9 days ago, I pushed to https://github.com/thepowersgang/mrustc/pull/151 a more complete mrustc on macOS branch including Apple Silicon support, to have an alternative Rust compiler available for the platform.
The patch-set is tested with Rust 1.29.0. mrustc’s Rust 1.39.0 support is not quite complete yet at this point in time.
Notes: Rust 1.29.0 does not officially support Apple Silicon Macs. And my port of the standard libraries to that platform could be quite incomplete. (Tested with building Cargo for the platform only)