"Traitify" the Style struct

[nicoburns]

Objective

Allow users of Taffy to bring their own style representation(s).

Context

While we have support for custom node tree representations, we currently enforce usage of a concrete style representation. Additionally, that style representation is shared across all algorithms and thus must include the styles for Flexbox, CSS Grid and Block layout, making it quite large. A problem which will only get worse if non-css algorithms are supported in future.

Feedback wanted

• Feedback on the trait hierarchy design

Todo

• Figure out how to handle the Display style. Algorithms typically only need to know whether the Display is None or not. I am considering creating a new enum with just None and Normal (name tbd) variants for this purpose. This would need to be extended with Contents if we implement that, but would never need to know about individual algorithms. See also: style::Display is not according to specifications #115
• Maybe: Blanket impl of the style traits for &T if they are implemented for T
• Update the Flexbox algorithm to use the new traits
• Update the CSS Grid algorithm to use the new traits
• Update the Block algorithm to use the new traits
• Update the Leaf algorithm to use the new traits
• Remove the get_style method from the base PartialLayoutTree trait Turns out this is still required for root layout
• Test implementing new traits for stylo types
• Fix examples

[nicoburns]

@vkurchatkin-miro How does this design for traitifying Style look to you?

[vkurchatkin-miro]

@nicoburns great stuff, even more flexible than what I thought of

[Patryk27]

btw, is it still worth doing considering rust-lang/rust#116505 exists? 👀