error: derive proc macro has incorrect signature
  --> $DIR/signature.rs:10:1
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected normal fn, found unsafe fn
   |
   = note: expected signature `fn(proc_macro::TokenStream) -> proc_macro::TokenStream`
              found signature `unsafe extern "C" fn(i32, u32) -> u32`

error: aborting due to 1 previous error