Function is_raw_string_prefix 

fn is_raw_string_prefix<T>(iter: &mut MultiPeek<T>) -> bool
where
    T: Iterator,
    T::Item: RichChar,

Returns true if the r just consumed opens a raw string literal, i.e. the run of #s that follows it ends in a ". Peeking a single # is not enough to tell a raw string apart from a raw identifier such as r#struct.