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Suggest bounds in more cases, accounting for type parameters referenced in predicate #144801

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@estebank estebank commented Aug 1, 2025

Use a ty::Visitor to see if the failed predicate references a type parameter. If it does, then we only suggest adding a bound to an (associated) item only if the referenced parameter is present in its generics.

Provide adding bound suggestion in trait and impl associated functions in cases we previously weren't:

error[E0277]: `?` couldn't convert the error to `ApplicationError`
  --> $DIR/suggest-complex-bound-on-method.rs:18:16
   |
LL |         t.run()?;
   |           -----^ the trait `From<<T as Trait>::Error>` is not implemented for `ApplicationError`
   |           |
   |           this can't be annotated with `?` because it has type `Result<_, <T as Trait>::Error>`
   |
note: `ApplicationError` needs to implement `From<<T as Trait>::Error>`
  --> $DIR/suggest-complex-bound-on-method.rs:12:1
   |
LL | enum ApplicationError {
   | ^^^^^^^^^^^^^^^^^^^^^
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
LL |     fn thing<T: Trait>(&self, t: T) -> Result<(), ApplicationError> where ApplicationError: From<<T as Trait>::Error> {
   |                                                                     +++++++++++++++++++++++++++++++++++++++++++++++++

Fix #144734.

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rustbot commented Aug 1, 2025

r? @spastorino

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@rustbot 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 Aug 1, 2025
@Kivooeo
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Kivooeo commented Aug 1, 2025

Mind to put a link for an issue into the test? So it will be easier to follow it later (wheter for someone or while possible future test reorganization process)

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where clause with generic is suggested at the impl block before the generic is introduced
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