`FindParamInClause` handle edge-cases by lcnr · Pull Request #153614 · rust-lang/rust

lcnr · 2026-03-09T15:22:12Z

If normalization is ambiguous, we want to treat the where-bound as non-global. The affected code should pretty much always have other errors in that case. It does make reasoning about trait solver cycles and rerunning more confusing and just feels wrong ™️ I encountered this while looking into ml-kem (rust-lang/trait-system-refactor-initiative#246). I don't know whether this matters in practice and don't care to look into it too deeply.

We also want to properly handle concrete aliases which mention to something mentioning a generic parameter due to another where-bound.

r? @BoxyUwU

rustbot · 2026-03-20T15:37:38Z

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lcnr · 2026-03-20T15:37:50Z

r? BoxyUwU

rustbot · 2026-03-20T15:37:53Z

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BoxyUwU · 2026-03-23T18:22:36Z

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

+                TypeFlags::HAS_PLACEHOLDER | TypeFlags::HAS_INFER | TypeFlags::HAS_ALIAS,
+            ) =>
+            {
+                ct.super_visit_with(self)


why the not doing recursion depth stuff here 🤔

existing xd :3 because the only way constants recurse is through types atm

I guess this has to change with mgca

up to u if u want to handle it in this PR, it'd be a fairly reasonable FIXME(mgca) to get some new-ish contributor to work on as onboarding

added a FIXME

BoxyUwU · 2026-03-23T18:24:57Z

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

-                }));
+            ty::Infer(_) => ControlFlow::Break(Ok(Certainty::AMBIGUOUS)),
+            _ if ty.has_type_flags(
+                TypeFlags::HAS_PLACEHOLDER | TypeFlags::HAS_INFER | TypeFlags::HAS_ALIAS,


so there are two meaningful changes here:

we now recurse if there are aliases

we now recurse if there are non-region-infers

recursing if there are aliases makes sense (that's certainly the point of this PR) but I don't fully understand why we care about infer vars or not.

to treat ambiguous normalization as Global. You could imagine normalizing T::Assoc to Vec<T::AmbiguousAlias> which ends up as Vec<?infer>

I guess we could always force GLobal if there are infer vars instead of recursing

do we have a test with ambiguous aliases? I guess you could only really get ambiguity here via overflow (are opaque types relevant?)?

no, it's hard to test (and I don't care enough 😅)

lcnr · 2026-04-02T10:33:23Z

@rustbot ready

rustbot · 2026-04-02T10:33:25Z

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.

BoxyUwU · 2026-04-02T10:35:07Z

r=me if CI passes

see the added test

lcnr · 2026-04-02T16:06:21Z

@bors r=BoxyUwU rollup

rust-bors · 2026-04-02T16:06:25Z

📌 Commit 69148fd has been approved by BoxyUwU

It is now in the queue for this repository.

@BoxyUwU

…=BoxyUwU `FindParamInClause` handle edge-cases If normalization is ambiguous, we want to treat the where-bound as non-global. The affected code should pretty much always have other errors in that case. It does make reasoning about trait solver cycles and rerunning more confusing and just feels wrong ™️ I encountered this while looking into `ml-kem` (rust-lang/trait-system-refactor-initiative#246). I don't know whether this matters in practice and don't care to look into it too deeply. We also want to properly handle concrete aliases which mention to something mentioning a generic parameter due to another where-bound. r? @BoxyUwU

