Strings may not contain a double quote unless it is escaped.
Reference:
https://doc.rust-lang.org/reference/tokens.html#string-literals
(This will still not colorize multi-line strings with an unescaped
newline, will still colorize "this\" as if it were a valid string,
and will still miscolor things when there is another string after
the closing quote of a two-line string. But those things can't
be helped -- line-based regexes cannot emulate a full parser.)
This fixes https://savannah.gnu.org/bugs/?61361.
Reported-by: Elias Jonsson <e@ejon.eu>
Bug existed since version 2.6.1, since the Rust syntax was introduced.
d57bfc057f