Rust 1.20.0-nightly ICE with never_type: Broken MIR

[ngc0202]

The following error results from dereferencing a *const ! as in this code: https://is.gd/SsSDiG

rustc 1.20.0-nightly (734c83642 2017-07-03)
error: internal compiler error: broken MIR (_3 = ()): bad assignment (! = ()): Sorts(ExpectedFound { expected: !, found: () })
 --> <anon>:4:5
  |
4 | /     unsafe {
5 | |         *x;
6 | |     }
  | |_____^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.20.0-nightly (734c83642 2017-07-03) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:426:8

[SimonSapin]

What’s the correct fix? Compiler error with a nice error message?

[arielb1]

@SimonSapin

I think this should compile and UB, but a lint against this is probably a good idea.

[varkor]

Reduced test case:

// error: internal compiler error: broken MIR in NodeId(4) (_2 = ()):
//     bad assignment (! = ()): Sorts(ExpectedFound { expected: !, found: () })
#![crate_type = "lib"]
#![feature(never_type)]

fn blah(x: !) {
    x;
    // () // Uncommenting this explicit unit return avoids the ICE
}

[varkor]

I'm working on a fix for this. Progress on this branch. The idea is to avoid the typing issues entirely by terminating the blocks with unreachable as soon as a !-value is constructed. Currently the examples given here are fixed, but some more convoluted examples still crash.