DST: trait objects should automatically implement the trait

[aturon]

This is a planned part of DST. Example from RFC:

fn foo<Sized? T: SomeTrait>(y: &T) { ... }

fn bar(x: &SomeTrait) {
    foo(x)
}

should be permitted even if there is no explicit impl of SomeTrait for &SomeTrait.

[aturon]

cc @nick29581

[nrc]

cc @nikomatsakis (last I heard he was starting to implement this)

[eddyb]

This seems perilous, especially around corner cases like Any.
Has there been an explanation for how this feature can be implemented so it isn't prone to creating unsoundness?

[aturon]

@eddyb Can you elaborate? This was part of the motivation for the "object safety" RFC I mentioned, which should also guarantee that this can only be done in appropriate cases.

[nikomatsakis]

I'm working on this, yes.

[nikomatsakis]

This was implemented in #20341