Make Rust SDK build to WebAssembly for Fastly Compute

[pishposh]

Hi Statsig gang, this PR gates a few pieces of statsig-rust behind compiler directives for Wasm:

• Switches to single-threaded Tokio runtime
• Strips out local file event logging adapter
• Stubs process ID

With these changes, I managed to get a Rust project using this crate to build to wasm32-wasip1 for deployment to Fastly Compute.

Notes

For LocalFileEventLoggingAdapter, I ended up gating the whole thing, but I guess another option for Wasm would be to just skip file locking. Apparently WASIp1 has no notion of it, and the file-guard dep refuses to compile to Wasm, while the new file-locking methods in Rust’s stdlib (1.89+) fail at runtime.

reqwest doesn’t build to wasm, but we need to use a custom network provider for Fastly Compute anyway. Here’s how we’ve configured Statsig in our app’s Cargo.toml:

statsig-rust = { ... default-features = false, features = ["custom_network_provider"] }

Also app-side, it seemed necessary to annotate main() with #[tokio::main(flavor = "current_thread")] to get exposure events properly queued and sent.

I’m new to Rust, so please let me know if there’s better ways of doing these things!

[daniel-statsig]

Thanks for the PR @pishposh, I have merged it internally and it should got out with the next release. I'll leave the issue open until the release is public

[pishposh]

Thanks again @daniel-statsig!

[daniel-statsig]

Closing this issue as this was published in 0.9.0-rc.1 as 37a22ad