Make #[rustc_deprecated] (and #[deprecated]) on re-exports either work properly or cause a compiler error

[bstrie]

#82080 laid undetected for years due to the silent inability of #[rustc_deprecated] to apply properly to re-exports. For now people simply have to remember not to use it with re-exports, but it would be nice if it either 1) worked as expected, or at least 2) caused a compiler error so that it couldn't pass silently.

Open question: should the chosen behavior extend to the stable #[deprecated] attribute as well? If made to work on re-exports this will be a slight increase to the API surface area, but if made into an error it could potentially be a breaking change (although easily understood and justified as a bugfix).

Test case for #[deprecated]:

mod foo {
    #[deprecated]
    pub use crate::bar::Bar;
}

mod bar {
    pub struct Bar;
}

fn main() {
    let _ = foo::Bar; // should warn
    let _ = bar::Bar; // should not warn
}

Once (if) this is fixed, then we should go through the stdlib and use this to replace all of the wrapper functions/type aliases that are currently used to trigger deprecated lints (e.g. the items in #82122 ). Better to have a deprecated path than a full-fledged item and symbol.

[Pratyush]

Relevant: #30827

[bstrie]

Good catch, I'll close this as a duplicate.