Rust

Ah, I thought GAT was a misspelling of GADT. And traits sound more like type classes... So essentially this is another version of ML module signatures / Haskell type classes. I'm guessing it looks more friendly to folks coming from C/C++.

Stability is pre-standardization like C/C++ attributes, or would be if there were multiple compilers out there. (gcc was on its way at some point, right?) Many mostly-single-vendor languages have stable v. implementation gizmos. At least this term is fairly well shared.

Rust isn't useful in my daily life (heavily shared memory structures being changed rapidly), so I haven't really poked at it.