Function legacy_doc_line_error 

pub fn legacy_doc_line_error(source: &str, span: Span) -> CompileError

#2203 (DECISION E): the bynk.lex.legacy_doc_line error for the legacy --| doc line at span (a TokenKind::DocLine token), with the fix to --- text. A --| with no text has no one-line spelling (a bare --- opens a block), so it gets no fix.