thread::spawn with function with no return result type causes SIGSEGV

[LunaBorowska]

Messages are never printed, the program just crashes.

This only happens in release mode.

What I did?

Ran following program in release mode.

use std::thread;

fn worker() -> ! {
    loop {
        println!("Printing messages");
    }
}

fn main() {
    thread::spawn(worker);
    loop {
        thread::park();
    }
}

What was expected?

Flood of "Printing messages" messages.

What happened instead.

SIGSEGV

Meta

This was confirmed to happen on stable (on IRC and in playpen), however I don't have stable installed.

rustc 1.16.0-nightly (df8debf6d 2017-01-25)
binary: rustc
commit-hash: df8debf6d9afc431adbbd8311dcaf2b70eb9762e
commit-date: 2017-01-25
host: x86_64-unknown-linux-gnu
release: 1.16.0-nightly
LLVM version: 3.9

[pmarcelll]

This is probably #28728.

[nagisa]

This is probably #28728.

It is not.

use std::thread;

fn worker() -> ! {
    panic!("woah");
}

fn main() {
    thread::spawn(worker).join().unwrap();
}

still sigsegvs. The issue here is the ! type itself.

[nagisa]

More minimal:

fn worker() -> ! {
    panic!()
}

fn main() {
    std::panic::catch_unwind(worker);
}

[alexcrichton]

This is due to this causing UB:

#![feature(never_type)]

fn main() {
	unsafe {
		let a = std::mem::uninitialized::<!>();
	}
}

I vaguely remember there being an issue for that but I can't recall where. The precise call to this function happens here.

[alexcrichton]

This is technically not a regression apparently because this was rejected in 1.11.0 and just started faulting in 1.12.0:

fn worker() -> ! {
    panic!()
}

fn main() {
    std::panic::catch_unwind(worker);
}

Could you fix this by changing Data<F, R> to be a union now that they exist?