How to match against a flags/bitmasks in Rust?

Question

Take a typical check for bit-flags:

if      (v & (1 << 0)) != 0 { foo(); }
else if (v & (1 << 1)) != 0 { bar(); }
else if (v & (1 << 2)) != 0 { baz(); }

How would this be written as a match statement?

I don't think it makes sense to use `match` when you are testing single bits. — starblue, Aug 27 '16 at 08:10

score 13 · Answer 1 · answered Aug 27 '16 at 09:38

13

if      (v & (1 << 0)) != 0 { foo(); }
else if (v & (1 << 1)) != 0 { bar(); }
else if (v & (1 << 2)) != 0 { baz(); }

Such concrete code can be rewritten like this:

match v.trailing_zeros() {
    0 => foo(),
    1 => bar(),
    2 => baz(),
    _ => {},
}

answered Aug 27 '16 at 09:38

fghj

1

That's really clever! I wrote some tests to check that this is indeed correct (and they pass): https://play.rust-lang.org/?gist=bcbd5fb07635e0601747abf06b8d3f57&version=stable&backtrace=0 – Dogbert Aug 27 '16 at 09:48
1

Is the generated code as efficient as a bitmask check? – ideasman42 Aug 29 '19 at 13:41

1 Answers1