Rust

Well you're writing a Compiler with extra features to tell you where compilation failed but without actually executing the code. If the language you are trying to implement has defined a BNF for its language its quite doable and depending on the features you want to implement almost easy. There are BNF interpreters and chumsky e.g. is close to reading like a BNF. But if you're building a language where you gotta find out the rules yourself it can take a while and finding all the edgecases can be a problem... But the project is big enough that most probably don't finish theirs