Make ui/borrowck/borrowck-unboxed-closures.rs robust w.r.t. NLL.

src/test/ui/borrowck/borrowck-unboxed-closures.nll.stderr

@@ -1,3 +1,13 @@
+error[E0502]: cannot borrow `f` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-unboxed-closures.rs:13:5
+   |
+LL |     let g = &mut f;
+   |             ------ mutable borrow occurs here
+LL |     f(1, 2);    //~ ERROR cannot borrow `f` as immutable
+   |     ^ immutable borrow occurs here
+LL |     use_mut(g);
+   |             - mutable borrow later used here
+
 error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
   --> $DIR/borrowck-unboxed-closures.rs:17:5
    |
@@ -16,7 +26,7 @@ LL |     f(1, 2);    //~ ERROR use of moved value
    |
    = note: move occurs because `f` has type `F`, which does not implement the `Copy` trait
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-Some errors occurred: E0382, E0596.
+Some errors occurred: E0382, E0502, E0596.
 For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-unboxed-closures.rs

@@ -11,8 +11,8 @@
 fn a<F:Fn(isize, isize) -> isize>(mut f: F) {
     let g = &mut f;
     f(1, 2);    //~ ERROR cannot borrow `f` as immutable
+    use_mut(g);
 }
-
 fn b<F:FnMut(isize, isize) -> isize>(f: F) {
     f(1, 2);    //~ ERROR cannot borrow immutable argument
 }
@@ -23,3 +23,5 @@ fn c<F:FnOnce(isize, isize) -> isize>(f: F) {
 }
 
 fn main() {}
+
+fn use_mut<T>(_: &mut T) { }

src/test/ui/borrowck/borrowck-unboxed-closures.stderr

@@ -5,6 +5,7 @@ LL |     let g = &mut f;
    |                  - mutable borrow occurs here
 LL |     f(1, 2);    //~ ERROR cannot borrow `f` as immutable
    |     ^ immutable borrow occurs here
+LL |     use_mut(g);
 LL | }
    | - mutable borrow ends here