It's a reasonable question. Aside from the nice data encapsulation of enums (which make them more like unions), the answer is basically "they're enforced". Mos

It's a reasonable question. Aside from the nice data encapsulation of enums (which make them more like unions), the answer is basically "they're enforced". Most C libraries and system calls return integers not enums. Often the meaning of those integers is hard coded in a header somewhere. There's no guarantee that the number returned will be one of the values in the header. When a library does return enums, there's no guarantee the value returned is actually in the enum. It could be anything. When you pattern match an enum in C, you have no checks to make sure you catch all the cases, or don't drop a break;. All of the above means that in C, you're relying on the programmer to cross check they haven't made a mistake. Rust does it all for you - if you match against an enum type, you have to handle all the cases. You can't return anything other than the type you said you would. It basically does all the work that the programmer has to do in C. And most importantly, it does it every time you recompile. Even if the first person to call a function in C is meticulous, the members in the enum might change later.