RFC: Add a matches!(expression, pattern) macro.

[rust-highfive]

Rendered view: https://github.com/SimonSapin/rfcs/blob/matches-macro/active/0000-matches-macro.md

Please note: This is different from the if let proposal. I believe the two proposals are complementary rather than competing.

Tracking issue for postponed PR #163

[masklinn]

@alexcrichton maybe this PR can be either accepted & merged or just closed since the feature was merged in 1.42 (apparently directly via PR without referring back to RFCs): https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#matches

[kennytm]

matches! was tracked in rust-lang/rust#65721 and is already stabilized. Closing.