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/document.rs

@@ -28,7 +28,6 @@ use dom::bindings::num::Finite;
 use dom::bindings::refcounted::Trusted;
 use dom::bindings::reflector::{Reflectable, reflect_dom_object};
 use dom::bindings::str::{DOMString, USVString};
-use dom::bindings::trace::RootedVec;
 use dom::bindings::xmlname::XMLName::InvalidXMLName;
 use dom::bindings::xmlname::{validate_and_extract, namespace_from_domstring, xml_name_type};
 use dom::browsingcontext::BrowsingContext;
@@ -115,6 +114,7 @@ use std::cell::{Cell, Ref, RefMut};
 use std::collections::HashMap;
 use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::default::Default;
+use std::iter::once;
 use std::mem;
 use std::ptr;
 use std::rc::Rc;
@@ -1009,18 +1009,15 @@ impl Document {
             }
         }
 
-        let mut touches = RootedVec::new();
+        rooted_vec!(let mut touches);
         touches.extend(self.active_touch_points.borrow().iter().cloned());
-
-        let mut changed_touches = RootedVec::new();
-        changed_touches.push(JS::from_ref(&*touch));
-
-        let mut target_touches = RootedVec::new();
+        rooted_vec!(let mut target_touches);
         target_touches.extend(self.active_touch_points
                                   .borrow()
                                   .iter()
                                   .filter(|t| t.Target() == target)
                                   .cloned());
+        rooted_vec!(let changed_touches <- once(touch));
 
         let event = TouchEvent::new(window,
                                     DOMString::from(event_name),