a.rs

mod private {
    pub trait Super {
        fn foo(self);
    }
    pub trait Child: Super {}

    impl Super for u32 {
        fn foo(self) {}
    }
    impl Child for u32 {}
}

use private::Child;

fn huh<T: Child>(x: T) {
    x.foo(); // OK
}

fn hah(x: u32) {
    huh(x); // OK
    x.foo(); // error: no method named `foo` found
             // for type `u32` in the current scope
}