Fail if singleline string has multiple lines

mesonbuild/mparser.py

@@ -141,6 +141,9 @@ class Lexer:
                     elif tid == 'dblquote':
                         raise ParseException('Double quotes are not supported. Use single quotes.', self.getline(line_start), lineno, col)
                     elif tid == 'string':
+                        # Handle here and not on the regexp to give a better error message.
+                        if match_text.find("\n") != -1:
+                            raise ParseException("Use ''' (three single quotes) for multiline strings.", self.getline(line_start), lineno, col)
                         value = match_text[1:-1].replace(r"\'", "'")
                         value = newline_rx.sub(r'\1\n', value)
                         value = value.replace(r" \\ ".strip(), r" \ ".strip())