Require trait definition to be in scope to be able to call trait methods on type.

[TomAFrench]

In the below program we have a trait Foo which is private to the module private_mod, however we can call the foo() method on Bar in main despite the root module not being supposed to know about Foo at all.

fn main() {
    assert_eq(Bar::foo(), 42);
}

pub struct Bar {}

mod private_mod {
    trait Foo {
        fn foo() -> Field;
    }

    impl Foo for super::Bar {
        fn foo() -> Field {
            42
        }
    }
}

This means that a trait used for internal implementation details (such as in #6845) leaks into user code. The way that Rust addresses this issue is by requiring the trait definition to be in scope in order to call trait methods.