Remove the use of unsafe code in the layout wrappers of the DOM. The
main change here is that `unsafe_get()` no longer needs to be an unsafe
method, which allows us to transitively remove or reduce unsafe blocks
from callers. The function itself is not renamed, because it's still
a bit dangerous to start removing the layers of abstraction from actual
DOM nodes.
In addition `init_style_and_opaque_layout_data` can be merged into
`initialize_data`, which removes one more unsafe method.
Finally, a "Safety" section is added to some unsafe methods.
* Add `no_trace` option to JSTraceable derive
* NoTrace wrapper
* Port some types to no_trace schematics
* Fixing my unsafe mistakes (not tracing traceables)
* Add docs & safety guards for no_trace
Safety guards (trait shenanigans) guarantees safety usage of `no_trace`
* Port canvas_traits to no_trace
* Port servo_media to no_trace
* Port net_traits to no_trace
* Port style to no_trace
* Port webgpu to no_trace
* Port script_traits to no_trace
* Port canvas_traits, devtools_traits, embedder_traits, profile_traits to no_trace
* unrooted_must_root lint in seperate file
* Add trace_in_no_trace_lint as script_plugin
* Composable types in must_not_have_traceable
* Introduced HashMapTracedValues wrapper
* `HashMap<NoTrace<K>,V>`->`HashMapTracedValues<K,V>`
* Port rest of servo's types to no_trace
* Port html5ever, euclid, mime and http to no_trace
* Port remaining externals to no_trace
* Port webxr and Arc<Mutex<_>>
* Fix spelling in notrace doc
http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.
This is expected since `Box::new` is defined as:
```rust
impl<T> Box<T> {
#[inline(always)]
pub fn new(x: T) -> Box<T> {
box x
}
}
```
With inlining, it should compile to the same as box syntax.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.
I don't want to do such a gratuitous rename, but with all the other types
now having "Dom" as part of their name, and especially with "DomOnceCell",
I feel like the other cell type that we already have should also follow
the convention. That argument loses weight though when we realise there
is still DOMString and other things.
It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
