Earlier in this chapter, we used a struct as a parameter. In C, there is nothing to stop the developer passing a structure as a parameter:

struct MyStruct; 
struct MyOtherStruct; 
void pass_struct(struct MyStruct *arg); 
void pass_struct2(struct MyOtherStruct *arg2); 

MyStruct and MyOtherStruct are known as opaque structs. The name is exposed, but the private parts aren't.

Handling a struct within Rust is not as simple as you'd first think, but then it's not that difficult either. The only difference is that we use an empty enum instead of a struct when interfacing with the C library. This creates our opaque type that stores the information from the C opaque type. As the enum is empty, we can't instantiate it and, more importantly, as MyStruct and MyOtherStruct aren't the same, we have type-safety and so can't get them mixed up:

enum MyStruct {}; 
enum MyOtherStruct {}; 
extern "C" 
{ 
    pub fn pass_struct(arg: *mut MyStruct); 
    pub fn pass_struct2(arg: *mut MyOtherStruct); 
}