[gpderetta]

Option 1 could be easily solved by having an atomic {} blocks that statically error if call any potentially async function in it. This is better as it document where an externally visible invariant is temporarily broken (i.e. reentrancy is required), instead of being implied by the code and potentially being broken a a future code change.

Implicit thread safety across async blocks of course break if you introduce actual shared multithreading in the language, while if you have atomic blocks at least you can build transactional memory on top.