Compiling garbage-game-lib for WASM is non-trivial. Instructions for doing this are here:

https://godot-rust.github.io/book/exporting/html5.html

However, I found some tiny differences and specific details which I needed to do to get the instructions to work. For one, I needed to get this specific release of emscripten (as recommended by the godot-rust book guide: Emscripten 3.1.21-git (3ce1f726b449fd1b90803a6150a881fc8dc711da)):

https://github.com/emscripten-core/emsdk/releases/tag/3.1.21

Also, I needed to run the following instead of ./emsdk install tot:

./emsdk install 3.1.21

This may be obvious for someone familiar with emscripten, but it was not for me. Essentially, even if downloading the 3.1.21 release, if one uses tot (tip-of-tree), they end up installing a later version of emscripten - and that will have compatibility issues.

Next, I was not able to compile the Rust code with the following:

cargo +nightly build --target=wasm32-unknown-emscripten --release

Indeed, one must specify the specific nightly version recommended by the godot-rust book guide (rustc 1.65.0-nightly (8c6ce6b91 2022-09-02)). However, even doing that did not work for me:

cargo +nightly-2022-09-02 build --target=wasm32-unknown-emscripten --release

So how did I get it to work? Well, I made rust-toolchain.toml with the following contents:

[toolchain]
channel = "nightly-2022-09-02"
components = [ "rustfmt", "clippy" ]

And then I could compile the WASM version with:

cargo build --target=wasm32-unknown-emscripten --release

Compiling the non-WASM version was pretty normal (only setup for Linux):

cargo build --release