I have some Rust structs that are made visible to C code via opaque pointers. The C code just passes these pointers around and never needs to dereference them. I provide new/free functions for the C code to use, and of course functions which actually do some work with this struct.
// This is explicitly not #[repr(C)] since it is meant to be opaque
struct MyRustStruct {
...
}
#[no_mangle]
pub extern fn my_rust_struct_new() -> *const MyRustStruct {
Box::into_raw(Box::new(MyRustStruct { ... }))
}
#[no_mangle]
pub extern fn my_rust_struct_free(x: *mut MyRustStruct) {
unsafe { Box::from_raw(x); }
}
Sometimes, for the Rust code to get access to this struct, I need to call a C function that will get me that pointer back from somewhere:
extern "C" fn get_the_struct_from_some_c_context(...) -> *mut MyRustStruct;
However, for each of these declarations of C-side extern functions, I get a warning like this on the return value, which corresponds to my Rust-side opaque struct:
warning: found enum without foreign-function-safe representation annotation in foreign module, consider adding a #[repr(...)] attribute to the type
--> src/state.rs:120:58
|
120 | extern "C" fn rsvg_state_get_stroke(state: *const RsvgState) -> *const PaintServer;
| ^^^^^^^^^^^^^^^^^^
How can I tell the compiler that even if I got a pointer to that struct from an extern function, the external code does not need to dereference it, and thus I don't need to #[repr(C)] my struct?
