Tracking issue for `#![feature(slice_patterns)]`

[Centril]

This is a tracking issue for the feature gate #![feature(slice_patterns)] covering the following RFCs:

• "Finalize syntax for slice patterns with subslices" (RFC: Finalize syntax for slice patterns with subslices rfcs#2359),
• "Make .. a pattern syntactically" (Mini-RFC: Make .. a pattern syntactically rfcs#2707),
• "Array pattern adjustments" (RFC: Array pattern adjustments rfcs#495).

For the old tracking issue see #23121.

Steps:

• Implement the changes in RFC: Finalize syntax for slice patterns with subslices rfcs#2359 and Mini-RFC: Make .. a pattern syntactically rfcs#2707
  (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on rustc-guide)
  • Reference documentation is tracked in Document subslice patterns reference#739.
  • Rustc-guide documentation is tracked in Document slice patterns rustc-dev-guide#563.
  • Book documentation in the book is tracked in Document subslice patterns book#2225.
• Stabilization PR (Stabilize #![feature(slice_patterns)] in 1.42.0 #67712)

Unresolved questions:

None thus far.

[Centril]

For now, I'm working on the syntactic aspect of supporting .. as a valid pattern syntactic pattern.

[Centril]

The syntax change in rust-lang/rfcs#2707 was implemented on the 2019-07-28 in #62550 (author: @Centril, reviewer: @petrochenkov).

What remains now is:

• Adding more tests for the syntax. Add syntactic and semantic tests for rest patterns, i.e. .. #63111

• Verifying the semantics in terms of type checking and making changes as needed. Subslice patterns: Test passing static & dynamic semantics. #63129

Before / during stabilization, we must also:

• Organize the tests.