #153611 (interpret: go back to regular string interpolation for error messages) - #153635 (Unify same-span labels in move error diagnostics) - #153660 (mir-opt: Drop invalid debuginfos after SingleUseConsts.) - #153685 (Introduce `for_each_query_vtable!` to move more code out of query macros) - #153722 (miri-test-libstd: use --tests and update some comments) - #153671 (Make Enzyme has dependent on LLVM hash) - #153710 (remove `.ftl` checks from tidy) - #153720 (doc/rustc: clarify how to contact arm-maintainers)

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nnethercote force-pushed the rm-cycle_stash branch from 45a010c to 3399ed3 Compare March 11, 2026 10:56

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JonathanBrouwer mentioned this pull request Mar 11, 2026

Rollup of 4 pull requests #153714

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JonathanBrouwer mentioned this pull request Mar 11, 2026

Rollup of 9 pull requests #153723

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JonathanBrouwer mentioned this pull request Mar 11, 2026

Rollup of 12 pull requests #153741

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rust-bors bot merged commit 4e64a8b into rust-lang:main Mar 12, 2026
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rustbot added this to the 1.96.0 milestone Mar 12, 2026

nnethercote deleted the rm-cycle_stash branch March 12, 2026 01:08

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