Not sure if this is really a bug, but I tried searching and couldn't find any hits on the warning message. Feel free to close the issue if this is intended/expected behaviour.

Input:

fn main() { [(); loop {}] } 

Output:

$ rustc - error[E0019]: constant contains unimplemented expression type --> <anon>:2:10 | 2 | [(); loop {}] | ^^^^^^^ warning: Constant evaluating a complex constant, this might take some time --> <anon>:2:10 | 2 | [(); loop {}] | ^^^^^^^ 

At this point the rustc process hangs seemingly forever while consuming 100% CPU.

I'm on commit c166b03.