I considered that (talking about clang-format here, not rustfmt), but there are cases where some files are formatted while others are not, such that, you can't go around formatting the whole project to your liking and then expect it to return to the same, once you go back to the project's configuration, for committing.

Here, the difference between clang-format and rustfmt comes up, because unlike clang-format, which has the required configurations, rustfmt does not have the configuration I require. The little part that is there, has not been fully implemented and a lot of the configuration hasn't even been defined.
Meaning that even if I were to put the time into contributing to rustfmt, I will soon reach a point where I will have to be setting up groundwork. That's where I will fail, because I am too new to Rust to be able to define formatting for undecided conditions, many of which I don't even know about yet.