Calling ".join().unwrap()" on a JoinHandle does not work

[RalfJung]

I would expect the following code to compile:

use std::thread;
pub fn spawn_and_wait_nothing() {
    std::thread::spawn(||{ let i = 3*4; }).join().unwrap();
}

However, compilation fails, complaining about unwrap not being found. As .ok().unwrap() works, the problem is probably that no implementation for Debug could be found. (#19950 tracks the unhelpful error message.)

[alexcrichton]

Dupe of #21291, but thanks!