Tracking Issue for pattern types (`pattern_types`)

This is a tracking issue for the feature gate #![feature(pattern_types).

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label. You can also start a thread on zulip in the pattern type stream

Steps

• Implementation (#120131)
• BLOCKER FOR ALL OF THE BELOW: write a proposal to T-types and implement a subtyping experiment for pattern types. If successful, write an RFC to lang
• non-macro syntax impl & rustfmt impl
• Write an RFC and get it approved
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Pattern type ranges cause monomorphization time errors on wraparound ranges (u8 is 10..5)
  • Same problems as having N + 2 in an array length for u8 is N..5 or even u8 is A..B
• should we forbid full-range patterns like u8 is 0..?
• Are pattern types always ABI compatible with their base type? Miri currently accepts them as ABI-compatible, we should either stably document that or remove this allowance from Miri.

Implementation history

• #120131
• #136922
• #137715

Start with the unchecked checklist in this tracking issue and the linked rustc-dev-guide, Style Guide, and Zulip resources. The remaining work includes a subtyping experiment and proposal, non-macro syntax and rustfmt support, an RFC, documentation, formatting guidance, and stabilization; completion depends on resolving the listed design questions and advancing those tracked items.