Auto merge of #11872 - eddyb:back-to-roots, r=Ms2ger

Replace return_address usage for rooting with stack guards and convenience macros.

The existing `Rooted` and `RootedVec` users were migrated the the following two macros:
```rust
let x = Rooted::new(cx, value);
// Was changed to:
rooted!(in(cx) let x = value);
// Which expands to:
let mut __root = Rooted::new_unrooted(value);
let x = RootedGuard::new(cx, &mut __root);
```
```rust
let mut v = RootedVec::new();
v.extend(iterator);
// Was changed to:
rooted_vec!(let v <- iterator);
// Which expands to:
let mut __root = RootableVec::new();
let v = RootedVec::new(&mut __root, iterator);
```

The `rooted!` macro depends on servo/rust-mozjs#272.
These APIs based on two types, a container to be rooted and a rooting guard, allow implementing both `Rooted`-style rooting and `Traceable`-based rooting in stable Rust, without abusing `return_address`.

Such macros may have been tried before, but in 1.9 their hygiene is broken, they work only since 1.10.

Sadly, `Rooted` is a FFI type and completely exposed, so I cannot prevent anyone from creating their own, although all fields but the value get overwritten by `RootedGuard::new` anyway.
`RootableVec` OTOH is *guaranteed* to be empty when not rooted, which makes it harmless AFAICT.

By fixing rust-lang/rust#34227, this PR enables Servo to build with `-Zorbit`.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix rust-lang/rust#34227
- [x] These changes do not require tests because they are not functional changes

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components/script/dom/xmlhttprequest.rs

@@ -40,7 +40,7 @@ use hyper::mime::{self, Mime, Attr as MimeAttr, Value as MimeValue};
 use ipc_channel::ipc;
 use ipc_channel::router::ROUTER;
 use js::jsapi::JS_ClearPendingException;
-use js::jsapi::{JSContext, JS_ParseJSON, RootedValue};
+use js::jsapi::{JSContext, JS_ParseJSON};
 use js::jsval::{JSVal, NullValue, UndefinedValue};
 use msg::constellation_msg::{PipelineId, ReferrerPolicy};
 use net_traits::CoreResourceMsg::Fetch;
@@ -771,7 +771,7 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
     // https://xhr.spec.whatwg.org/#the-response-attribute
     fn Response(&self, cx: *mut JSContext) -> JSVal {
         unsafe {
-            let mut rval = RootedValue::new(cx, UndefinedValue());
+            rooted!(in(cx) let mut rval = UndefinedValue());
             match self.response_type.get() {
                 XMLHttpRequestResponseType::_empty | XMLHttpRequestResponseType::Text => {
                     let ready_state = self.ready_state.get();
@@ -808,7 +808,7 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
                     self.response.borrow().to_jsval(cx, rval.handle_mut());
                 }
             }
-            rval.ptr
+            rval.get()
         }
     }
 
@@ -1176,7 +1176,7 @@ impl XMLHttpRequest {
         let json_text = UTF_8.decode(&bytes, DecoderTrap::Replace).unwrap();
         let json_text: Vec<u16> = json_text.encode_utf16().collect();
         // Step 5
-        let mut rval = RootedValue::new(cx, UndefinedValue());
+        rooted!(in(cx) let mut rval = UndefinedValue());
         unsafe {
             if !JS_ParseJSON(cx,
                              json_text.as_ptr(),
@@ -1187,7 +1187,7 @@ impl XMLHttpRequest {
             }
         }
         // Step 6
-        self.response_json.set(rval.ptr);
+        self.response_json.set(rval.get());
         self.response_json.get()
     }