Sandbox::https://github.com/rust-lang/rust/pull/165207

estebank · 2026-10-30T20:39:34Z

The lines showing traits A and B are new:

error[E0221]: ambiguous associated function `f` in bounds of `T`
  --> $DIR/path-ambiguous-late-bound-vars.rs:27:5
   |
LL | trait A {
LL |     fn f() -> impl Sized;
   |     --------------------- ambiguous `f` from `A`
...
LL | trait B {
LL |     fn f<'b>() -> impl Sized;
   |     ------------------------- ambiguous `f` from `B`
...
LL |     T::f(..):,
   |     ^^^^^^^^ ambiguous associated function `f`
   |
help: use fully-qualified syntax to disambiguate
   |
LL -     T::f(..):,
LL +     <T as B>::f(..):,
   |
help: use fully-qualified syntax to disambiguate
   |
LL -     T::f(..):,
LL +     <T as A>::f(..):,
   |

Show the place of the assoc items even when non-local.

On ambiguous assoc type, suggest alternative with a structured suggestion:

help: consider being explicit about the bounds
   |
LL - fn foo<T: Trait<Assoc = u32>>() {}
LL + fn foo<T: Trait<> + Super<i32, Assoc = u32> + Super<u32, Assoc = u32>>() {}
   |

When encountering ambiguous associated type in bounds, provide structured suggestion for where bounds with correct syntax:

help: consider introducing a new type parameter `U` and adding `where` constraints
   |
LL ~ fn foo<T>(_: U, _: T) where
LL +         U: Trait,
LL +         U: Super<i32, Assoc = u32>,
LL +         U: Super<u32, Assoc = u32>
   |

Do not suggest introducing a where bound on local binding.

Fix #97583.

rustbot · 2026-10-30T20:39:37Z

HIR ty lowering was modified

cc @fmease

rustbot · 2026-10-30T20:39:40Z

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estebank · 2026-11-01T15:47:50Z

-                where_bounds.join(",\n"),
-            ));
+        if !suggestions.is_empty() {
+            suggestions.push((span, String::new()));


This doesn't account for impl Trait<A=(), B=()>, which would leave impl Trait<, B=()> in the suggested code.

View changes since the review

fmease · 2026-11-01T15:52:08Z

You seem to be in the process of fixing #97583. Note that I've never completed the work because there are so many edge cases to consider.

My WIP branch is / was: main...fmease:rust:fix-diag-sugg-on-ambig-assoc-item-constraint.

See the (messy) test file in that patch for some edge cases (but feel free to avert your eyes, it wasn't meant for others to see). IIRC I gave up several times trying to compute the correct higher-ranked binders (for<...>) for the structured suggestion and maybe due to some other things, too.

estebank · 2026-11-01T16:13:51Z

@fmease I'll try to incorporate as much as possible both on this PR and follow ups. When it comes to the trickiest structured suggestions, I think a plain message as a fallback is reasonable instead of burn out from san loss.

estebank · 2026-11-01T17:50:49Z

To reviewer: Review per commit, some of the changes are independent of each other, but were noticed while seeing the diagnostics change, and separate PRs will conflict. I can split things out as needed.

estebank · 2026-11-02T00:40:08Z

@fmease would you mind taking a look? I haven't fully handled the suggestions in the face of malformed input (could just hide the suggestion in that case, or even the whole error). I'm pretty happy with the results, but you'll have a fresh perspective given you've been looking at the problem already.

estebank · 2026-11-02T00:41:19Z

We could point at type Bar here. ~~Also, E0191 also needs context Span.~~ Edit: I'd done that already ^_^'

View changes since the review

The lines showing traits A and B are new: ``` error[E0221]: ambiguous associated function `f` in bounds of `T` --> $DIR/path-ambiguous-late-bound-vars.rs:27:5 | LL | trait A { LL | fn f() -> impl Sized; | --------------------- ambiguous `f` from `A` ... LL | trait B { LL | fn f<'b>() -> impl Sized; | ------------------------- ambiguous `f` from `B` ... LL | T::f(..):, | ^^^^^^^^ ambiguous associated function `f` | help: use fully-qualified syntax to disambiguate | LL - T::f(..):, LL + <T as B>::f(..):, | help: use fully-qualified syntax to disambiguate | LL - T::f(..):, LL + <T as A>::f(..):, | ```

Provide structured suggestion: ``` help: consider being explicit about the bounds | LL - fn foo<T: Trait<Assoc = u32>>() {} LL + fn foo<T: Trait<> + Super<i32, Assoc = u32> + Super<u32, Assoc = u32>>() {} | ``` When suggesting to add a `where` clause, use proper syntax: ``` = help: consider introducing a new type parameter `T` and adding `where` constraints: where T: BoxCar, T: Vehicle<Color = COLOR>, T: Box<Color = COLOR> ```

When encountering ambiguous associated type in bounds, provide structured suggestion: ``` help: consider introducing a new type parameter `U` and adding `where` constraints | LL ~ fn foo<T>(_: U, _: T) where LL + U: Trait, LL + U: Super<i32, Assoc = u32>, LL + U: Super<u32, Assoc = u32> | ```

Make it so the suggestion message still has all information even without the suggested code being visible.

