During migration to 0.20 -> 0.21, I found it impossible to rounds trip Expression. There is Expression::raw() but I could not find "into_raw" or write capabilities.

See requirement at https://github.com/bytecodealliance/wasmtime/blob/7d88384c0fe087cd80a3d8ae1bbc80950b2b7d91/cranelift/codegen/src/isa/unwind/systemv.rs#L25