Rust's Fn traits feel weird. The argument tuple is a generic parameter, but the return type is an associated type, even though Rust is supposed to use Hindley-Milner type inference, so inferring through return type should always fail if this were a regular trait.

Then, this would mean that blanket implementations for Fn(T) and Fn(T, U) should conflict because AnyTrait<(T)> and AnyTrait<(T, U)> aren't mutually exclusive. I tried, they work just fine.

There's some weird and I suspect unnecessary special case magic here, and I'd like to uncover it.