Warn on shadowing generics

[carllerche]

Given the following:

pub struct WrittenOutputStream<'a, 'b, W> {
  priv writer: &'a W,
  priv sizes: &'b SerializedSizeInfo
}

impl<'a, 'b, W: Writer> WrittenOutputStream<'a, 'b, W> {
  pub fn new<'a, 'b, W>(writer: &'a W, sizes: &'b SerializedSizeInfo) -> WrittenOutputStream<'a, 'b, W> {
    WrittenOutputStream { writer: writer, sizes: sizes }
  }
}

I would suggest that shadowing the generics in the new function should cause a warning. I'm still getting my mind wrapped around rust and the error message that I got in the previous case was pretty cryptic. Once I saw what was happening, it was obvious. I think a warning would have helped a lot.

[huonw]

Example of the errors:

struct Foo<T>(T);

impl<T> Foo<T> {
     fn new<T>(x: T) -> Foo<T> { Foo(x) }
}

fn main() {
     Foo::new(1i);
}

11658.rs:8:6: 8:14 error: cannot determine a type for this expression: unconstrained type
11658.rs:8      Foo::new(1i);
                ^~~~~~~~

It's the <T> from the impl that's unconstrained.

(This cannot be just a lint, since typechecking failures kill compilation well before lints run.)

[ben0x539]

A solution for this would probably also fix #9914.

[FreeFull]

DST seems to have made the error messages for this a lot less clearer than they have already been.

[steveklabnik]

We do in fact warn for this now. From @huonw 's example:

hello.rs:4:10: 4:46 error: type parameter `T` shadows another type parameter of the same name
hello.rs:4          fn new<T>(x: T) -> Foo<T> { Foo(x) }
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~