Superseded by #101728 due to the landing of RFC 3307

Tracking issue for rust-lang/rfcs#803. Important points:

• Implement the expr:Type syntax
• Ensure soundness with respect to ref positions:
  • let ref x = <expr> / let mut ref = <expr>
  • match <expr>: Type { ref x => ... }
  • (<expr>: Type).method_with_ref_self()
• Permit coercions like &[1, 2, 3]: &[u8] -- permit coercion in type ascription #78248