We have two things, both custom: bytecode (what our VM uses) and IR (what Python is parsed into); there's no stable ABI yet since we practically don't need it, yet, IR is specified as a protobuf schema and bytecode is a specified via rust-native types in our framework for building instruction sets.

In principle, where WASM would go is it would be our compilation target instead of our own bytecode. We've thought about using WASM and, independent of that, about using LLVM compiler facilities. They don't have to go together, and, arguably, tapping into LLVM would be more interesting than WASM (I'm mentioning them together one of the intuitive perks from using WASM would seemingly be using WASM compilers too).

The thing is, we would, most likely, not have too much benefits from using LLVM or existing compilers from various languages into WASM; we compile things in a slightly different manner, and, I suspect, it would be possible to re-use some of the existing optimizations for control graph - but not the low-level ones.

As for WASM - we use a stackless VM, and WASM has an execution stack as a requirement - so we can't seemingly use WASM. That said, in practice we could use the same technique rust uses for compiling futures, and run on a WASM VM with a single stack frame holding our state machine and just doing everything in it. That's pretty close conceptually to how we have implemented in our custom VM now.