r[type.never]

Never type

r[type.never.syntax]

NeverType -> `!`

r[type.never.intro] The never type ! is a type with no values, representing the result of computations that never complete.

r[type.never.coercion] Expressions of type ! can be coerced into any other type.

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

fn foo() -> ! {
    panic!("This call never returns.");
}

unsafe extern "C" {
    pub safe fn no_return_extern_func() -> !;
}