Can create trait objects with self-containing bounds

[arielb1]

Meta

rustc 1.15.0-nightly (71c06a56a 2016-12-18)

STR

trait Q<T:?Sized> {}
trait Foo where u32: Q<Self> {
    fn foo(&self);
}

impl Q<()> for u32 {}
impl Foo for () {
    fn foo(&self) {
        println!("foo!");
    }
}

fn main() {
    let _f: Box<Foo> = Box::new(());
}

Expected Result

Box<Foo> is not well-formed, because u32: Q<Foo> does not hold.

Actual Result

Code compiles.

This does not seem to cause any trouble, but is a potential future-compatibility hazard if we start elaborating these bounds.

[nikomatsakis]

Discussed in compiler team meeting. This is also an @rust-lang/lang thing, so cc @rust-lang/lang. It does seem like it'd be better to make any trait that references Self in bounds as being not object-safe, as much as a precautionary measure as anything else, but we decided to first run a crater run and measure impact in any case. @arielb1 has a branch that can be adapted.

[aturon]

I agree with taking this conservative step.