Function parse_in 

pub fn parse_in<'a, T>(
    psess: &'a ParseSess,
    tts: TokenStream,
    name: &'static str,
    f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
) -> PResult<'a, T>

Runs the given subparser f on the tokens of the given attr’s item.