error: proc macro functions may not be `extern "C"`
  --> $DIR/signature.rs:10:1
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: proc macro functions may not be `unsafe`
  --> $DIR/signature.rs:10:1
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: mismatched derive proc macro signature
  --> $DIR/signature.rs:10:49
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   |                                                 ^^^ found u32, expected type `proc_macro::TokenStream`
   |
   = note: derive proc macros must have a signature of `fn(TokenStream) -> TokenStream`

error: mismatched derive proc macro signature
  --> $DIR/signature.rs:10:33
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   |                                 ^^^ found i32, expected type `proc_macro::TokenStream`
   |
   = note: derive proc macros must have a signature of `fn(TokenStream) -> TokenStream`

error: mismatched derive proc macro signature
  --> $DIR/signature.rs:10:38
   |
LL | pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
   |                                      ^^^^^^ found unexpected argument

error: aborting due to 5 previous errors