Graal Native Mill Executables

[lihaoyi]

I just landed experimental support for a Graal native-image client for Mill, download the latest ./mill script from the Mill repo and set your Mill version to 0.12.5-46-bbcc16-native to try it out. It seems to cut about 100ms off the Mill command overhead, and opens the door to having a smarter client going forward (e.g. better argument parsing, downloading the necessary JVM, etc.)

Currently supports mac-aarch64 and linux-amd64, Windows support and other OS-CPU combinations is WIP #4274