It would be nice if JoinHandle<T> were must-use for T != ()

[scottmcm]

One IRC today, someone had code not working because they were doing

thread::spawn(|| mylib::my_function);

Which of course returns the function rather than running it.

Arguably it's almost always mistake to not keep the JoinHandle if the thread is going to return a non-unit value, since if you didn't care the closure could just be -> (), so this should lint, but AFAIK #[must_use] on the function or type would warn for JoinHandle<()> too today.

Repro link: https://play.rust-lang.org/?gist=fd8973579bf0bf474a7e2b9e135ff946&version=nightly

[varkor]

This looks like #48820, which was closed.

[jscinoz]

I think this is subtly different - #48820 was for JoinHandle to be marked with #[must_use] in general, whereas this issue is for the more narrow scenario where the spawned thread returns value (implying that there's something still to be done with this value?.