Exhaustiveness check

[tamaroning]

This issue tracks the implementation status of the exhaustiveness check.
All unimplemented patterns are converted to wildcards when match arms are lowered to matrix for now.

ADT

• enum
  • pattern analysis #3115
• union
• struct
  • pattern analysis #3115

Pattern

• paths
• identifier pattern
  • pattern analysis #3115
• wildcard
  • pattern analysis #3115
• tuple struct
  • pattern analysis #3115
• struct struct
  • pattern analysis #3115
• & and ref
• integer literal
• floating point literal
• range pattern
• string literal
• char literal
• or pattern (pat | pat)
• slice pattern
• grouped pattern

Others

• if guard
• memory optimization
  • See https://doc.rust-lang.org/nightly/nightly-rustc/rustc_pattern_analysis/usefulness/struct.Matrix.html#method.unspecialize
  • and https://doc.rust-lang.org/nightly/nightly-rustc/rustc_pattern_analysis/usefulness/struct.WitnessMatrix.html#method.apply_constructor