unboxed closure ICE with debug format qualifier

[jeremyletang]

Here is the code:

#![feature(unboxed_closures)]
#![feature(unboxed_closure_sugar)]

extern crate debug;

fn main() {
    let f = move |:| {};
    println!("{:?}", f)
}

and the produced error:

error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'couldn't find visit method for err', /build/rust-git/src/rust/src/libcore/option.rs:315

[bkoropoff]

The type visitor glue generation code does not support unboxed closures:

          ty::ty_unboxed_closure(..) => self.leaf("err"),

A quick fix would mimic the visitor glue for boxed closures. Ideally, the TyVisitor interface could be extended to handle unboxed closures separately, which would allow for visting upvars in the closure environment.

[bkoropoff]

Working on the quick fix

[aturon]

Closing as a dupe of #16808