Auto merge of #11697 - mrmiywj:form-index-getter, r=Ms2ger

indexed getter of form elements

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11405 (github issue number if applicable).

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components/script/dom/htmlformelement.rs

@@ -6,6 +6,7 @@ use dom::bindings::codegen::Bindings::BlobBinding::BlobMethods;
 use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
 use dom::bindings::codegen::Bindings::EventBinding::EventMethods;
 use dom::bindings::codegen::Bindings::HTMLButtonElementBinding::HTMLButtonElementMethods;
+use dom::bindings::codegen::Bindings::HTMLFormControlsCollectionBinding::HTMLFormControlsCollectionMethods;
 use dom::bindings::codegen::Bindings::HTMLFormElementBinding;
 use dom::bindings::codegen::Bindings::HTMLFormElementBinding::HTMLFormElementMethods;
 use dom::bindings::codegen::Bindings::HTMLInputElementBinding::HTMLInputElementMethods;
@@ -229,6 +230,12 @@ impl HTMLFormElementMethods for HTMLFormElement {
     fn Length(&self) -> u32 {
         self.Elements().Length() as u32
     }
+
+    // https://html.spec.whatwg.org/multipage/#dom-form-item
+    fn IndexedGetter(&self, index: u32, found: &mut bool) -> Option<Root<Element>> {
+        let elements = self.Elements();
+        elements.IndexedGetter(index, found)
+    }
 }
 
 #[derive(Copy, Clone, HeapSizeOf, PartialEq)]

components/script/dom/webidls/HTMLFormElement.webidl

@@ -17,7 +17,7 @@ interface HTMLFormElement : HTMLElement {
 
   [SameObject] readonly attribute HTMLFormControlsCollection elements;
   readonly attribute unsigned long length;
-  //getter Element (unsigned long index);
+  getter Element? (unsigned long index);
   //getter (RadioNodeList or Element) (DOMString name);
 
   void submit();