Deref coercion not working for `Box<Trait>`

[aturon]

The following fails to compile:

trait Trait {}

fn takes_ref(t: &Trait) {}
fn takes_box(t: Box<Trait>) {
    takes_ref(&t)
}
fn main() { }

with

<anon>:5:15: 5:17 error: the trait `Trait` is not implemented for the type `Box<Trait>` [E0277]
<anon>:5     takes_ref(&t)
                       ^~

despite the Deref implementation here.

I believe this should work based on the RFC.

[aturon]

cc @nick29581 @wycats

[emberian]

Bump!

[steveklabnik]

Triage: today, this complains about a lack of 'static, and I'm not 100% sure how to fix it; I thought adding + 'static would, but it doesn't seem to help.

[Michael-F-Bryan]

I also encountered this bug while trying to iterate over boxed trait objects and posted to the user forum.

[CAD97]

This came up again today.

Based on my reading, the unsize cooersion is only applying to the first level when it should pass through Deref cooersion as well.

[Spoonbender]

Triage: no change

[nielsle]

This version seems to compile

trait Trait {}
fn takes_ref(t: &dyn Trait) {}
fn takes_box(t: Box<dyn Trait>) {
    takes_ref(&*t)
}

[Michael-F-Bryan]

@nielsle the problem is that takes_ref(&t) won't compile even though t dereferences to dyn Trait, which is what you would expect according to the deref coersion rules.