Nightly/beta regression? Sized + UnwindSafe

[dpc]

use std::panic::UnwindSafe;

fn is_unwind_safe<T: UnwindSafe>() {}

trait Foo: UnwindSafe {}

fn main() {
    is_unwind_safe::<Box<[u8]>>();
    is_unwind_safe::<Box<Foo>>();
}

https://is.gd/xWwNn6

Compiles on stable, but fails on beta and nightly.

[alexcrichton]

I don't recall any changes on the libs side offhand, but @rust-lang/compiler does this look familiar?

[TimNN]

Introduced between nightly-2017-01-26 and nightly-2017-02-02 (Changes) (Sadly, no nightlies in between.

[eddyb]

This is actually @petrochenkov's #39230 exposing a library bug (missing ?Sized):

impl<T: UnwindSafe> UnwindSafe for Unique<T> {}

EDIT: I checked, and also talked to @nikomatsakis: it seems this trait is the only one affected, and the OIBIT/"auto trait" feature is still gated, so this is fine to break, it's just this regression that wasn't caught.

[alexcrichton]

Thanks for the investigation @eddyb!

[alexcrichton]

Sent a fix as #40020