It’s the reference implementation of the Raku programming language. https://rakudo.org/
The installer ran very quickly and the REPL worked out of the box. Nothing else to report yet, but I’m sure I’ll have more to say in the future.
Updated 1: Rakudo won’t build from source on Cygwin because libuv is still a bit dicey there. However, cascent/neovim-cygwin looks like a promising step forward.