[SOLVED] How can you link a Rust library with a C app into one WASM file using Emscripten?

How can you link a Rust library with a C app into one WASM file using Emscripten?

I have a project that links a C library with a C app using Emscripten. I would like to replace the C library with a library written in Rust. How can this be accomplished?

I know that there are two relevant WASM targets in Rust, the normal wasm32-unknown-unknown one and the wasm32-unknown-emscripten one. I'm guessing the latter target is necessary for it to work with the Emscripten compiler? But how do I tell Emscripten to use a .wasm library rather than a .a library?

There may be other ways of linking Rust and C code together into a .wasm but I think I do need to stick with Emscripten as I require its Asyncify feature (and in the future will use its WASMFS system too.)

Solution

This is actually quite straightforward!

So first, make sure your Rust project is set to build a static library:

[lib]
crate-type=["staticlib"]

Then compile your Rust project with the correct target, wasm32-unknown-emscripten:

cargo build --target=wasm32-unknown-emscripten

Then you can simply link the static lib into your C/C++ app. Note that you'll probably want to use a variable to handle the different paths for dev and release builds. I'm using CMake:

if (${CMAKE_BUILD_TYPE} STREQUAL Release)
    set(REMGLK_RS_LIB_FOLDER ${CMAKE_CURRENT_SOURCE_DIR}/remglk/target/wasm32-unknown-emscripten/release)
else()
    set(REMGLK_RS_LIB_FOLDER ${CMAKE_CURRENT_SOURCE_DIR}/remglk/target/wasm32-unknown-emscripten/debug)
endif()

target_link_libraries(app_target ${REMGLK_RS_LIB_FOLDER}/libremglk_capi.a)

Lastly, I needed to change the linker language to C++, as otherwise using panic!() seems to result in exception errors when the final app is only C, not C++. Depending on what features of Rust you use this may not be necessary.

set_target_properties(app_target PROPERTIES LINKER_LANGUAGE CXX)