Add C++ Support for WebAssembly

[NathanKolbas]

Is your feature request related to a problem? Please describe.
I have a Rust crate that uses some C++ code. The C++ code is compiled when the Rust code is compiled using build.rs with cc-rs. Since the target is wasm32-unknown-unknown when compiling for web with flutter_rust_bridge_codegen build-web cc-rs uses the C compiler. However, wasm32-unknown-unknown is needed since wasm-bindgen uses this build to generate the JavaScript glue. I can compile the code just fine using emscripten with target wasm32-unknown-emscripten.

Describe the solution you'd like
Would it be possible to support emscripten so that we can take advantage of C++ in WebAssembly? What would it take for flutter_rust_bridge to support C++?

Describe alternatives you've considered
I tried to compile separately and then link after the fact but I run into error: cannot import from modules (env) with --no-modules (missing symbols) and the linker ends up using the C linker instead of the C++ due to the target.
Perhaps there is a workaround that I am not aware of? Maybe there is something better that I should be using instead of emscripten.

Additional context
I wouldn't mind helping out with this or working on a PR but would like some direction first or how web support currently works today.

Thanks!

[welcome]

Hi! Thanks for opening your first issue here! 😄

[fzyzcjy]

Good question! Theoretically speaking this should work, because frb's build-web does not do anything special, and calls the standard toolchains under the hood. My naive guess is that, maybe firstly try to check how to build a normal rust + c++ + web project, and then see how the logic of build-web command needs to be changed for that.

[NathanKolbas]

Gotcha, sounds good. I can already compile the Rust + C++ code and have a working example on the web. I can mess with the logic of the build-web command and see where that goes. I am going to assume from what I've tested so far that it will be more complicated than that.

Some questions, what does frb use wasm_bindgen for? In the generated frb_generated.rs file I notice a wasm section:

/// cbindgen:ignore
#[cfg(target_family = "wasm")]
mod web {
   ...
}
#[cfg(target_family = "wasm")]
pub use web::*;

How does frb use this?

[fzyzcjy]

looking forward to it!

Some questions, what does frb use wasm_bindgen for?

to generate some low-level bindings that frb can further use