Function rustc_builtin_macros::source_util::expand_file

pub fn expand_file(
    cx: &mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream
) -> Box<dyn MacResult + 'static>

file!(): expands to the current filename */ The source_file (loc.file) contains a bunch more information we could spit out if we wanted.