Never type

^Syntax
NeverType : !

The never type ! is a type with no values, representing the result of computations that never complete. Expressions of type ! can be coerced into any other type.

The ! type can only appear in function return types presently, indicating it is a diverging function that never returns.

#![allow(unused)]
fn main() {
fn foo() -> ! {
    panic!("This call never returns.");
}
}

#![allow(unused)]
fn main() {
extern "C" {
    pub fn no_return_extern_func() -> !;
}
}