At first sight, it looks like a lot of stuff borrowed from Zig, Rust, of course C, and a few others.
One thing that got me curious though, is how lean the whole tooling is. They are using QBE, a (kind of funky) back-end compiler that is not LLVM (for once). While it only has 3 targets, 64-bit only for now, and with limited optimization, it's excessively small. Could be interesting to look at and experiment with:
https://c9x.me/compile/