Function query_at_caret 

pub fn query_at_caret(text: &str, caret: usize) -> Option<MentionQuery>

The @-run ending at caret in text, if the caret is in one: an @ that opens a word (at the start, after whitespace or after punctuation that is not part of a word), followed by at most MAX_QUERY_LEN bytes with no newline and no other @. An @ inside a word — an email’s a@b — opens nothing.