1375525 - Stylo: Fix servo's animation cascade so we can take rule nodes in the common case

Status: NEW

(Core :: CSS Parsing and Computation, enhancement, P4)

Description

[J. Ryan Stinnett [:jryans] (Use needinfo, replies may be slow)]

For the normal match and cascade path, we'd like to continue `take()`ing rule nodes, to avoid refcount traffic, etc.

However, it's currently difficult to do so after bug 1370719 which places the rule nodes inside `ComputedValues` during the cascade.  The main problem is the custom iterator that animation can pass to `apply_declarations`, so we should fix this and resume taking the rules in the common case again.

Quoting :emilio from bug 1370719 comment 34:
> The lifetime issue seems unavoidable as things stand. But the root cause is
> that still we have that apply_declarations call from Servo's animation code,
> which instead of creating a rule node, it goes and applies them directly.
> Seems like we should really fix that and use a rule node there instead. Then
> all the iterator madness is moot and can go away, and the cascade() function
> can just take rule nodes.

Comment 1

[Hiroyuki Ikezoe (:hiro)]

I think we should fix servo's animation code, right? We need to change both of them?

Comment 2

[J. Ryan Stinnett [:jryans] (Use needinfo, replies may be slow)]

(In reply to Hiroyuki Ikezoe (:hiro) from comment #1)
> I think we should fix servo's animation code, right? We need to change both
> of them?

The issue is in `animation::compute_style_for_animation_step`, which looks like it's only called in Servo (if I am reading things correctly).  Essentially, anyone calling `apply_declarations` might need some adjustment.  At the moment, it looks like that's the main `cascade` path and also `compute_style_for_animation_step`.

Comment 3

[Firefox Bug Husbandry Bot]

https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Move_fix-optionals