error[E0277]: expected a `Fn<(proc_macro::TokenStream,)>` closure, found `unsafe extern "C" fn(i32, u32) -> u32 {foo}`
  --> $DIR/signature.rs:10:1
   |
LL | / pub unsafe extern "C" fn foo(a: i32, b: u32) -> u32 {
LL | |
LL | |     loop {}
LL | | }
   | |_^ call the function in a closure: `|| unsafe { /* code */ }`
   |
   = help: the trait `Fn<(proc_macro::TokenStream,)>` is not implemented for `unsafe extern "C" fn(i32, u32) -> u32 {foo}`
   = note: unsafe function cannot be called generically without an unsafe block
note: required by a bound in `ProcMacro::custom_derive`
  --> $SRC_DIR/proc_macro/src/bridge/client.rs:LL:COL
   |
LL |         expand: impl Fn(crate::TokenStream) -> crate::TokenStream + Copy,
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ProcMacro::custom_derive`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.