Update E0007 to new format

Author: srdja

src/librustc_const_eval/check_match.rs

@@ -1112,7 +1112,10 @@ fn check_legality_of_move_bindings(cx: &MatchCheckCtxt,
 
         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
         if sub.map_or(false, |p| pat_contains_bindings(&p)) {
-            span_err!(cx.tcx.sess, p.span, E0007, "cannot bind by-move with sub-bindings");
+            struct_span_err!(cx.tcx.sess, p.span, E0007,
+                             "cannot bind by-move with sub-bindings")
+                .span_label(p.span, &format!("binds an already bound by-move value by moving it"))
+                .emit();
         } else if has_guard {
             struct_span_err!(cx.tcx.sess, p.span, E0008,
                       "cannot bind by-move into a pattern guard")

src/test/compile-fail/E0007.rs

@@ -11,8 +11,10 @@
 fn main() {
     let x = Some("s".to_string());
     match x {
-        op_string @ Some(s) => {}, //~ ERROR E0007
-                                   //~| ERROR E0303
+        op_string @ Some(s) => {},
+        //~^ ERROR E0007
+        //~| NOTE binds an already bound by-move value by moving it
+        //~| ERROR E0303
         None => {},
     }
 }