Cargo Build hangs forever with recent nighlies

[jamesmunns]

With the following steps, I see the build hang indefinitely. This caused a 6 hour CI timeout for the nrf-rs organization. This does not happen with the stable/beta CI jobs, and did not happen previously with nightly. The following is a minimal repro of the CI steps of nrf-rs/nrf-hal#189.

git clone https://github.com/kalkyl/nrf-hal.git
cd nrf-hal
git checkout comp
rustup override set nightly-2020-08-02-x86_64-unknown-linux-gnu
rustup target add thumbv7em-none-eabihf
cargo build --manifest-path examples/rtic-demo/Cargo.toml

[jonas-schievink]

#0  0x00007ffff53b4ea6 in rustc_mir::transform::simplify::CfgSimplifier::simplify ()
   from /home/jonas/.rustup/toolchains/nightly-2020-08-02-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-edb22b7dff97b8bd.so
#1  0x00007ffff53b4728 in rustc_mir::transform::simplify::simplify_cfg ()
   from /home/jonas/.rustup/toolchains/nightly-2020-08-02-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-edb22b7dff97b8bd.so

yep, same issue as #75051

[jonas-schievink]

Reduced to just:

fn main() {
    loop {
        continue;
    }
}