Excitingly, today! If you run the rustup install script on an arm64 device it should "Just Work", but if it doesn't end up detecting arm64 you can download rustup and/or the compiler manually.
* rustup - https://static.rust-lang.org/rustup/dist/aarch64-unknown-lin...
* rustc+cargo - https://static.rust-lang.org/dist/rust-nightly-aarch64-unkno...
Currently arm64 isn't a Tier 1 platform for us, however, so you may hit some bumps along the way. Please feel free to file issues so we know what to fix if you do!