Can I run embedded wasmer on a wasm client?

[seniorjoinu]

Summary

My environment is webassembly itself (wasmtime). I write programs in rust, compile them to wasm32-unknown-unknown and then execute them. This flow works fine.
But now I need to write a program which can take another wasm program as an input, execute it and process the result. Such a wasmception!

How could I achieve that?

Additional details

I tried to compile hello_world to wasm32-unknown-unknown, but it looks like it can't be compiled that way.

[aristotaloss]

Seconding this, any way to get wasmer working as embedded library when targeting wasm32-unknown-unknown or even wasm32-wasi?

[JoshFourie]

tldr: I think that this is possible but painful and requires an overhaul of the wasmer design.

If you are working with rust, you can do this with wasm-bindgen (example). I found managing imports/exports to wasm binaries painful and I like the wasmer api, so I would like support for this feature.

---

I saw in-tree that wasmer/api/src/js was recently added so that wasmer will be compiled to WebAssembly to run in a JavaScript host. I pulled that out of the wasmer tree and rolled a patch to expose the js api ahead of the wasmer release schedule.

This only worked for wasm binaries that were less than 4kb and a StackOverflow question recommended using either a web worker or WebAssembly.InstantiateStreaming.

I did not want to use a web worker at this stage, so I had to work around a few things:

1. WebAssembly.InstantiateStreaming takes a Promise of a Response of the source of WebAssembly.Module not a compiled WebAssembly.Module: docs
2. wasmer::Instance constructor takes a wasmer::Module which polyfills the ModuleTypeHints struct with import/export information by parsing the binary, which is not available with WebAssembly.InstantiateStreaming

To get this to work, I had to build the ModuleTypeHints information without parsing the binary. In my case, this information was known at compile-time, so I rolled a new resolver implementation and wasmer::Instance constructor. See an example.

I hacked out a patch and an example repository but I am not maintaining or working with that library. It does not properly handle results returned from wasm exports. I think that the wasmer team can probably fix this much more intelligently.