hang: issue-55608-captures-empty-region.rs with -Zmir-opt-level=2

[matthiaskrgr]

Code

./src/test/ui/impl-trait/issues/issue-55608-captures-empty-region.rs

// This used to ICE because it creates an `impl Trait` that captures a
// hidden empty region.

// check-pass

fn server() -> impl FilterBase2 {
    segment2(|| { loop { } }).map2(|| "")
}

trait FilterBase2 {
    fn map2<F>(self, _fn: F) -> Map2<F> where Self: Sized { loop { } }
}

struct Map2<F> { _func: F }

impl<F> FilterBase2 for Map2<F> { }

fn segment2<F>(_fn: F) -> Map2<F> where F: Fn() -> Result<(), ()> {
    loop { }
}

fn main() { server(); }

Meta

repo @ 12799ad

Error output

Rustc simply hangs when running
build/x86_64-unknown-linux-gnu/stage1/bin/rustc ./src/test/ui/impl-trait/issues/issue-55608-captures-empty-region.rs -Zmir-opt-level=2

Edit: perf top while rustc is crunching on the hang:

  45,75%  librustc_driver-3c13214d88d90089.so  [.] rustc_mir::transform::simplify::CfgSimplifier::collapse_goto_chain
  28,35%  librustc_driver-3c13214d88d90089.so  [.] rustc_mir::transform::simplify::CfgSimplifier::simplify
   7,00%  [kernel]                             [k] mwait_idle_with_hints.constprop.0
   2,16%  librustc_driver-3c13214d88d90089.so  [.] rustc_middle::mir::terminator::TerminatorKind::successors_mut
   1,00%  librustc_driver-3c13214d88d90089.so  [.] rustc_middle::mir::terminator::Terminator::successors_mut

[matthiaskrgr]

cc @oli-obk ?

edit: I assume this is due to aba0251

[oli-obk]

O_o yea, I'll have a look

[jonas-schievink]

Wild speculation: #75074 might have the same cause

(EDIT: Backtrace says yes)