Warn on all erroneous constants

src/test/ui/const-eval/conditional_array_execution.rs

@@ -14,6 +14,7 @@ const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
 //~^ WARN attempt to subtract with overflow
+//~| WARN this constant cannot be used
 
 fn main() {
     println!("{}", FOO);