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Does it handle macros_rules? (rustfmt doesn't)
Not all of it, the "transformed" righthand side part isn't supported yet. Formatting that part requires tinkering with the parser it's based on,
jinx-rust. It can be done and I'll get to it if Prettier Rust gets traction -
Does it format in a way that rustfmt does not need to do further changes if it runs?
It probably could if you added some lines to your rustfmt config, I might try to find out if that's something people want