Can't they just expose the parser as a library? Actually, it looks like they did, with the rustc crate.
Hand-writing a parser for some other language leads to madness - just ask the folks who've done SWIG, GDB, or most IDE syntax-checkers. You'll inevitably get some corner-cases wrong, or the language definition will change underneath you long after you've ceased to maintain the tool. Instead, the language should just expose its compiler front-end as a library, and then you can either serialize the AST to some common format for analysis outside the language or build your tools directly on top of that library.
Hand-writing a parser for some other language leads to madness - just ask the folks who've done SWIG, GDB, or most IDE syntax-checkers. You'll inevitably get some corner-cases wrong, or the language definition will change underneath you long after you've ceased to maintain the tool. Instead, the language should just expose its compiler front-end as a library, and then you can either serialize the AST to some common format for analysis outside the language or build your tools directly on top of that library.