Tracking issue for RFC 2345, "Allow panicking in constants" (const_panic)

[Centril]

This is a tracking issue for the RFC "Allow panicking in constants" (rust-lang/rfcs#2345).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• Should there be some additional message in the error about this being a panic turned error?
  Or do we just produce the exact message the panic would produce?

• This change becomes really useful if Result::unwrap and Option::unwrap become const fn, doing both in one go might be a good idea.

Blockers:

• ICE: const_panic: ICE on non-&str panic payload #66693

[clarfonthey]

Don't forget expect and all the other unwrap-like methods. :P

[vi]

I suppose unimplemented!() and unreachable!() should also be included?

[SimonSapin]

I think they would be "automatically", since macros don’t have a separation similar to fn v.s. const fn.

[oli-obk]

I'll add some tests to #52011, but yes, it just works.

[Centril]

Triage: @oli-obk so this is implemented now, right?