Rust: Associated types

[paldepind]

This PR improves calls to methods in trait blocks (not methods in impl blocks) that refer to associated types.

One approach to implementing this would be: When a method call resolves to a method inside a trait we find the impl block that makes the self argument implement the trait. This impl block contains the definition of the associated type, and when we read off the declared type of the method we convert the associated type to the specific definition of the type.

However, this change would have to be done inside getDeclaredType which right now doesn't depend on the call site at all. Hence, instead of doing that, this PR handles associated types by treating them as type parameters. I.e. a trait like this:

trait MyTrait {
  type AssociatedType;
  // ...
}

is treated as-if it was

trait MyTrait<AssociatedType> {
  // ...
}

One downside of this approach is that it's not immediately clear to me how to handle associated types with generics like this:

trait MyTraitAssoc2 {
    type GenericAssociatedType<AssociatedParam>;

    fn put<A>(&self, a: A) -> Self::GenericAssociatedType<A>;
}

Here Self::GenericAssociatedType refers to a type parameter of put. So what should we do for the A passed to it?The GenericAssociatedType type parameter would have to be handled as a sort of higher-kinded type.

My impression is that associated types with generics is a feature that is not used very much. So I suggest that it's ok to not handle that feature, unless there's some straightforward way to do it.

[paldepind]

This missing type is probably worth fixing. But since it's not for a method in a trait block and not a regression I didn't look into it for this PR.