Thread Safe Status: Send Sync? about melior HOT 2 OPEN

I haven't really thought about it yet. But Melior is just a thin layer on top of the MLIR C API. So I think at least it's easy to make basic objects Send and Sync (we can just mark them with the traits unsafely) if they are at the C++ level.

For the context and dialect loading, when we mark the context, module, and other related types Sync and invoke the same passes at the same time, I believe that would not be thread-safe according to the documentation. So if basic objects like Operation, Type, etc. should be marked Sync, we might have no choice but to introduce dynamic checks for that and mark a context object Sync as well.

Also, I think at least everything is Send because parallel passes exist.

from melior.

Just for notes, currently, regions and blocks can't be Sync because of #347.

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