cxx-rust-demo

What is this?

This repo is mostly so I can more easily share my hacking experiments.

What is your actual goal?

I'm trying to set up a build where:

1. Emscripten is driving the build.
2. My C++ has a Rust dep.
3. Emscripten is using embind to interface with JS.
4. Rust is using wasm-bindgen to interface with JS.

To do this, my goal is to make wasm-bindgen output JS that can be consumed by Emscripten. Changes to both wasm-bindgen and Emscripten will be necessary.

How do I build this?

Install cargo and rust and emscripten and stuff.

You'll need Emscripten from https://github.com/walkingeyerobot/emscripten. Note the branch is called wbg-walkingeyerobot.

You'll need wasm-bindgen from https://github.com/walkingeyerobot/wasm-bindgen

I could probably do some more setup instructions here.

Build the Rust code.

cargo build -v

Build the C++ code and link in the Rust.

EMCC_DEBUG=1 EMCC_DEBUG_SAVE=1 em++ embind.cc random_point_generator.cc -lembind target/wasm32-unknown-emscripten/debug/libcxxrustdemo.a --post-js=post.js -sEXPORTED_FUNCTIONS=___wbg_randompointgeneratorrs_free,___wbg_rspoint_free,___wbindgen_describe_randompointgeneratorrs_get_random_point,___wbindgen_describe_randompointgeneratorrs_new,___wbindgen_describe_rs_add,___wbindgen_describe_rspoint_get_x,___wbindgen_describe_rspoint_get_y,_random_rs,_randompointgeneratorrs_get_random_point,_randompointgeneratorrs_new,_rs_add,_rspoint_get_x,_rspoint_get_y,___externref_drop_slice,___externref_heap_live_count,___externref_table_alloc,___externref_table_dealloc,___wbindgen_exn_store,___wbindgen_free,___wbindgen_malloc,___wbindgen_realloc -Wno-undefined -sWASM_BINDGEN

Run it

node a.out.js

You should see some random numbers get printed out like this:

testing js -> c++, random point:
58.11117446948042, 30.315140343167112
testing js -> rust, add:
7
testing js -> rust: random point:
16.48646923280117, 89.43444161227008

What's next?

For Emscripten

1. There's an issue with running stripping on the wasm-bindgen produced .wasm. llvm-objcopy errors out with invalid function export. I just commented out that part in Emscripten for now, but it needs to be fixed.

For wasm-bindgen

1. Start writing tests. Every test that wasm-bindgen currently has should have an --output=emscripten equivalent.
2. Fix all the tests that we just wrote that are broken.

For rustc

1. Automatically generate exports from the rust compiles to pass to Emscripten during link. Maybe see about the tmpdir argument here or maybe use llvm-nm on rust object files or possibly this exists already in some compiler output somewhere? Absolute worst case we can write a fake linker that's just a python script that forwards along the rust objects and -sEXPORTED_FUNCTIONS.

Once we can automatically pass the -sEXPORTED_FUNCTIONS stuff to emscripten's linker, -Wno-undefined will also become unnecessary. Several of those symbols (i.e. the describe ones) are ones that wasm-ld needs to keep alive but that wasm-bindgen will remove. These symbols do not need to be passed all the way through like other -sEXPORTED_SYMBOLS need to be.