It's binding a variable `even`, and then using a pattern guard to conditionally take that arm depending on the value of `even`; it's (basically) the same as

It's binding a variable `even`, and then using a pattern guard to conditionally take that arm depending on the value of `even`; it's (basically) the same as match val { even => { if even % 2 == 0 { Some(even) } else { None } } _ => None } which is an absurd construct (and the example pretty absurd too). It should be written as just if val % 2 == 0 { Some(val) } else { None } or, if you really like the formatting a match provides, just avoid creating a new name binding match () { _ if val % 2 == 0 => Some(val), _ => None } `match` and pattern guards make far more sense when not just binding directly to a single variable like that, e.g. let val: Option<int> = ...; match val { // even Some(x) if x % 2 == 0 => Some(x), // odd Some(x) => Some(x + 1), None => None } (Of course this may still be clearer using `.map` instead `match` but it's a better example than the original one.)