Bug 1719824 - Inline GeckoNode::prev_sibling. r=smaug

It's very hot when matching some kind of selectors like the ones in bug
1717267, and the two function calls show up in the profiles.

Differential Revision: https://phabricator.services.mozilla.com/D119505

layout/style/GeckoBindings.cpp

@@ -130,10 +130,6 @@ const nsINode* Gecko_GetLastChild(const nsINode* aNode) {
   return aNode->GetLastChild();
 }
 
-const nsINode* Gecko_GetPreviousSibling(const nsINode* aNode) {
-  return aNode->GetPreviousSibling();
-}
-
 const nsINode* Gecko_GetFlattenedTreeParentNode(const nsINode* aNode) {
   return aNode->GetFlattenedTreeParentNodeForStyle();
 }

layout/style/GeckoBindings.h

@@ -76,8 +76,6 @@ void Gecko_Snapshot_DebugListAttributes(const mozilla::ServoElementSnapshot*,
 bool Gecko_IsSignificantChild(const nsINode*, bool whitespace_is_significant);
 
 const nsINode* Gecko_GetLastChild(const nsINode*);
-const nsINode* Gecko_GetPreviousSibling(const nsINode*);
-
 const nsINode* Gecko_GetFlattenedTreeParentNode(const nsINode*);
 const mozilla::dom::Element* Gecko_GetBeforeOrAfterPseudo(
     const mozilla::dom::Element*, bool is_before);

servo/components/style/dom.rs

@@ -152,7 +152,7 @@ pub trait TNode: Sized + Copy + Clone + Debug + NodeInfo + PartialEq {
     /// Get this node's first child.
     fn first_child(&self) -> Option<Self>;
 
-    /// Get this node's first child.
+    /// Get this node's last child.
     fn last_child(&self) -> Option<Self>;
 
     /// Get this node's previous sibling.

servo/components/style/gecko/wrapper.rs

@@ -414,9 +414,11 @@ impl<'ln> TNode for GeckoNode<'ln> {
     #[inline]
     fn prev_sibling(&self) -> Option<Self> {
         unsafe {
-            bindings::Gecko_GetPreviousSibling(self.0)
-                .as_ref()
-                .map(GeckoNode)
+            let prev_or_last = GeckoNode::from_content(self.0.mPreviousOrLastSibling.as_ref()?);
+            if prev_or_last.0.mNextSibling.raw::<nsIContent>().is_null() {
+                return None;
+            }
+            Some(prev_or_last)
         }
     }