Auto merge of #20265 - Xanewok:fix-js-objects-in-unions, r=jdm

Fix JS object conversion in unions

<!-- Please describe your changes on the following line: -->
Requires safe `Heap::boxed` constructor from https://github.com/servo/rust-mozjs/pull/395 (more info on it is in the PR).

Since unions currently assume that their respective members root themselves and can be stored on heap, I modified the union member object conversion branch to convert to a `RootedTraceableBox<Heap<*mut JSObject>>` (which is the currently generated type for objects in said unions).

I did it only for Unions and not dictionaries, since some dictionaries had bare `*mut JSObject` members - is this a mistake and something that needs further fixing?

Does this need a test, e.g. passing a union with object to a function that returns said object, and comparing the results for equality?

r? @jdm

Generated code with this patch (for `StringOrObject`):
```rust
impl FromJSValConvertible for StringOrObject {
    type Config = ();
    unsafe fn from_jsval(cx: *mut JSContext,
                         value: HandleValue,
                         _option: ())
                         -> Result<ConversionResult<StringOrObject>, ()> {
        if value.get().is_object() {
            match StringOrObject::TryConvertToObject(cx, value) {
                Err(_) => return Err(()),
                Ok(Some(value)) => return Ok(ConversionResult::Success(StringOrObject::Object(value))),
                Ok(None) => (),
            }

        }
        // (...)
    }
}

impl StringOrObject {
    // (...)
    unsafe fn TryConvertToObject(cx: *mut JSContext, value: HandleValue) -> Result<Option<RootedTraceableBox<Heap<*mut JSObject>>>, ()> {
        Ok(Some(RootedTraceableBox::from_box(Heap::boxed(value.get().to_object()))))
    }
}
```

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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #17011 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/bindings/codegen/CodegenRust.py

@@ -1075,20 +1075,24 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
     if type.isObject():
         assert not isEnforceRange and not isClamp
 
-        # TODO: Need to root somehow
-        # https://github.com/servo/servo/issues/6382
+        templateBody = "${val}.get().to_object()"
         default = "ptr::null_mut()"
-        templateBody = wrapObjectTemplate("${val}.get().to_object()",
-                                          default,
-                                          isDefinitelyObject, type, failureCode)
 
-        if isMember in ("Dictionary", "Union"):
+        # TODO: Do we need to do the same for dictionaries?
+        if isMember == "Union":
+            templateBody = "RootedTraceableBox::from_box(Heap::boxed(%s))" % templateBody
+            default = "RootedTraceableBox::new(Heap::default())"
+            declType = CGGeneric("RootedTraceableBox<Heap<*mut JSObject>>")
+        elif isMember == "Dictionary":
             declType = CGGeneric("Heap<*mut JSObject>")
         else:
             # TODO: Need to root somehow
             # https://github.com/servo/servo/issues/6382
             declType = CGGeneric("*mut JSObject")
 
+        templateBody = wrapObjectTemplate(templateBody, default,
+                                          isDefinitelyObject, type, failureCode)
+
         return handleOptional(templateBody, declType,
                               handleDefaultNull(default))
 
@@ -4291,11 +4295,13 @@ class CGUnionConversionStruct(CGThing):
         else:
             mozMapObject = None
 
-        hasObjectTypes = interfaceObject or arrayObject or dateObject or object or mozMapObject
+        hasObjectTypes = object or interfaceObject or arrayObject or dateObject or mozMapObject
         if hasObjectTypes:
             # "object" is not distinguishable from other types
             assert not object or not (interfaceObject or arrayObject or dateObject or callbackObject or mozMapObject)
             templateBody = CGList([], "\n")
+            if object:
+                templateBody.append(object)
             if interfaceObject:
                 templateBody.append(interfaceObject)
             if arrayObject:
@@ -4363,11 +4369,6 @@ class CGUnionConversionStruct(CGThing):
         returnType = "Result<Option<%s>, ()>" % actualType
         jsConversion = templateVars["jsConversion"]
 
-        # Any code to convert to Object is unused, since we're already converting
-        # from an Object value.
-        if t.name == 'Object':
-            return CGGeneric('')
-
         return CGWrapper(
             CGIndenter(jsConversion, 4),
             pre="unsafe fn TryConvertTo%s(cx: *mut JSContext, value: HandleValue) -> %s {\n"

components/script/dom/bindings/trace.rs

@@ -765,7 +765,12 @@ unsafe impl<T: JSTraceable + 'static> JSTraceable for RootedTraceableBox<T> {
 impl<T: JSTraceable + 'static> RootedTraceableBox<T> {
     /// DomRoot a JSTraceable thing for the life of this RootedTraceable
     pub fn new(traceable: T) -> RootedTraceableBox<T> {
-        let traceable = Box::into_raw(Box::new(traceable));
+        Self::from_box(Box::new(traceable))
+    }
+
+    /// Consumes a boxed JSTraceable and roots it for the life of this RootedTraceable.
+    pub fn from_box(boxed_traceable: Box<T>) -> RootedTraceableBox<T> {
+        let traceable = Box::into_raw(boxed_traceable);
         unsafe {
             RootedTraceableSet::add(traceable);
         }

components/script/dom/testbinding.rs

@@ -293,6 +293,14 @@ impl TestBindingMethods for TestBinding {
     fn ReceiveInterfaceSequence(&self) -> Vec<DomRoot<Blob>> {
         vec![Blob::new(&self.global(), BlobImpl::new_from_bytes(vec![]), "".to_owned())]
     }
+    #[allow(unsafe_code)]
+    unsafe fn ReceiveUnionIdentity(
+        &self,
+        _: *mut JSContext,
+        arg: UnionTypes::StringOrObject,
+    ) -> UnionTypes::StringOrObject {
+        arg
+    }
 
     fn ReceiveNullableBoolean(&self) -> Option<bool> { Some(false) }
     fn ReceiveNullableByte(&self) -> Option<i8> { Some(0) }

components/script/dom/webidls/TestBinding.webidl

@@ -228,6 +228,8 @@ interface TestBinding {
   TestDictionary receiveTestDictionaryWithSuccessOnKeyword();
   boolean dictMatchesPassedValues(TestDictionary arg);
 
+  (DOMString or object) receiveUnionIdentity((DOMString or object) arg);
+
   void passBoolean(boolean arg);
   void passByte(byte arg);
   void passOctet(octet arg);