Auto merge of #12980 - jdm:proxychanges, r=nox

Fix ridiculous DOM proxy getter performance

This implements the missing shadowing checks that were causing us to take many slow paths when dealing with proxy objects. Verified by running `tests/html/binding_perf.html` before and after and observing a 12x improvement.

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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 #12357
- [X] These changes do not require tests because we can't test performance yet.

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components/script/dom/bindings/proxyhandler.rs

@@ -10,21 +10,51 @@ use dom::bindings::conversions::is_dom_proxy;
 use dom::bindings::utils::delete_property_by_id;
 use js::glue::GetProxyExtra;
 use js::glue::InvokeGetOwnPropertyDescriptor;
-use js::glue::{GetProxyHandler, SetProxyExtra};
+use js::glue::{GetProxyHandler, SetProxyExtra, GetProxyHandlerFamily};
 use js::jsapi::GetObjectProto;
 use js::jsapi::GetStaticPrototype;
 use js::jsapi::JS_GetPropertyDescriptorById;
 use js::jsapi::MutableHandleObject;
 use js::jsapi::{Handle, HandleId, HandleObject, MutableHandle, ObjectOpResult};
-use js::jsapi::{JSContext, JSObject, JSPROP_GETTER, PropertyDescriptor};
-use js::jsapi::{JSErrNum, JS_StrictPropertyStub};
-use js::jsapi::{JS_DefinePropertyById, JS_NewObjectWithGivenProto};
+use js::jsapi::{JSContext, JSObject, JSPROP_GETTER, PropertyDescriptor, DOMProxyShadowsResult};
+use js::jsapi::{JSErrNum, JS_StrictPropertyStub, JS_AlreadyHasOwnPropertyById};
+use js::jsapi::{JS_DefinePropertyById, JS_NewObjectWithGivenProto, SetDOMProxyInformation};
 use js::jsval::ObjectValue;
 use libc;
 use std::{mem, ptr};
 
 static JSPROXYSLOT_EXPANDO: u32 = 0;
 
+/// Determine if this id shadows any existing properties for this proxy.
+pub unsafe extern "C" fn shadow_check_callback(cx: *mut JSContext,
+                                               object: HandleObject,
+                                               id: HandleId)
+                                               -> DOMProxyShadowsResult {
+    // TODO: support OverrideBuiltins when #12978 is fixed.
+
+    rooted!(in(cx) let expando = get_expando_object(object));
+    if !expando.get().is_null() {
+        let mut has_own = false;
+        if !JS_AlreadyHasOwnPropertyById(cx, expando.handle(), id, &mut has_own) {
+            return DOMProxyShadowsResult::ShadowCheckFailed;
+        }
+
+        if has_own {
+            return DOMProxyShadowsResult::ShadowsViaDirectExpando;
+        }
+    }
+
+    // Our expando, if any, didn't shadow, so we're not shadowing at all.
+    DOMProxyShadowsResult::DoesntShadow
+}
+
+/// Initialize the infrastructure for DOM proxy objects.
+pub unsafe fn init() {
+    SetDOMProxyInformation(GetProxyHandlerFamily(),
+                           JSPROXYSLOT_EXPANDO,
+                           Some(shadow_check_callback));
+}
+
 /// Invoke the [[GetOwnProperty]] trap (`getOwnPropertyDescriptor`) on `proxy`,
 /// with argument `id` and return the result, if it is not `undefined`.
 /// Otherwise, walk along the prototype chain to find a property with that