…uwer Rollup of 21 pull requests Successful merges: - #153105 (Compute the result of a projection type with region errors) - #153286 (various fixes for scalable vectors) - #153532 (Attributes containing rustc) - #153960 (Make `layout_of` cycles fatal errors) - #154527 (Emit pre-expansion feature gate warnings for negative impls and specialization) - #154666 (Remove `StableHashContext` impls) - #154669 (Introduce #[diagnostic::on_move] on `Arc`) - #154710 (opaque_generic_const_args -> generic_const_args) - #154712 (Revert "`-Znext-solver` Remove the forced ambiguity hack from search graph") - #153614 (`FindParamInClause` handle edge-cases) - #154213 (tidy-alphabetical: fix line number in error message) - #154425 (Migrate transmute tests) - #154442 (Export `derive` at the crate root: `core::derive` and `std::derive`) - #154469 (mGCA: Lower spans for literal const args) - #154578 (Rename `probe_ty_var` to `try_resolve_ty_var`) - #154615 (Moving issues) - #154644 (rustdoc: seperate methods and associated functions in sidebar) - #154660 (Avoid creating async return opaques for foreign async fns) - #154671 (Add a test for a past ICE when calling a const fn of an unresolved type with the wrong number of args) - #154680 ([rustdoc] Replace `DocContext` with `TyCtxt` wherever possible) - #154709 (Revert `Ty` type alias in `rustc_type_ir`)

@BoxyUwU

…=BoxyUwU `FindParamInClause` handle edge-cases If normalization is ambiguous, we want to treat the where-bound as non-global. The affected code should pretty much always have other errors in that case. It does make reasoning about trait solver cycles and rerunning more confusing and just feels wrong ™️ I encountered this while looking into `ml-kem` (rust-lang/trait-system-refactor-initiative#246). I don't know whether this matters in practice and don't care to look into it too deeply. We also want to properly handle concrete aliases which mention to something mentioning a generic parameter due to another where-bound. r? @BoxyUwU

…uwer Rollup of 20 pull requests Successful merges: - #153105 (Compute the result of a projection type with region errors) - #153532 (Attributes containing rustc) - #153960 (Make `layout_of` cycles fatal errors) - #154527 (Emit pre-expansion feature gate warnings for negative impls and specialization) - #154666 (Remove `StableHashContext` impls) - #154669 (Introduce #[diagnostic::on_move] on `Arc`) - #154710 (opaque_generic_const_args -> generic_const_args) - #154712 (Revert "`-Znext-solver` Remove the forced ambiguity hack from search graph") - #153614 (`FindParamInClause` handle edge-cases) - #154213 (tidy-alphabetical: fix line number in error message) - #154425 (Migrate transmute tests) - #154442 (Export `derive` at the crate root: `core::derive` and `std::derive`) - #154469 (mGCA: Lower spans for literal const args) - #154578 (Rename `probe_ty_var` to `try_resolve_ty_var`) - #154615 (Moving issues) - #154644 (rustdoc: seperate methods and associated functions in sidebar) - #154660 (Avoid creating async return opaques for foreign async fns) - #154671 (Add a test for a past ICE when calling a const fn of an unresolved type with the wrong number of args) - #154680 ([rustdoc] Replace `DocContext` with `TyCtxt` wherever possible) - #154709 (Revert `Ty` type alias in `rustc_type_ir`)

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lcnr force-pushed the find-param-in-clause-ambig branch from 3564a3f to 755953f Compare March 20, 2026 14:56

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lcnr force-pushed the find-param-in-clause-ambig branch from 755953f to a4564e7 Compare March 20, 2026 15:31

lcnr changed the title ~~FindParamInClause global on overflow~~ FindParamInClause handle edge-cases Mar 20, 2026

rustbot assigned BoxyUwU Mar 20, 2026

lcnr marked this pull request as ready for review March 20, 2026 15:37

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 Mar 20, 2026

BoxyUwU reviewed Mar 23, 2026

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FindParamInClause non-global on overflow

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lcnr force-pushed the find-param-in-clause-ambig branch from a4564e7 to d59709b Compare April 2, 2026 10:33

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lcnr added 2 commits April 2, 2026 16:29

handle concrete alias norm to generic

b3b29ed

see the added test

add mgca fixme

69148fd

lcnr force-pushed the find-param-in-clause-ambig branch from d59709b to 69148fd Compare April 2, 2026 14:29

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 Apr 2, 2026

JonathanBrouwer mentioned this pull request Apr 2, 2026

Rollup of 21 pull requests #154718

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JonathanBrouwer mentioned this pull request Apr 2, 2026

Rollup of 20 pull requests #154727

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