The reason not to hide `RustcInternal` and `Stable` traits

[zjp-CN]

These two traits used to be public, while they are hidden in the doc.

The troubles are:

• internal fn needs RustcInternal trait bound which doesn't have a direct definition to jump in.
• RustcInternal becomes harder to search and understand:
  • Users will never be sure that types like DefId or Instance can be converted into the internal conterparts, unless they read the source code of RustcInternal and the implementations.
  • Being convertible to an internal item is an important property for these types, and should be reflected in the API doc of the types to help readers be aware of it.
• The same troubles apply to the Stable case.

[celinval]

They are marked as hidden because they are not part of the supported API. I.e.: Version semantic will not apply to them. There's also no guarantee on its future support and coverage. Note that they also expose internal compiler structures. They are only there to help users with migration and for lack of coverage from existing APIs.

Unfortunately, Rust doesn't have a mechanism today to mark APIs as unstable, and marking them as doc(hidden) is the current convention.