Do not leave stray commas behind even if there are type parameters after assoc type. ``` help: consider being explicit about the bounds | LL - fn f(_: impl C<T = (), bool>) {} LL + fn f(_: impl C<bool> + with_args_after::B<(bool,), T = ()> + with_args_after::module::A<((bool,),), T = ()>) {} | ```

rustbot · 2026-11-02T16:04:13Z

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.

``` error[E0222]: ambiguous associated type `T` in bounds of `gat::C` --> $DIR/dyn-trait-in-param-or-body.rs:85:20 | LL | trait A { type T<Q, K>; } | ------------ ambiguous `T` from `gat::A` LL | trait B: A { type T<Q>; } | --------- ambiguous `T` from `gat::B` ... LL | fn f(_: impl C<T<i32> = ()>) {} | ^^^^^^^^^^^ ambiguous associated type `T` | help: consider being explicit about the bounds | LL - fn f(_: impl C<T<i32> = ()>) {} LL + fn f(_: impl C + gat::B<T<i32> = ()> + gat::A<T<i32> = ()>) {} | ```

rustbot · 2026-11-02T17:52:01Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

estebank · 2026-11-03T17:42:48Z

+                            let args = rustc_hir_pretty::generic_args_to_string(
+                                &(&tcx as &dyn HirTyCtxt<'_>) as &dyn PpAnn,
+                                constraint.gen_args,
+                            );


If we want to avoid the new crate dep, we can instead use the textual snippet, which I would still produce applicable suggestions.

View changes since the review

Doing this gets us some nice benefits for free: bounds and equality terms are both handled the same way, we don't need to evaluate the term, we don't need the new dep to handle GATs, and we get around the incorrect rendering of consts using the gca! macro.

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 Oct 30, 2026

rustbot assigned jieyouxu Oct 30, 2026

estebank changed the title ~~[tiny] Provide context when pointing at ambiguous assoc item~~ Tweak diagnostic when encountering ambiguous associated function in trait type Nov 1, 2026

estebank commented Nov 1, 2026

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Comment thread tests/ui/const-generics/associated-const-bindings/ambiguity.stderr Resolved

estebank commented Nov 1, 2026

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Comment thread tests/ui/associated-type-bounds/dyn-trait-in-param-or-body.stderr Resolved

fmease assigned fmease and unassigned jieyouxu Nov 1, 2026

estebank force-pushed the ambiguous-context branch from 8e84fdc to db8d19c Compare November 1, 2026 17:49

fmease changed the title ~~Tweak diagnostic when encountering ambiguous associated function in trait type~~ Tweak diagnostic when encountering ambiguous associated items at the type level Nov 2, 2026

estebank commented Nov 2, 2026

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Comment thread tests/ui/privacy/sysroot-private.default.stderr Resolved

estebank commented Nov 2, 2026

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estebank force-pushed the ambiguous-context branch from e389f6b to a8f12d9 Compare November 2, 2026 02:53

estebank commented Nov 2, 2026

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Comment thread tests/ui/associated-type-bounds/dyn-trait-in-param-or-body.stderr Resolved

estebank commented Nov 2, 2026

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Comment thread tests/ui/associated-type-bounds/dyn-trait-in-param-or-body.stderr Resolved

estebank added 6 commits November 2, 2026 15:54

Add tests

fcea00b

Do not suggest introducing a where bound on local binding

a614075

Silence E0191 if E0222 was emitted earlier

104a7c1

estebank added 7 commits November 2, 2026 15:54

Make suggestion verbose

1947fa8

Tweak suggestion wording

421f674

Make it so the suggestion message still has all information even without the suggested code being visible.

Do not leave stray commas after applying suggestion

e6ca960

Fix suggestion missing type parameter

61c4b30

Fix ICE

7ebf8fd

Do not suggest specifying assoc type if it is already present

7cfbafd

estebank force-pushed the ambiguous-context branch from 985174a to a79427d Compare November 2, 2026 16:04

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estebank added 2 commits November 2, 2026 17:51

Remove outdated comments

1027495

estebank force-pushed the ambiguous-context branch from 4189602 to 1027495 Compare November 2, 2026 17:51

estebank commented Nov 3, 2026

View reviewed changes

Use snippet for suggestion

4b9d9ba

estebank force-pushed the ambiguous-context branch from 27d5bb4 to 4b9d9ba Compare November 3, 2026 22:37

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