Function complete_known_attribute_input 

pub(crate) fn complete_known_attribute_input(
    acc: &mut Completions,
    ctx: &CompletionContext<'_>,
    colon_prefix: &bool,
    fake_attribute_under_caret: &Attr,
    extern_crate: Option<&ExternCrate>,
) -> Option<()>

Complete inputs to known builtin attributes as well as derive attributes