Impls discriminating on associated types do not cause rustc consider them separate

[nagisa]

Consider code like this:

impl<C> fmt::Display for Stuff<C>
where C: SomeTrait<Type=()>
{
    fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        panic!()
    }
}

impl<C> fmt::Display for Stuff<C>
where C: SomeTrait<Type=Vec<C>>
{
    fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        panic!()
    }
}

Play here

It is obvious, that C cannot in any way implement the same trait so the associated Type is both () and Vec<C> at the same time. The compiler should be able to infer that these trait implementations are completely independent. Alas, in the current nightly you get something like

error[E0119]: conflicting implementations of trait `std::fmt::Display` for type `Stuff<_>`:
  --> <anon>:24:1
   |
15 | impl<C> ::std::fmt::Display for Stuff<C>
   | - first implementation here
...
24 | impl<C> ::std::fmt::Display for Stuff<C>
   | ^ conflicting implementation for `Stuff<_>`

instead.

cc @nikomatsakis @eddyb

[nagisa]

Seems to be rust-lang/rfcs#1672