It absolutely can be done [1] but it is a lot of effort to do high quality error recovery. It's easier in languages with natural "synchronization points" [2], harder in languages which don't have them.

Having built a lot of protobuf tooling, I'd estimate that protobuf largely falls into the former camp; most of your time working with .proto files is operating on fields which terminate using `;` at the end of the line (though multi-line is also possible).

[1] source: I've done it for SQLite SQL at https://github.com/LalitMaganti/syntaqlite/

[2] e.g. SQL naturally has this at statement and expression boundaries which covers almost all of the cases people care about.