Tracking issue for tweaks to object safety (RFC 2027)

[withoutboats]

This is the tracking issue for RFC 2027, tweaks to object safety (rust-lang/rfcs#2027)

TODO

• Implement
• Document
• Stabilize

[withoutboats]

If anyone can mentor I'd be excited to try to implement this.

From my naive perspective, it seems like what we need to do is stop checking object safety at certain points (possibly determining if a type is well formed?) and start checking it at other points (checking if the object type implements its corresponding trait). More insight would be great, cc'ing @eddyb

[eddyb]

I'm afraid @nikomatsakis understands the details here much better than me.

[withoutboats]

I experimented with implementing this, but I definitely don't know what I'm doing. I noticed some interesting cases.

trait Foo: Sized { }

fn foo(_: &Foo) { panic!() }

fn main {
    let x: Box<Foo>;
}

Today these types are just invalid types, but I believe this RFC would make them valid. You would just never be able to initialize x or call foo. I didn't consider these in the RFC, but it seems fine to me.