Lazily compute and cache the classnames of elements in the style sharing cache.

[emilio]

When #12668 lands, we can compute the StyleSharingCandidate class names the same way as we're computing the CommonStyleAffectingAttributes instead of once per test.

Code:

• components/style/matching.rs

Tests:

• Not needed, if it passes tests and review, it's fine.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[ashrwin]

I'd like to work on this :)

[emilio]

Hi! The mentioned bug landed, so it's yours!

Let me know if you need any help :)

[ashrwin]

Hi,
I need a bit of help on where exactly the changes should go. This is the first time am dealing with Rust code. Thanks :)

[emilio]

No problem!

So, in the file components/style/matching.rs, we have a function that compares classes between two elements, called have_same_class.

That function currently takes two elements and compares the two vectors. The key point here is that one element comes straight from the cache, and we might be computing its class attribute a lot of times even when we know it shouldn't change.

So what we'd have to do is adding a field to the StyleSharingCandidate struct, something like our current common_style_affecting_attributes, but for classes.

This field should be an Option<Vec<Atom>>, that is, an optional vector of strings (atoms are the optimized representation for commonly used strings in Servo).

We should add this field, and then write have_same_class in the same way have_same_common_style_affecting_attributes is written, that is, something like:

if candidate.class_attribute.is_none() {
    // compute the class attribute
}
return candidate.class_attribute.unwrap() == element_class_attribute

I hope that helps, thanks!