1 files changed, 64 insertions, 0 deletions
diff --git a/components/layout_2020/flow_ref.rs b/components/layout_2020/flow_ref.rs
new file mode 100644
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+++ b/components/layout_2020/flow_ref.rs
@@ -0,0 +1,64 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Reference-counted pointers to flows.
+//!
+//! Eventually, with dynamically sized types in Rust, much of this code will
+//! be superfluous. This design is largely duplicating logic of Arc<T> and
+//! Weak<T>; please see comments there for details.
+
+use crate::flow::Flow;
+use std::ops::Deref;
+use std::sync::{Arc, Weak};
+
+#[derive(Clone, Debug)]
+pub struct FlowRef(Arc<dyn Flow>);
+
+impl Deref for FlowRef {
+    type Target = dyn Flow;
+    fn deref(&self) -> &dyn Flow {
+        self.0.deref()
+    }
+}
+
+impl FlowRef {
+    /// `FlowRef`s can only be made available to the traversal code.
+    /// See https://github.com/servo/servo/issues/14014 for more details.
+    pub fn new(mut r: Arc<dyn Flow>) -> Self {
+        // This assertion checks that this `FlowRef` does not alias normal `Arc`s.
+        // If that happens, we're in trouble.
+        assert!(Arc::get_mut(&mut r).is_some());
+        FlowRef(r)
+    }
+    pub fn get_mut(this: &mut FlowRef) -> Option<&mut dyn Flow> {
+        Arc::get_mut(&mut this.0)
+    }
+    pub fn downgrade(this: &FlowRef) -> WeakFlowRef {
+        WeakFlowRef(Arc::downgrade(&this.0))
+    }
+    pub fn into_arc(mut this: FlowRef) -> Arc<dyn Flow> {
+        // This assertion checks that this `FlowRef` does not alias normal `Arc`s.
+        // If that happens, we're in trouble.
+        assert!(FlowRef::get_mut(&mut this).is_some());
+        this.0
+    }
+    /// WARNING: This should only be used when there is no aliasing:
+    /// when the traversal ensures that no other threads accesses the same flow at the same time.
+    /// See https://github.com/servo/servo/issues/6503
+    /// Use Arc::get_mut instead when possible (e.g. on an Arc that was just created).
+    #[allow(unsafe_code)]
+    pub fn deref_mut(this: &mut FlowRef) -> &mut dyn Flow {
+        let ptr: *const dyn Flow = &*this.0;
+        unsafe { &mut *(ptr as *mut dyn Flow) }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct WeakFlowRef(Weak<dyn Flow>);
+
+impl WeakFlowRef {
+    pub fn upgrade(&self) -> Option<FlowRef> {
+        self.0.upgrade().map(FlowRef)
+    }
+}