Manual impls of Foo for Foo should be illegal

[huonw]

A manual implementation of a trait for its own DST is currently legal:

trait Foo { fn gen<A>(&self) {} }

impl<'a> Foo for Foo+'a {}

fn main() {}

but shouldn't be. An object-safe Foo gives an ICE #20939, but should also be plain illegal.

[nikomatsakis]

I actually think that a manual impl for a _non-_object-safe trait is ok. It's the object-safe case that should give a better failure.

[nikomatsakis]

(The fix is presumably to modify coherence to be less focused on impls.)

[huonw]

Closing in favour of #20939 then.