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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1 files changed, 2 insertions, 2 deletions

diff --git a/components/script/dom/websocket.rs b/components/script/dom/websocket.rs
index 1409936c6c7..cc8c6f74d54 100644
--- a/components/script/dom/websocket.rs
+++ b/components/script/dom/websocket.rs
@@ -23,7 +23,7 @@ use dom::eventtarget::EventTarget;
 use dom::messageevent::MessageEvent;
 use dom::urlhelper::UrlHelper;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
-use js::jsapi::{JSAutoCompartment, RootedValue};
+use js::jsapi::JSAutoCompartment;
 use js::jsapi::{JS_GetArrayBufferData, JS_NewArrayBuffer};
 use js::jsval::UndefinedValue;
 use libc::{uint32_t, uint8_t};
@@ -585,7 +585,7 @@ impl Runnable for MessageReceivedTask {
         unsafe {
             let cx = global.r().get_cx();
             let _ac = JSAutoCompartment::new(cx, ws.reflector().get_jsobject().get());
-            let mut message = RootedValue::new(cx, UndefinedValue());
+            rooted!(in(cx) let mut message = UndefinedValue());
             match self.message {
                 MessageData::Text(text) => text.to_jsval(cx, message.handle_mut()),
                 MessageData::Binary(data) => {