need to check privacy for constants with `adt_const_params` and mgce

[lcnr]

What is this

This is a design document for const generics. Any discussions about its content should be on zulip. The conclusions of these discussions should then be edited back into this issue. Please do not post any comments directly in this issue.

Content

trait Trait<T> {}

mod private {
    use crate::Trait;
    struct Private;
    impl Trait<Private> for () {}
}

fn fetch<T: Trait<U>, U>() -> U {
    todo!()
}

fn main() {
    let x = fetch::<(), _>();  //~ ERROR type `Private` is private
}

we have to make sure we also consider constants violate our privacy rules, most notably:

• with mgce we could infer a constant to be a private generic const item
• with adt_const_params we need to handle the type of const params, even if these could be checked eagerly as part of the function definition