Skip generating unnecessary @supports

[sapphi-red]

This PR introduces TargetsWithSupportsScope struct that tracks the parent @supportss. This struct has current field that contains the current target that is based on the target option and the parent @supportss.

This makes lightningcss to skip generating unnecessary @supports.

closes #711

[devongovett]

This would only work if the value is an exact match. Perhaps a future enhancement would be to attempt to parse the value, and extract the features from that. For example, if any lab() color was used, set that feature rather than only lab(0% 0 0)

[sapphi-red]

Yeah, it would be nice to expand this detection in future.

[devongovett]

You could probably get away with just a Vec here. Since Features is a bit set, it should only accumulate new items the deeper you go. So you could track each @supports you enter in a stack, and modify current in place:

fn enter_supports(&mut self, features: Features) -> bool {
  if features.is_empty() || self.current.exclude.contains(features) {
    // Already excluding all features
    return false;
  }

  self.stack.push(supported);
  self.current.exclude.insert(features);
  true
}

fn exit_supports(&mut self) {
  if let Some(last) = self.stack.pop() {
    self.current.exclude.remove(last);
  }
}

Then you wouldn't need to iterate over the stack each time and could avoid the hashing cost of IndexSet.

Do you think this would work or am I missing something?

[sapphi-red]

That's true! I updated the code to use Vec.
I added this line to make cases like what I added as a test case work.
394930a#diff-05702e78430359853e5b38404035fa394540368bee185a2cdc936a62b9cd65b0R271

[devongovett]

Should this just be part of Targets?

[sapphi-red]

Do you mean putting the fields in Targets and making it stateful?

[devongovett]

Sorry for the delay! I implemented improved detection of used features by parsing the value in d398c1b. That also applies the exclusions during printing so that features like light-dark that are compiled during printing are excluded when in a @supports rule as well.

[sapphi-red]

Thanks! That sounds great. It'd be great if I can have your input on #880, too.