add note when there's a type mismatch due to param_env shadowing #149910
Description
trait Trait {
type Assoc;
}
impl<T> Trait for T {
type Assoc = T;
}
fn foo<T: Trait>(x: T) -> T::Assoc {
x
}results in the following error
error[E0308]: mismatched types
--> src/lib.rs:10:5
|
9 | fn foo<T: Trait>(x: T) -> T::Assoc {
| - -------- expected `<T as Trait>::Assoc` because of return type
| |
| found this type parameter
10 | x
| ^ expected associated type, found type parameter `T`
|
= note: expected associated type `<T as Trait>::Assoc`
found type parameter `T`
help: consider further restricting this bound
|
9 | fn foo<T: Trait<Assoc = T>>(x: T) -> T::Assoc {
| +++++++++++
The reason this fails is that we do not normalize via impls if there is a where-bound in scope https://rustc-dev-guide.rust-lang.org/solve/candidate-preference.html#where-bounds-shadow-impls
If we get a type equality error and one side is an alias, we should check whether the alias would successfully normalize without param_env shadowing (using a ProofTreeVisitor in the new solver and maybe looking at all candidates and checking for impl ones in the old), and then add a note explaining why this error happens and suggesting ways to avoid it (mostly by dropping trivial where-clauses)