script: Reduce the use of unsafe in LayoutDom (#31979)

Remove the use of unsafe code in the layout wrappers of the DOM. The
main change here is that `unsafe_get()` no longer needs to be an unsafe
method, which allows us to transitively remove or reduce unsafe blocks
from callers. The function itself is not renamed, because it's still
a bit dangerous to start removing the layers of abstraction from actual
DOM nodes.

In addition `init_style_and_opaque_layout_data` can be merged into
`initialize_data`, which removes one more unsafe method.

Finally, a "Safety" section is added to some unsafe methods.

components/script/layout_dom/document.rs

@@ -64,11 +64,11 @@ impl<'ld, LayoutDataType: LayoutDataTrait> ServoLayoutDocument<'ld, LayoutDataTy
     }
 
     pub fn needs_paint_from_layout(&self) {
-        unsafe { self.document.needs_paint_from_layout() }
+        self.document.needs_paint_from_layout()
     }
 
     pub fn will_paint(&self) {
-        unsafe { self.document.will_paint() }
+        self.document.will_paint()
     }
 
     pub fn style_shared_lock(&self) -> &StyleSharedRwLock {

components/script/layout_dom/element.rs

@@ -323,7 +323,11 @@ impl<'dom, LayoutDataType: LayoutDataTrait> style::dom::TElement
 
     unsafe fn clear_data(&self) {
         if self.get_style_and_opaque_layout_data().is_some() {
-            drop(self.as_node().take_style_and_opaque_layout_data());
+            drop(
+                self.as_node()
+                    .get_jsmanaged()
+                    .take_style_and_opaque_layout_data(),
+            );
         }
     }
 
@@ -758,7 +762,7 @@ impl<'dom, LayoutDataType: LayoutDataTrait> ThreadSafeLayoutElement<'dom>
         self.as_node().type_id()
     }
 
-    unsafe fn unsafe_get(self) -> ServoLayoutElement<'dom, LayoutDataType> {
+    fn unsafe_get(self) -> ServoLayoutElement<'dom, LayoutDataType> {
         self.element
     }
 

components/script/layout_dom/node.rs

@@ -207,21 +207,17 @@ impl<'dom, LayoutDataType: LayoutDataTrait> LayoutNode<'dom>
     }
 
     unsafe fn initialize_data(&self) {
-        if self.get_style_and_opaque_layout_data().is_none() {
-            let opaque = StyleAndOpaqueLayoutData::new(
-                StyleData::default(),
-                AtomicRefCell::new(LayoutDataType::default()),
-            );
-            self.init_style_and_opaque_layout_data(opaque);
-        };
-    }
+        if self.get_style_and_opaque_layout_data().is_some() {
+            return;
+        }
 
-    unsafe fn init_style_and_opaque_layout_data(&self, data: Box<StyleAndOpaqueLayoutData>) {
-        self.get_jsmanaged().init_style_and_opaque_layout_data(data);
-    }
+        let opaque = StyleAndOpaqueLayoutData::new(
+            StyleData::default(),
+            AtomicRefCell::new(LayoutDataType::default()),
+        );
 
-    unsafe fn take_style_and_opaque_layout_data(&self) -> Box<StyleAndOpaqueLayoutData> {
-        self.get_jsmanaged().take_style_and_opaque_layout_data()
+        self.get_jsmanaged()
+            .init_style_and_opaque_layout_data(opaque);
     }
 
     fn is_connected(&self) -> bool {
@@ -402,7 +398,7 @@ impl<'dom, LayoutDataType: LayoutDataTrait> ThreadSafeLayoutNode<'dom>
         }
     }
 
-    unsafe fn unsafe_get(self) -> Self::ConcreteNode {
+    fn unsafe_get(self) -> Self::ConcreteNode {
         self.node
     }