DOM element has parent and sibling pointer, so there is nothing particularly difficult about adjacent sibling, later sibling, or even parent selector as far as selector matching goes. Avoiding restyling becomes more tricky, but nothing serious. "Parent before child" is for CSS inheritance, not for selector matching.
As soon as there are parent selectors, the pointers don't help. Specifically, if I restyle a parent, all children of parent need to be adjusted, right?
Siblings... I'm assuming there is nothing hard there. Positions, maybe?
I still don't see what problem parent selectors cause. Restyling parent happens anyway without parent selectors (say, by JavaScript), so it is handled. In https://github.com/servo/servo/blob/master/components/style/..., MustCascadeChildren is returned when you... must.
I was basically restating the restriction for the parallel scan. It goes from parent down so that during the scan you don't do something that causes the scan to have to restart.
I was also shooting from the hip for things I'm interested in, here. I'm not sure it matters. But, if on scanning, you pick relevant rules to apply based on the path to a node, then I could see some trickiness on having to consider both the top down and the bottom up paths to a node.
Siblings.., I don't think has this problem. Not sure why I thought it might.
And javascript is ultimately a different topic. So I wasn't worried about that here.
I missed your point about selector matching and style application. I think that is ultimately where my hip shot missed on this. (I have never had to reserve the right to be wrong. Just to assume it. :)
I still feel there is some danger there, but I think that is just clinging to an initial shot. I am curious why rust's help was needed to get this, now. A basic thread pool seems like it would have been somewhat easy to wire up in any language.
Yeah, the key reason Rust is necessary here is that there is an insane amount of complexity in the heart of modern web engines. Injecting concurrency into the intersection of DOM and Layout is only realistic with some kind of static guarantees against data races, and Rust is the only tool I'm aware of to do that.
One neat thing is that we use rust-bindgen to walk C++ data structures, which extends Rust's concurrency guarantees into C++. We also have an FFI layer for invoking C++ code, and we have some careful static analysis of that callgraph from the entry points to be sure we're not mutating anything.
Right, you don't need Rust to do this. In fact, Qualcomm did this with C++ and TBB in 2013, see http://dl.acm.org/citation.cfm?id=2442543. Rust does give you peace of mind that parallel styling won't be endless source of bugs, or worse, vulnerabilities.
For not-very-difficult details, read https://github.com/servo/servo/blob/master/components/select... searching for NotMatchedAndRestartFromClosestLaterSibling and HAS_SLOW_SELECTOR_LATER_SIBLINGS.