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/node.rs

@@ -28,7 +28,6 @@ use dom::bindings::js::RootedReference;
 use dom::bindings::js::{JS, LayoutJS, MutNullableHeap};
 use dom::bindings::reflector::{Reflectable, reflect_dom_object};
 use dom::bindings::str::{DOMString, USVString};
-use dom::bindings::trace::RootedVec;
 use dom::bindings::xmlname::namespace_from_domstring;
 use dom::characterdata::{CharacterData, LayoutCharacterDataHelpers};
 use dom::document::{Document, DocumentSource, IsHTMLDocument};
@@ -1562,7 +1561,7 @@ impl Node {
                 parent.ranges.increase_above(parent, index, count);
             }
         }
-        let mut new_nodes = RootedVec::new();
+        rooted_vec!(let mut new_nodes);
         let new_nodes = if let NodeTypeId::DocumentFragment = node.type_id() {
             // Step 3.
             new_nodes.extend(node.children().map(|kid| JS::from_ref(&*kid)));
@@ -1606,9 +1605,9 @@ impl Node {
             Node::adopt(node, &*parent.owner_doc());
         }
         // Step 2.
-        let removed_nodes = parent.children().collect::<RootedVec<_>>();
+        rooted_vec!(let removed_nodes <- parent.children());
         // Step 3.
-        let mut added_nodes = RootedVec::new();
+        rooted_vec!(let mut added_nodes);
         let added_nodes = if let Some(node) = node.as_ref() {
             if let NodeTypeId::DocumentFragment = node.type_id() {
                 added_nodes.extend(node.children().map(|child| JS::from_ref(&*child)));
@@ -2152,7 +2151,7 @@ impl NodeMethods for Node {
         };
 
         // Step 12.
-        let mut nodes = RootedVec::new();
+        rooted_vec!(let mut nodes);
         let nodes = if node.type_id() == NodeTypeId::DocumentFragment {
             nodes.extend(node.children().map(|node| JS::from_ref(&*node)));
             nodes.r()