I am writing a Rust program targeting wasm (it will *only* run in wasm and I know this). I access a "global style" variable such that you'd normally need to use arc<mutex<T>> or arc<refcell<T>>. Which of the following statements are true?

1. Rust in wasm currently runs single threaded and I don't need to worry about concurrent access, use refcell.
2. Rust in wasm is multithreaded but that's okay because arc<mutex<T>> is available
3. I'm screwed.