style: Add a Document::elements_with_id API.

MozReview-Commit-ID: BNtbJp0RI68
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/dom.rs

@@ -146,6 +146,17 @@ pub trait TDocument : Sized + Copy + Clone {
 
     /// Returns the quirks mode of this document.
     fn quirks_mode(&self) -> QuirksMode;
+
+    /// Get a list of elements with a given ID in this document.
+    ///
+    /// Can return an error to signal that this list is not available, or also
+    /// return an empty slice.
+    fn elements_with_id(
+        &self,
+        _id: &Atom,
+    ) -> Result<&[<Self::ConcreteNode as TNode>::ConcreteElement], ()> {
+        Err(())
+    }
 }
 
 /// The `TNode` trait. This is the main generic trait over which the style

components/style/gecko/wrapper.rs

@@ -110,6 +110,26 @@ impl<'ld> TDocument for GeckoDocument<'ld> {
     fn quirks_mode(&self) -> QuirksMode {
         self.0.mCompatMode.into()
     }
+
+    fn elements_with_id(&self, id: &Atom) -> Result<&[GeckoElement<'ld>], ()> {
+        unsafe {
+            let array = bindings::Gecko_GetElementsWithId(self.0, id.as_ptr());
+            if array.is_null() {
+                return Ok(&[]);
+            }
+
+            let elements: &[*mut RawGeckoElement] = &**array;
+
+            // NOTE(emilio): We rely on the in-memory representation of
+            // GeckoElement<'ld> and *mut RawGeckoElement being the same.
+            #[allow(dead_code)]
+            unsafe fn static_assert() {
+                mem::transmute::<*mut RawGeckoElement, GeckoElement<'static>>(0xbadc0de as *mut _);
+            }
+
+            Ok(mem::transmute(elements))
+        }
+    }
 }
 
 /// A simple wrapper over a non-null Gecko node (`nsINode`) pointer.