Unnecessary indirection for 'unsafe' boxed vecs in layout

[frewsxcv]

servo/components/layout/parallel.rs

Lines 84 to 86 in 0f597d3

	pub type UnsafeLayoutNodeList = (Box<Vec<UnsafeLayoutNode>>, usize);
	pub type UnsafeFlowList = (Box<Vec<UnsafeLayoutNode>>, usize);

Vec is already on the heap, is Box necessary?

[mbrubeck]

run_queue_with_custom_work_data_type uses transmute to convert between this type and struct WorkQueueData(usize, usize). So it needs to be exactly two pointers wide. Maybe we could use Box<[T]> instead of (Box<Vec<T>>, usize). (The usize element is unused.)

[Manishearth]

Are we pushing to the vector? If not, Box<[T]> makes more sense.

[eefriedman]

If #7524 ever lands, the relevant code will become a lot simpler.

[Ms2ger]

Box<[T]> should work; I'm pretty sure I've written that code at some point. Can't find that branch now, though.