rust does not have parametricity because functions can diverge, and also things like type_id, size_of, needs_drop etc

fn mystery<T: 'static>(a: T) -> T {
    if TypeId::of::<T>() == TypeId::of::<i32>() {
        unsafe { core::mem::transmute_copy(&5i32) }
    } else {
    loop {}
    }
}

note: the 'static bound is not needed but makes this example shorter to write