The #[repr(C)] attribute on the callback example is not necessary, since the type is not used in C.

Author: detly

src/ffi.md

@@ -310,7 +310,6 @@ referenced Rust object.
 Rust code:
 
 ```rust,no_run
-#[repr(C)]
 struct RustObject {
     a: i32,
     // Other members...