C attribute: nodiscard (since C23)

struct [[nodiscard]] error_info { int status; /*...*/ };
struct error_info enable_missile_safety_mode() { /*...*/ return (struct error_info){0}; }
void launch_missiles() { /*...*/ }
void test_missiles() {
 enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
 launch_missiles();
}
struct error_info* foo() { static struct error_info e; /*...*/ return &e; }
void f1() {
 foo(); // nodiscard type itself is not returned, no warning
}
// nodiscard( string-literal ):
[[nodiscard("PURE FUN")]] int strategic_value(int x, int y) { return x ^ y; }
 
int main()
{
 strategic_value(4,2); // compiler may warn on discarding a nodiscard value
 int z = strategic_value(0,0); // OK: return value is not discarded
 return z;
}

game.cpp:5:4: warning: ignoring return value of function declared with 'nodiscard' attribute
game.cpp:17:5: warning: ignoring return value of function declared with 'nodiscard' attribute: PURE FUN