script: Replace use of UnsafeCell in WeakRangeVec. (#37327)

I can't find any historical motivation for the use of UnsafeCell in the
implementation of WeakRangeVec from #8506. We can replace all the uses
of unsafe in this code by using RefCell, and the resulting code is
easier to understand.

Testing: Existing WPT tests using Ranges show no behaviour changes.
Fixes: #37276

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/range.rs

@@ -2,14 +2,13 @@
  * 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/. */
 
-use std::cell::UnsafeCell;
+use std::cell::RefCell;
 use std::cmp::{Ordering, PartialOrd};
 use std::iter;
 
 use dom_struct::dom_struct;
 use js::jsapi::JSTracer;
 use js::rust::HandleObject;
-use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 
 use crate::dom::abstractrange::{AbstractRange, BoundaryPoint, bp_position};
 use crate::dom::bindings::cell::DomRefCell;
@@ -1194,14 +1193,15 @@ impl RangeMethods<crate::DomTypeHolder> for Range {
     }
 }
 
+#[derive(MallocSizeOf)]
 pub(crate) struct WeakRangeVec {
-    cell: UnsafeCell<WeakRefVec<Range>>,
+    cell: RefCell<WeakRefVec<Range>>,
 }
 
 impl Default for WeakRangeVec {
     fn default() -> Self {
         WeakRangeVec {
-            cell: UnsafeCell::new(WeakRefVec::new()),
+            cell: RefCell::new(WeakRefVec::new()),
         }
     }
 }
@@ -1210,7 +1210,7 @@ impl Default for WeakRangeVec {
 impl WeakRangeVec {
     /// Whether that vector of ranges is empty.
     pub(crate) fn is_empty(&self) -> bool {
-        unsafe { (*self.cell.get()).is_empty() }
+        self.cell.borrow().is_empty()
     }
 
     /// Used for steps 2.1-2. when inserting a node.
@@ -1235,8 +1235,7 @@ impl WeakRangeVec {
 
         let offset = context.index();
         let parent = context.parent;
-        unsafe {
+        let ranges = &mut *self.cell.borrow_mut();
-            let ranges = &mut *self.cell.get();
 
         ranges.update(|entry| {
             let range = entry.root().unwrap();
@@ -1253,8 +1252,12 @@ impl WeakRangeVec {
             }
         });
 
-            (*context.parent.ranges().cell.get()).extend(ranges.drain(..));
+        context
-        }
+            .parent
+            .ranges()
+            .cell
+            .borrow_mut()
+            .extend(ranges.drain(..));
     }
 
     /// Used for steps 7.1-2. when normalizing a node.
@@ -1264,8 +1267,7 @@ impl WeakRangeVec {
             return;
         }
 
-        unsafe {
+        let ranges = &mut *self.cell.borrow_mut();
-            let ranges = &mut *self.cell.get();
 
         ranges.update(|entry| {
             let range = entry.root().unwrap();
@@ -1282,8 +1284,7 @@ impl WeakRangeVec {
             }
         });
 
-            (*sibling.ranges().cell.get()).extend(ranges.drain(..));
+        sibling.ranges().cell.borrow_mut().extend(ranges.drain(..));
-        }
     }
 
     /// Used for steps 7.3-4. when normalizing a node.
@@ -1295,10 +1296,10 @@ impl WeakRangeVec {
         child: &Node,
         new_offset: u32,
     ) {
-        unsafe {
+        let child_ranges = child.ranges();
-            let child_ranges = &mut *child.ranges().cell.get();
+        let mut child_ranges = child_ranges.cell.borrow_mut();
 
-            (*self.cell.get()).update(|entry| {
+        self.cell.borrow_mut().update(|entry| {
             let range = entry.root().unwrap();
 
             let node_is_start = range.start().node() == node;
@@ -1332,7 +1333,6 @@ impl WeakRangeVec {
             }
         });
     }
-    }
 
     /// Used for steps 8-11. when replacing character data.
     /// <https://dom.spec.whatwg.org/#concept-cd-replace>
@@ -1360,10 +1360,10 @@ impl WeakRangeVec {
         offset: u32,
         sibling: &Node,
     ) {
-        unsafe {
+        let sibling_ranges = sibling.ranges();
-            let sibling_ranges = &mut *sibling.ranges().cell.get();
+        let mut sibling_ranges = sibling_ranges.cell.borrow_mut();
 
-            (*self.cell.get()).update(|entry| {
+        self.cell.borrow_mut().update(|entry| {
             let range = entry.root().unwrap();
             let start_offset = range.start_offset();
             let end_offset = range.end_offset();
@@ -1400,13 +1400,11 @@ impl WeakRangeVec {
             }
         });
     }
-    }
 
     /// Used for steps 7.4-5. when splitting a text node.
     /// <https://dom.spec.whatwg.org/#concept-text-split>
     pub(crate) fn increment_at(&self, node: &Node, offset: u32) {
-        unsafe {
+        self.cell.borrow_mut().update(|entry| {
-            (*self.cell.get()).update(|entry| {
             let range = entry.root().unwrap();
             if range.start().node() == node && offset == range.start_offset() {
                 range.report_change();
@@ -1418,11 +1416,9 @@ impl WeakRangeVec {
             }
         });
     }
-    }
 
     fn map_offset_above<F: FnMut(u32) -> u32>(&self, node: &Node, offset: u32, mut f: F) {
-        unsafe {
+        self.cell.borrow_mut().update(|entry| {
-            (*self.cell.get()).update(|entry| {
             let range = entry.root().unwrap();
             let start_offset = range.start_offset();
             if range.start().node() == node && start_offset > offset {
@@ -1436,33 +1432,21 @@ impl WeakRangeVec {
             }
         });
     }
-    }
 
     pub(crate) fn push(&self, ref_: WeakRef<Range>) {
-        unsafe {
+        self.cell.borrow_mut().push(ref_);
-            (*self.cell.get()).push(ref_);
-        }
     }
 
     fn remove(&self, range: &Range) -> WeakRef<Range> {
-        unsafe {
+        let mut ranges = self.cell.borrow_mut();
-            let ranges = &mut *self.cell.get();
         let position = ranges.iter().position(|ref_| ref_ == range).unwrap();
         ranges.swap_remove(position)
     }
-    }
-}
-
-#[allow(unsafe_code)]
-impl MallocSizeOf for WeakRangeVec {
-    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
-        unsafe { (*self.cell.get()).size_of(ops) }
-    }
 }
 
 #[allow(unsafe_code)]
 unsafe impl JSTraceable for WeakRangeVec {
     unsafe fn trace(&self, _: *mut JSTracer) {
-        (*self.cell.get()).retain_alive()
+        self.cell.borrow_mut().retain_alive()
     }
 }