single_match should be smarter for known enums

[Manishearth]

    match my_option {
      Some(an_int) => println!("{}", an_int),
      None => {}
    }

this can be replaced with an if let, but we don't suggest it, because there's no wildcard. This makes sense for arbitrary enums -- replacing an exhaustive match with an if let can be problematic if the original enum gets updated. But for known enums like Option/Result/Cow, this isn't an issue.

[llogiq]

With Cow, the cases using only one match arm will be rare anyway. Regarding Option/Result, I agree. Perhaps I can add the OptionBool type from my own optional crate to the list? I'm positive that it won't get additional variants in the future 😄

[Manishearth]

Yeah, basically anything where we're sure that it won't be extended.

[llogiq]

The implementation will need to call expr_ty on the matched expression, then use utils::match_ty to get the type, then loop to match the type against a white-list of known two-variant enums.

[mcarton]

Closing as fixed in #547.