Update E0301 to the new format

src/librustc_const_eval/check_match.rs

@@ -1175,8 +1175,10 @@ impl<'a, 'gcx, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'gcx> {
               _: LoanCause) {
         match kind {
             MutBorrow => {
-                span_err!(self.cx.tcx.sess, span, E0301,
+                struct_span_err!(self.cx.tcx.sess, span, E0301,
                           "cannot mutably borrow in a pattern guard")
+                    .span_label(span, &format!("borrowed mutably in pattern guard"))
+                    .emit();
             }
             ImmBorrow | UniqueImmBorrow => {}
         }

src/test/compile-fail/E0301.rs

@@ -12,6 +12,7 @@ fn main() {
     match Some(()) {
         None => { },
         option if option.take().is_none() => {}, //~ ERROR E0301
+        //~| NOTE borrowed mutably in pattern guard
         Some(_) => { }
     }
 }