Single threaded as a feature (supporting WASM/WebAssembly?)

[TiemenSch]

Would you consider allowing a single-threaded approach where rayon/multithreading could be disabled by means of a feature flag?

My underlying use case would be that multithreading is unsupported for the regular WebAssembly target.

For the WebAssembly use case specifically, the alternative would be using a rayon adapter relying on SharedArrayBuffers and Rust nightly (https://github.com/RReverser/wasm-bindgen-rayon). Although that requires nightly Rust and specific headers (when embedding on websites), so that may be a bit much.

[TiemenSch]

I believe this would require reworking significant parts of the crate that currently support multi-threading/concurrency.
Even though multithreading is a feature toggle, a quick look around showed that concurrent programming crates like parking_lot and rayon are omnipresent, which might prove challenging to abstract away behind cfg feature/target_arch flags.

Anyway, to run (the currently failing) tests locally in WASM, you can do the following:

1. Add the wasm32-wasip1 target:

rustup target add wasm32-wasip1

2. Add the following to a config.toml file in a .cargo directory in your repo at .cargo/config.toml:

[target.wasm32-wasip1]
runner = "wasmtime run --dir ."

3. Run tests with:

cargo test --target wasm32-wasip1