Code change:
<ins> // Copyright 2016 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. use std::fmt::Debug; const CONST_0: Debug+Sync = *(&0 as &(Debug+Sync)); //~^ ERROR `std::fmt::Debug + Sync + 'static: std::marker::Sized` is not satisfied //~| NOTE does not have a constant size known at compile-time //~| NOTE constant expressions must have a statically known size const CONST_FOO: str = *"foo"; //~^ ERROR `str: std::marker::Sized` is not satisfied //~| NOTE does not have a constant size known at compile-time //~| NOTE constant expressions must have a statically known size static STATIC_1: Debug+Sync = *(&1 as &(Debug+Sync)); //~^ ERROR `std::fmt::Debug + Sync + 'static: std::marker::Sized` is not satisfied //~| NOTE does not have a constant size known at compile-time //~| NOTE constant expressions must have a statically known size static STATIC_BAR: str = *"bar"; //~^ ERROR `str: std::marker::Sized` is not satisfied //~| NOTE does not have a constant size known at compile-time //~| NOTE constant expressions must have a statically known size fn main() { println!("{:?} {:?} {:?} {:?}", &CONST_0, &CONST_FOO, &STATIC_1, &STATIC_BAR); } </ins>

Related discussion:
text: This looks unintentional, and has been broken in all cases, until a few releases ago, AFAICT. Should this be covered by WF rules? cc Unsized constants cause an ICE, in both old trans and MIR trans: Unsized statics appear to work, but only in old trans: I believe this is possible because a constant DST lvalue in old trans is merely the fat pointer. It seems useful to allow an unsized , but IMO it should require a sized RHS instead, i.e.: And, of course, this should all go through the RFC process, to figure out all the finer details.
Should it be ? I don't think A needs to be an lvalue.
It makes more sense for constants, but I gave the example with because that's the only one that works today (although by accident).
Hmm? I mean the static is desugared into a const. works today.
The idea being that a static and a const reference both give access to one lvalue.
No, has different semantics: it allows an somewhere within and is guaranteed not to be duplicated.
Hmm? Both of these gives me a "error: cannot borrow a constant which contains interior mutability, create a static instead". This is what I'd suspect as the lvalue from the promoted has nothing to do with being a or a .
Let me rewrite my desugaring to account for that, oops.
Oh and my confusion really just stemmed from forgetting about the constants referring to statics rule. [Kind of a funny rule when works, but this is an orthogonal issue.]
I think in the meantime unsized statics should probably error, perhaps dependent on a crater run, though I doubt there's many people relying on said bug. It's much easier to relax restrictions than tighten them.
Will prepare a patch to turn this into an error, hopefully nobody has been abusing it.