Rustfmt produces code with syntax error when r# pattern is used for names in macros

Consider the following Rust code:

macro_rules! example_macro {
    ($r#example: tt) => {
        $r#example
    };
}

fn main() {
    println!(example_macro!("Hello, world!"));
}

It's very strange, but it should work just fine, and print out its greeting. Rustc aggrees, happily compiling and running this code when asked to via cargo run. But, when I run cargo fmt on this code, it gets modified to the following:

macro_rules! example_macro {
    ($r#example: tt) => {
        r#e$xample
    };
}

fn main() {
    println!(example_macro!("Hello, world!"));
}

which, as can be seen in the macro's expansion, is wrong. This seems like a bug, since rustfmt is not supposed to break my code.

Start by reproducing the issue with the Rust macro snippet in the report and compare the source before and after cargo fmt. Trace the formatter behavior for raw-identifier macro metavariables; done means formatting preserves $r#example, and the formatted code still compiles and prints the greeting.