The problem is not that you can write bad stuff in rust, the problem is that this particular usage pattern can't be sanely coded in C. This is a public API that should inform users and enforce certain properties. A Results type does exactly that, you would have to go way out of your way to force invalid stuff through that type alone.
int crypt2(const char *phrase, const char *setting, char *out);
It would have averted the bug in a program which checks the int return value for error.
But we had that already: the null value, different from any valid string. That had to be done away with because programmers couldn't be relied upon to check for it.
Those same programmers are not going to check an integer value which doesn't even threaten them with a segfault.
(However, there is a GCC attribute that can be given to functions such that a diagnostic is produced if their value is not used: that could be profitably used with the above API.)