macro_rules! rand {
     ()=> { {
        enum Foo { Bar() };
        Foo::Bar as usize
        }
    };
}


fn main() {
    println!("{} {}", rand!(), rand!())
}

cpp_gcc

void f() {}
constexpr int g() {
    return reinterpret_cast<long long>(&f);
}
int x[g()];

423316089/source.cpp:5:10: error: size of array ‘x’ is not an integral constant-expression
 int x[g()];
          ^

cpp_gcc

void f() {}
template <decltype(&f) A> struct some{};
some<&f> x; //это другое

/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status