r[type.never]

Never type

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

r[type.never.syntax]

NeverType -> `!`

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

fn foo() -> ! {
    loop {}
}

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

let _: ! = loop {};

fn always_ok() -> Result<u32, !> {
    Ok(42)
}

[!NOTE] The standard library type Infallible is a type alias for !.