I don't think I have any specific resources to recommend, sorry.
I wrote most of the initial implementation of geckodriver and mostly learnt Rust from reading the official book, writing some small patches for Servo, and working on this project (of course); pretty much the same advice you would get coming from any other language.
I just chatted with one of the other major contributors to the project who said that he was able to learn on the job by reading the existing code, looking up concepts in the book or via web searches, and asking questions when necessary; although some parts of Rust undoubtedly have a significant learning curve it is empirically possible to contribute to an existing project without a significant amount of upfront study. This arguably isn't too different from learning most languages, although you are going to need to read up on the rules around e.g. references and borrowing sooner than you would need to look something up if you took the same approach to writing your first Python.
The type system and borrow checker mean that the compiler will tell you if you're doing something wrong which is both a blessing and a curse; it can be dispiriting to get dozens of compile errors when you are getting started, but once you have satisfied the compiler it's possible to be more confident that your code won't break in ways that are relatively common in Python (e.g. missing or broken code for error handling).
So I'm not sure that I answered your question, but in practice it didn't seem to be a major problem. Of course in a different environment — one where people were less enthusiastic about learning a technology their colleagues were raving about, for example — you might have a different experience.