Updated E0054 to new format

Author: Knight

src/librustc_typeck/check/cast.rs

@@ -161,6 +161,7 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
             }
             CastError::CastToBool => {
                 struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`")
+                    .span_label(self.span, &format!("unsupported cast"))
                     .help("compare with zero instead")
                     .emit();
             }

src/test/compile-fail/cast-rfc0401.rs

@@ -59,10 +59,12 @@ fn main()
     //~^ ERROR casting
     //~^^ HELP through a usize first
     let _ = 3_i32 as bool;
-    //~^ ERROR cannot cast as `bool`
+    //~^ ERROR cannot cast as `bool` [E0054]
+    //~| unsupported cast
     //~| HELP compare with zero
     let _ = E::A as bool;
-    //~^ ERROR cannot cast as `bool`
+    //~^ ERROR cannot cast as `bool` [E0054]
+    //~| unsupported cast
     //~| HELP compare with zero
     let _ = 0x61u32 as char; //~ ERROR only `u8` can be cast