Auto merge of #13134 - servo:archery, r=emilio

Add lots of Arc’s in style, and prepare for using DOMRefCell

<!-- Please describe your changes on the following line: -->

`DOMRefCell` usage is not there year because of thread-safety questions, but I have this much already that I’d like to land before it bitrots.

r? @emilio

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because refactor

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13134)
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ports/cef/Cargo.lock

@@ -1691,11 +1691,6 @@ dependencies = [
  "rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
-[[package]]
-name = "ref_filter_map"
-version = "1.0.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-
 [[package]]
 name = "ref_slice"
 version = "1.0.0"
@@ -1781,7 +1776,6 @@ dependencies = [
  "profile_traits 0.0.1",
  "rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "range 0.0.1",
- "ref_filter_map 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "ref_slice 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "regex 0.1.73 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -2720,7 +2714,6 @@ dependencies = [
 "checksum quickersort 2.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e952ea7699262481636004bc4ab8afaccf2bc13f91b79d1aee6617bd8fc39651"
 "checksum rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)" = "2791d88c6defac799c3f20d74f094ca33b9332612d9aef9078519c82e4fe04a5"
 "checksum rayon 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "8e501871917624668fe601ad12a730450414f9b0b64722a898b040ce3ae1b0fa"
-"checksum ref_filter_map 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "2b5ceb840e4009da4841ed22a15eb49f64fdd00a2138945c5beacf506b2fb5ed"
 "checksum ref_slice 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "24c91f8f8903c37f0525112df98ef53b1985abca5702972e5e00854cd874baf2"
 "checksum regex 0.1.73 (registry+https://github.com/rust-lang/crates.io-index)" = "56b7ee9f764ecf412c6e2fff779bca4b22980517ae335a21aeaf4e32625a5df2"
 "checksum regex-syntax 0.3.4 (registry+https://github.com/rust-lang/crates.io-index)" = "31040aad7470ad9d8c46302dcffba337bb4289ca5da2e3cd6e37b64109a85199"

ports/geckolib/glue.rs

@@ -356,7 +356,7 @@ pub extern "C" fn Servo_StyleSet_Drop(data: *mut RawServoStyleSet) -> () {
 }
 
 pub struct GeckoDeclarationBlock {
-    pub declarations: Option<PropertyDeclarationBlock>,
+    pub declarations: Option<Arc<PropertyDeclarationBlock>>,
     // XXX The following two fields are made atomic to work around the
     // ownership system so that they can be changed inside a shared
     // instance. It wouldn't provide safety as Rust usually promises,
@@ -377,7 +377,7 @@ pub extern "C" fn Servo_ParseStyleAttribute(bytes: *const u8, length: u32,
                                             -> ServoDeclarationBlockStrong {
     let value = unsafe { from_utf8_unchecked(slice::from_raw_parts(bytes, length as usize)) };
     GeckoDeclarationBlock::from_arc(Arc::new(GeckoDeclarationBlock {
-        declarations: GeckoElement::parse_style_attribute(value),
+        declarations: GeckoElement::parse_style_attribute(value).map(Arc::new),
         cache: AtomicPtr::new(cache),
         immutable: AtomicBool::new(false),
     }))

ports/geckolib/wrapper.rs

@@ -35,7 +35,6 @@ use snapshot::GeckoElementSnapshot;
 use snapshot_helpers;
 use std::fmt;
 use std::marker::PhantomData;
-use std::mem::transmute;
 use std::ops::BitOr;
 use std::ptr;
 use std::sync::Arc;
@@ -462,8 +461,6 @@ lazy_static! {
     };
 }
 
-static NO_STYLE_ATTRIBUTE: Option<PropertyDeclarationBlock> = None;
-
 impl<'le> TElement for GeckoElement<'le> {
     type ConcreteNode = GeckoNode<'le>;
     type ConcreteDocument = GeckoDocument<'le>;
@@ -472,14 +469,16 @@ impl<'le> TElement for GeckoElement<'le> {
         unsafe { GeckoNode::from_raw(self.element as *mut RawGeckoNode) }
     }
 
-    fn style_attribute(&self) -> &Option<PropertyDeclarationBlock> {
+    fn style_attribute(&self) -> Option<&Arc<PropertyDeclarationBlock>> {
         let declarations = unsafe { Gecko_GetServoDeclarationBlock(self.element) };
         if declarations.is_null() {
-            &NO_STYLE_ATTRIBUTE
+            None
         } else {
-            GeckoDeclarationBlock::with(declarations, |declarations| {
-                unsafe { transmute(&declarations.declarations) }
-            })
+            let opt_ptr = GeckoDeclarationBlock::with(declarations, |declarations| {
+                // Use a raw pointer to extend the lifetime
+                declarations.declarations.as_ref().map(|r| r as *const Arc<_>)
+            });
+            opt_ptr.map(|ptr| unsafe { &*ptr })
         }
     }