We should forbid type parameters from shadowing other types

[erickt]

This is a weird case that we should probably not allow:

mod T {
    pub type Type = i32;
}

trait Trait {
    type Type;
}

impl Trait for i32 {
    type Type = i8;
}

struct Baz<T: Trait> {
    a: T::Type,
}

fn main() {
    let x: Baz<i32> = Baz { a: 0i8 };
}

The rule appears to be that the <T: Trait> has precedence over the module, it would be easiest if we just errored out in this circumstance.

[kmcallister]

Nominating for reasons of backwards-incompatibility.

[kmcallister]

(Though adding a lint would be back-compat, aiui)

[nikomatsakis]

I for one am not convinced we should disallow this. We disallow type parameters from shadowing one another specifically because it came up very frequently. This seems less likely to arise frequently (it relies on not following our suggested naming conventions for type parameters and modules, for one thing).

[nikomatsakis]

(Even if it did come up frequently, I'd probably rather just work on the error messages that result.)

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#707