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, 11 insertions, 11 deletions

diff --git a/components/script/devtools.rs b/components/script/devtools.rs
index 7533b159771..20394bf367a 100644
--- a/components/script/devtools.rs
+++ b/components/script/devtools.rs
@@ -22,7 +22,7 @@ use dom::element::Element;
 use dom::node::Node;
 use dom::window::Window;
 use ipc_channel::ipc::IpcSender;
-use js::jsapi::{JSAutoCompartment, ObjectClassName, RootedObject, RootedValue};
+use js::jsapi::{JSAutoCompartment, ObjectClassName};
 use js::jsval::UndefinedValue;
 use msg::constellation_msg::PipelineId;
 use script_thread::get_browsing_context;
@@ -38,24 +38,24 @@ pub fn handle_evaluate_js(global: &GlobalRef, eval: String, reply: IpcSender<Eva
         let cx = global.get_cx();
         let globalhandle = global.reflector().get_jsobject();
         let _ac = JSAutoCompartment::new(cx, globalhandle.get());
-        let mut rval = RootedValue::new(cx, UndefinedValue());
+        rooted!(in(cx) let mut rval = UndefinedValue());
         global.evaluate_js_on_global_with_result(&eval, rval.handle_mut());
 
-        if rval.ptr.is_undefined() {
+        if rval.is_undefined() {
             EvaluateJSReply::VoidValue
-        } else if rval.ptr.is_boolean() {
-            EvaluateJSReply::BooleanValue(rval.ptr.to_boolean())
-        } else if rval.ptr.is_double() || rval.ptr.is_int32() {
+        } else if rval.is_boolean() {
+            EvaluateJSReply::BooleanValue(rval.to_boolean())
+        } else if rval.is_double() || rval.is_int32() {
             EvaluateJSReply::NumberValue(FromJSValConvertible::from_jsval(cx, rval.handle(), ())
                                              .unwrap())
-        } else if rval.ptr.is_string() {
-            EvaluateJSReply::StringValue(String::from(jsstring_to_str(cx, rval.ptr.to_string())))
-        } else if rval.ptr.is_null() {
+        } else if rval.is_string() {
+            EvaluateJSReply::StringValue(String::from(jsstring_to_str(cx, rval.to_string())))
+        } else if rval.is_null() {
             EvaluateJSReply::NullValue
         } else {
-            assert!(rval.ptr.is_object());
+            assert!(rval.is_object());
 
-            let obj = RootedObject::new(cx, rval.ptr.to_object());
+            rooted!(in(cx) let obj = rval.to_object());
             let class_name = CStr::from_ptr(ObjectClassName(cx, obj.handle()));
             let class_name = str::from_utf8(class_name.to_bytes()).unwrap();