Stability on `pub use` items is mandatory but has no effect

Currently pub use items must have a stability attribute on them in the stdlib.
At the same time, this attribute does absolutely nothing and is ignored. That's weird and likely to result in bugs. It is possible someone will try to stabilize an item and try to keep its pub use unstable, not realizing this does not work.

We have two options:

1. Make this have an effect. When an item is used through a pub use, both the item itself and the pub use must be stable for it to work. Not sure how hard this is to implement, I haven't touched that part of the codebase.
2. Remove stability attributes on pub use items. I could easily implement this.

What is the preferred option here?

The issue names no files, tests, or entry points. Start by tracing how stability attributes on pub use items are validated and enforced, then compare the proposed enforcement and removal approaches. Done means the project has a decided behavior and the corresponding compiler handling and tests reflect it.