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

detly · Gankra · commit b9ca313e687c · 2021-06-22T12:02:20.000-04:00
diff --git a/src/ffi.md b/src/ffi.md
@@ -310,7 +310,6 @@ referenced Rust object.
 Rust code:
 
 ```rust,no_run
-#[repr(C)]
 struct RustObject {
     a: i32,
     // Other members...
