The following snippet formats with an extra space between pub and extern "C" if the arguments continue across multiple lines. i.e. rustfmt produces pub␣␣extern "C" instead of pub␣extern "C".

pub struct SomeCallback(
    pub extern "C" fn( 
        long_argument_name_to_avoid_wrap: u32,
        second_long_argument_name: u32,
        third_long_argument_name: u32,
    ),
);

Checked with

• rustfmt 1.5.1-stable (4b91a6e 2022-08-08) from 1.63.0 stable
• rustfmt 1.5.1-nightly (c07a8b4 2022-08-26) from a recent nightly