This change makes changes to allow Servo to compile with the 1.78
version of Rust:
- Dead code is removd (Rust seems to have gotten better at detecting
it).
- The `FlowRef` `DerefMut` is updated according to @SimonSapin's advice
[^1].
- The `imports.rs` now explicitly silences warnings about unused
imports.
[^1]: https://github.com/servo/servo/issues/6503#issuecomment-2066088179
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This creates a sharp distinction between `Arc<Flow>`s, which may be
owned by anyone, and `FlowRef`s, which may only be owned by the
traversal code. By checking the reference count, we ensure that a `Flow`
cannot be pointed to by `Arc`s and `FlowRef`s simultaneously.
This is not a complete fix for #6503, though it is a necessary start
(enforcing the no-aliasing rule of `FlowRef::deref_mut` will require far
more work).
Fixes#14014
Cycles were being created in the flow tree since absolutely positioned
descendants had pointers to their containing blocks. This adds
WeakFlowRef (based on Weak<T>) and makes these backpointers weak
references. This also harmonizes our custom Arc<T>, FlowRef, to be
consistent with the upstream implementation.
Fixes#4915.
