From 15db31769cb0172be610fe2d731533c30c40fa76 Mon Sep 17 00:00:00 2001
From: Anthony Ramine <nox@nox.paris>
Date: Mon, 27 Apr 2020 10:59:38 +0200
Subject: [PATCH] Make DomRefCell not mutate the borrow flag non-atomically

---
 components/script/dom/bindings/cell.rs | 9 +++++----
 components/script/dom/shadowroot.rs    | 2 +-
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/components/script/dom/bindings/cell.rs b/components/script/dom/bindings/cell.rs
index e3f5ad71bee2..5807138c948e 100644
--- a/components/script/dom/bindings/cell.rs
+++ b/components/script/dom/bindings/cell.rs
@@ -45,11 +45,12 @@ impl<T> DomRefCell<T> {
         &mut *self.value.as_ptr()
     }
 
-    /// Version of the above that we use during restyle while the script thread
-    /// is blocked.
-    pub fn borrow_mut_for_layout(&self) -> RefMut<T> {
+    /// Mutably borrow a cell for layout. Ideally this would use
+    /// `RefCell::try_borrow_mut_unguarded` but that doesn't exist yet.
+    #[allow(unsafe_code)]
+    pub unsafe fn borrow_mut_for_layout(&self) -> &mut T {
         debug_assert!(thread_state::get().is_layout());
-        self.value.borrow_mut()
+        &mut *self.value.as_ptr()
     }
 }
 
diff --git a/components/script/dom/shadowroot.rs b/components/script/dom/shadowroot.rs
index f1c0567a3991..c70c13d8cefd 100644
--- a/components/script/dom/shadowroot.rs
+++ b/components/script/dom/shadowroot.rs
@@ -281,7 +281,7 @@ impl<'dom> LayoutShadowRootHelpers<'dom> for LayoutDom<'dom, ShadowRoot> {
         quirks_mode: QuirksMode,
         guard: &SharedRwLockReadGuard,
     ) {
-        let mut author_styles = (*self.unsafe_get()).author_styles.borrow_mut_for_layout();
+        let author_styles = self.unsafe_get().author_styles.borrow_mut_for_layout();
         if author_styles.stylesheets.dirty() {
             author_styles.flush::<E>(device, quirks_mode, guard);
         }