Hey all! I'm one of the co-founders of CodeSandbox and a big fan of Rust. Rust support is something that we've been working on for a while, but the recent addition of Docker support in CodeSandbox really enabled it. Happy to answer any questions!
I know https://play.rust-lang.org/ is pretty limited as well.
I'd say that CodeSandbox has a strong focus on extending the existing workflow for developers. That's why we have a VSCode integration, a GitHub integration with a GitHub App that creates a running dev env for every branch/PR, and we make sure that generally all editor features you expect (autocomplete, go to definition, hover info, etc) are available for the languages that we support.