I think I've hinted my "secret" project in the past, but finally I can announce that I'm working on the type checker for Lua, dubbed Kailua, in my daily job. Written in Rust, of course!
I was working on this project for about 5 months by now, and I gave an initial talk on Kailua in some biggish gamedev conference last week. (The slides are, unfortunately, only available in Korean.) To give a rationale, we started using Lua for both client and server out of necessity and various constraints but it had been very hard to maintain more than 300K lines of Lua code base. As we didn't initially anticipate the possibility of type checkers (and we had some unique constraints that forbade most available solutions, including the consistent but prevalent usage of globals) I have officially started the research. I ended up with a practical type system based upon Tidal Lock but with a significant modification.
The current checker is still in the early stage but seems fine as a proof of concept. I'm not sure if it will be eventually released in public, but now you know that there is yet another company using Rust. 