[E0284] suggestion is useless

[jendrikw]

Code

trait S {
    type Assoc<U>: T;
    fn f<B, F>(self, f: Self::Assoc<F>) -> Self::Assoc<B>;
}

trait T: S { }

fn g<A>(a: A) where A: T {
    a.f::<A::Assoc<A>, _>(todo!());
}

Current output

error[E0282]: type annotations needed
 --> src/lib.rs:9:7
  |
9 |     a.f::<A::Assoc<A>, _>(todo!());
  |       ^ cannot infer type of the type parameter `F` declared on the method `f`
  |
help: consider specifying the generic arguments
  |
9 -     a.f::<A::Assoc<A>, _>(todo!());
9 +     a.f::<<A as S>::Assoc<A>, _>(todo!());
  |

Desired output

Rationale and extra context

The suggestion is just rewriting it and won't change anything.

Other cases

Rust Version

rustc 1.94.0 (4a4ef493e 2026-03-02)
binary: rustc
commit-hash: 4a4ef493e3a1488c6e321570238084b38948f6db
commit-date: 2026-03-02
host: x86_64-unknown-linux-gnu
release: 1.94.0
LLVM version: 21.1.8

Anything else?

No response

[eggyal]

It would be better if:

1. The indicated span was the _ wildcard in place of the uninferred F parameter;

2. The superfluous suggestion to change the specified B parameter was not part of this error.

@rustbot label +C-enhancement +D-confusing +D-imprecise-spans

[arferreira]

@rustbot claim