warning: thread support is experimental: the scheduler is not preemptive, and can get stuck in spin loops.
         (see https://github.com/rust-lang/miri/issues/1388)

thread '<unnamed>' panicked at 'Hello!', $DIR/simple.rs:LL:CC
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'childthread' panicked at 'Hello, world!', $DIR/simple.rs:LL:CC