These changes add compile-time assertions that:
* any type that implements the Serializable/Transferable trait has a
`[Serializable]` or `[Transferable]` annotation in the interface WebIDL
* any WebIDL interface with the `[Serializable]` or `[Transferable]`
annotation implements the corresponding trait
This is useful because it means that WebIDL definitions will be less
confusing if you're trying to figure out whether Servo supports
serializing/transferring a particular interface type. It also makes
fixing #21715 in the future a little bit easier, because the annotations
will remain up to date.
Testing: compile-time only; no point in writing tests for this since it
involves webidl codegen.
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Signed-off-by: Josh Matthews <josh@joshmatthews.net>
