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add note when there's a type mismatch due to param_env shadowing #149910
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example with worse error msg by @jdonszelmann
pub trait Supertrait { type Assoc; fn decoder(self) -> Self::Assoc; } pub trait Trait: Supertrait {} impl<T: Trait> Supertrait for T { type Assoc = (); fn decoder(self) -> Self::Assoc {} }
error[E0308]: mismatched types --> src/lib.rs:10:25 | 10 | fn decoder(self) -> Self::Assoc {} | ------- ^^^^^^^^^^^ expected associated type, found `()` | | | implicitly returns `()` as its body has no tail or `return` expression | = note: expected associated type `<T as Supertrait>::Assoc` found unit type `()` = help: consider constraining the associated type `<T as Supertrait>::Assoc` to `()` or calling a method that returns `<T as Supertrait>::Assoc` = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html- added and removed
on Dec 14, 2025 yes, when reporting type mismatch errors, check whether one side is an alias, if so, check whether the alias could have been normalized to another type via an impl, if so, check whether these types would be equal, in either case, add a note explaining that the alias is rigid due to a where-bound and could have otherwise been normalized
Reacted by xonxGreat , makes so much sense now . Thanks
- added a commit that references this issue
on Feb 15, 2026 - added a commit that references this issue
on Feb 16, 2026
results in the following error
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
ProofTreeVisitorin 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)