Associated type binding completions show up when they are not allowed

[jonas-schievink]

struct S;
trait Tr<T> {
    type Ty;
}

impl Tr<$0

This suggests using Ty = to specify the associated type, but that's not allowed when implementing a trait, only when using dyn Trait or impl Trait as a type, and when using a trait in a where-clause (perhaps whether it's allowed should be stored in the context).

[jonas-schievink]

Also they probably shouldn't show up when there are missing generic arguments