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Review nits: Narrowly scoping unsafety, and expanding comment.

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This commit is contained in:
Bobby Holley 2016-01-07 11:17:31 -08:00
parent 29987a6715
commit 3b33174163
2 changed files with 9 additions and 5 deletions
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11
components/layout/traversal.rs
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@ -4,8 +4,6 @@
//! Traversals over the DOM and flow trees, running the layout computations.
#![allow(unsafe_code)]
use construct::FlowConstructor;
use context::{LayoutContext, SharedLayoutContext};
use flow::{PostorderFlowTraversal, PreorderFlowTraversal};
@ -29,6 +27,7 @@ pub struct RecalcStyleAndConstructFlows<'lc> {
impl<'lc, 'ln, N: LayoutNode<'ln>> DomTraversalContext<'ln, N> for RecalcStyleAndConstructFlows<'lc> {
type SharedContext = SharedLayoutContext;
#[allow(unsafe_code)]
fn new<'a>(shared: &'a Self::SharedContext, root: OpaqueNode) -> Self {
// FIXME(bholley): This transmutation from &'a to &'lc is very unfortunate, but I haven't
// found a way to avoid it despite spending several days on it (and consulting Manishearth,
@ -48,7 +47,13 @@ impl<'lc, 'ln, N: LayoutNode<'ln>> DomTraversalContext<'ln, N> for RecalcStyleAn
// that would require higher-kinded types, which don't exist yet and probably aren't coming
// for a while.
//
// So we transmute. :-(
// So we transmute. :-( This is safe because the DomTravesalContext is stack-allocated on
// the worker thread while processing a WorkUnit, whereas the borrowed SharedContext is
// live for the entire duration of the restyle. This really could _almost_ compile: all
// we'd need to do is change the signature to to |new<'a: 'lc>|, and everything would
// work great. But we can't do that, because that would cause a mismatch with the signature
// in the trait we're implementing, and we can't mention 'lc in that trait at all for the
// reasons described above.
//
// [1] For example, the WorkQueue type needs to be parameterized on the concrete type of
// DomTraversalContext::SharedContext, and the WorkQueue lifetime is similar to that of the