Tracking issue for RFC 1758: Specify `repr(transparent)`

[aturon]

RFC

This is a tracking issue for the RFC "Specify repr(transparent)" (rust-lang/rfcs#1758).

Steps:

• Implement the RFC (done in Implement repr(transparent) #47158)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

[eddyb]

Mentoring instructions unavailable. This is sadly not that easy, as we have too much code that still relies on assumptions that such types do not exist. I have an in-progress branch which requires implementing the necessary support for "newtype unpacking" but no time frame for completion.

[RalfJung]

Is making types like Cell repr(transparent) (which e.g. rust-lang/rfcs#1789 relies on) part of the implementation of the RFC or would that require its own RFC?

[aturon]

@RalfJung For a change at that level, a PR approved by the libs team suffices.