False Positive? Mutable key type lint is emitted on HashSets containing associated types

[TimoFreiberg]

$ cargo clippy -V                                                                     
clippy 0.0.212 (e8642c7 2020-01-06)                                                   

Minimal example:

trait Trait {
    type AssociatedType;
    
    fn trait_fn(&self, set: std::collections::HashSet<Self::AssociatedType>);
}

A mutable key type lint is emitted on std::collections::HashSet<Self::AssociatedType> in the trait definition, but not in impls of that trait (interestingly not even in trait impls where Trait::AssociatedType is AtomicUsize).
While Trait::AssociatedType could be an internally mutable type, I think the correct place to lint this would be in impls that actually set Trait::AssociatedType to an internally mutable type.

Reproducible on the playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=094f6136a26c9911ee8dae0dcf94f150