How can I use wasm-bindgen from a program compiled with Emscripten? - c

Question

I am trying to link a Rust library containing code generated by wasm-bindgen against a program written in C which I would like to compile with Emscripten. My MRE is as follows:
On the Rust side, I have Cargo.toml:
[package]
name = "rust_project"
version = "0.1.0"
edition = "2021"
[lib]
crate-type = ["staticlib"]
[dependencies]
wasm-bindgen="0.2"
and in lib.rs I have:
use wasm_bindgen::prelude::*;
#[wasm_bindgen]
extern "C" {
#[wasm_bindgen(js_namespace = console)]
fn log(s: &str);
}
#[no_mangle]
pub extern "C" fn call_from_c() {
log("Hello, World!");
}
As a first step, I compile this with cargo build --target wasm32-unknown-unknown which produces a librust_project.a. I then set up the following C project with main.c:
/* forward declare the function from Rust */
void call_from_c();
/* call the function from main */
int main() {
call_from_c();
return 0;
}
and CMakeLists.txt:
cmake_minimum_required(VERSION 3.5)
project(c_project)
add_executable(c_project main.c)
target_link_libraries(c_project /path/to/librust_project.a)
Finally, I attempt to put it all together using the Emscripten toolchain as follows:
cmake -DCMAKE_TOOLCHAIN_FILE=path/to/Emscripten.cmake ../
make
Which is where something appears to go wrong at the linking stage with emcc reporting that __wbg_log_941ab916ed5a24bd is an undefined symbol. I suspect that this symbol (among others) is being stripped out as part of an optimization effort but I am not sure at what stage or how I can disable this optimization.
Adding the following linker options in CMake results in compilation with a warning about the undefined symbol:
target_link_libraries(c_project
path/to/librust_project.a
"-s EXPORTED_FUNCTIONS=[\"_main\",\"___wbg_log_941ab916ed5a24bd\"]"
"-s ERROR_ON_UNDEFINED_SYMBOLS=0")
but I believe these missing symbols are problematic and when I run wasm-bindgen (the CLI tool) over c_project.wasm I get the following error:
import of `__wbg_log_941ab916ed5a24bd` doesn't have an adapter listed
How can I prevent the wasm-bindgen imported/exported functions from being stripped during this process?

Answer 1

This is a work-in-progress answer, which hopefully I will be able to turn into a complete answer in the near future. If not perhaps, it will at least serve as a starting point for others who have come down this path.
I have been working on making my MWE even more minimal by removing CMake and Emscripten and just compiling with Clang directly. This is sufficient since I don't need to worry about the standard library in this MWE.
My command for compiling then becomes:
clang -Wall --target=wasm32-unknown-unknown --no-standard-libraries \
-Wl,--export-all \
-Wl,--no-entry \
-Wl,-L/path/to/librust_project_a
-Wl,-lrust_project
-o main.wasm main.c
Worthwhile noting is that I can trigger the following wasm-bindgen error by adding/removing the --export-all linker argument. Suggesting that the LLVM linker was responsible for removing this section before it could be processed with wasm-bindgen.
import of `__wbg_log_941ab916ed5a24bd` doesn't have an adapter listed