A proc macro would still need to be invoked as either

    cpp_call! { fn foo() { ... } }

or

   #[cpp_call] fn foo() { ... }

The former can be written even with a macro_rules macro. Regardless, both macros have to work by parsing the whole input tokentree `fn foo ( ) { ... }` and emitting it back with an `unsafe` prepended. The macro invocation cannot expand to just `unsafe`.