Status of rust+C bindings for wasm targets

[elichai]

Hi,
There are a lot of important dependencies in the rust ecosystem that use ffi to C fia the cc crate or similiar.
which tools and targets currently support that?
in the past asmjs-unknown-emscripten worked and together with cargo-web you could just run cargo web test to know if your library compiles+run on wasm/emscripten.
Sadly emscripten is broken on stable for a while now rust-lang/rust#66916.

wasm32-unknown-unknown works in combination with clang-8 only(probably a bug), not before and not later(doesn't work on clang-9 and clang-10) https://github.com/alexcrichton/cc-rs/issues/378
so wasm-pack can't be used here.

wasi seem to have the same problem, running cargo wasi test fails to find the sysroot(fatal error: 'string.h' file not found) although it seems like you can download sysroots for wasi or compile them yourself https://bytecodealliance.github.io/wasmtime/wasm-c.html

Is there any tool out there that can test rust+C code out of the box? (We had emscripten tests in the CI until it broke and we want to continue testing for wasm target in the CI)

[kripken]

@tlively has made a lot of progress on the emscripten side of things, and I think can update on the status there.

[tlively]

Currently the wasm32-unknown-emscripten and asmjs-unknown-emscripten targets are the only targets that can be linked with C code because all the other wasm targets use a different ABI for compatibility with wasm-bindgen. Unfortunately, I’m the only one maintaining the emscripten targets in Rust and I don’t have a lot of time to spend on it, so I’ve mostly just been trying to fix critical bugs as I become aware of them. I’ll take a look at the one you linked to above.

[DzmitryFil]

I am also struggling to compile simple c libraries like imgui to wasm.
@alexcrichton maybe you could help us, and share couple of words on how to build wasm32-unknown-unknown with wasi sysroot, or some other way? Or building rust+C to wasm is a lost cause for near future?

[alexcrichton]

If you want to integrate Rust and C on the web your best bet is likely going to be Emscripten. The next best bet is going to be using WASI (and wasi-sdk), but that requires getting a WASI implementation working on the best which isn't always as available. Finally you can try to get wasm32-unknown-unknown working. While that's technically possible for C/C++ there aren't really any maintained toolchains targeting that for C/C++, so you're on your own making such a toolchain.

[DzmitryFil]

Well, wasm32-emscripten target doesn't get much love from rust community either. Anyway, thanks for reply, it's nice to know status of these things.

[RReverser]

Hi, I'm a WebAssembly DevRel at Google and thought I'd chime as I've actively worked with both Rust (including wasm-bindgen) and Emscripten at various points.

Admittedly, this is a simple demo, but, as far as I can tell, wasm32-unknown-emscripten target works fine for combined Rust + C code.

Of course, you can't expect to use both #[wasm_bindgen] and Emscripten bindings at the same time - you need to choose a single tool that will be providing runtime bindings and generating the JavaScript wrapper - but as long as one side is a "driver" and another contains pure portable code, things should work.

Is there a specific API or minimal code sample you could show that you're struggling with?

[DzmitryFil]

Wow, thanks for offering help. My problem is compiling imgui-rs crate to wasm32-unknown-unknown target, i have no idea how to solve this, because my c++ knowledge is pretty limited.
As of emsripten, stuff like rust-lang/rust#66916 doesn't give much confidence. Also stdweb, which supports emscripten target, seems abandoned (no activity for 8 months).
Also, I do you use wasm_bindgen quite extensively. For example, how would i replace wasm-bindgen-futures crate and work with js promises from inside rust, if i choose to use wasm32-unknown-emscripten?

[RReverser]

I can't say much about cargo-web TBH because while I've seen it, personally never used it. In example above you can see that I'm just using regular cargo build command instead.

As of emsripten, stuff like rust-lang/rust#66916 doesn't give much confidence.

I haven't seen that error, and given that 1) it's from half a year ago and 2) examples I've tried work fine, I suspect it might have been some temporary issue that has been resolved since.

Also, I do you use wasm_bindgen quite extensively. For example, how would i replace wasm-bindgen-futures crate and work with js promises from inside rust, if i choose to use wasm32-unknown-emscripten?

Right, unfortunately, you can't, since these are two different toolchains. You need to either deal with Web APIs and JS values from Emscripten, or from Rust, but not both.

I've experimented in the past with using Embind in Rust (shameless plug: https://www.youtube.com/watch?v=zxIbTfsOJZE), but not too long afterwards wasm-bindgen appeared and it wasn't clear if there are benefits to using both, so never made it into a full package. Either way, if really necessary, it should be possible to build your own bindings from Rust to Emscripten's APIs or vice versa, but that would be complicated.

It might be easier to understand what issues you're running into with wasm32-unknown-unknown for your particular library, but that's probably best discussed in an issue on imgui-rs repo where author could chime in too and probably help out.

[DzmitryFil]

Yeah, i probably could successfully compile rust with emscripten, but wasm-bindgen absence seems like a showstopper.

It might be easier to understand what issues you're running into with wasm32-unknown-unknown for your particular library, but that's probably best discussed in an issue on imgui-rs repo where author could chime in too and probably help out.

The thing is, my issues aren't specific to this library. When you compile any c library that uses standard headers like stdio.h or string.h you get compilation errors.
servo/rust-stb-image#94
https://github.com/alexcrichton/cc-rs/issues/378#issuecomment-469331613
I tried manually inserting headers from wasi-libc, and including them into cc build, but then i got linking errors like

note: rust-lld: error: unable to find library -lstdc++

Imgui doesn't really need any os-specific stuff to work, i've seen it compiled with emscripten on the web, but i have no idea how to make it work with wasm32-unknown-unknown.