[NLL] ICE: unexpected region for local data with reference to closure

[matthewjasper]

The following code gives an internal compiler error. It is a normal compiler error without NLL.

#![feature(nll)]

fn foo(x: &i32) -> &i32 {
    let z = 4;
    let f = &|y| { y };
    let k = f(&z);
    f(x)
}

fn main() {}

error: internal compiler error: unexpected region for local data ReFree(DefId(0/0:3 ~ example[8787]::foo[0]), BrAnon(0))
 --> <source>:9:17
  |
6 |     let k = f(&z);
  |                ^
error: aborting due to previous error
Compiler returned: 101

[nikomatsakis]

This is a side-effect, I believe, of the hacky-ish way that we are enabling NLL while still running the old borrow check. In particular, this ICE originates from the old borrow check, I believe caused because we suppressed an error from the old region check (in favor of the new NLL check). We ought to be skipping the old borrow checker as well when NLL is enabled; but we can't do that until the unused_mut work that @KiChjang is working on is complete, I believe, since the borrow checker is still used to track which mut declarations are used. Maybe I can make it swallow the ICE though.

[KiChjang]

Work is tracked on this PR: #48605.

[nikomatsakis]

I have a simple fix for the ICE in the meantime.