Add tests for super let.

tests/ui/super-let.borrowck.stderr

@@ -0,0 +1,174 @@
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:30:28
+   |
+LL |             super let b = DropMe(&mut x);
+   |                                  ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `b` is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:46:28
+   |
+LL |             super let b = &DropMe(&mut x);
+   |                            --------------
+   |                            |      |
+   |                            |      `x` is borrowed here
+   |                            a temporary with access to the borrow is created here ...
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:64:32
+   |
+LL |             super let b = identity(&DropMe(&mut x));
+   |                                     --------------
+   |                                     |      |
+   |                                     |      `x` is borrowed here
+   |                                     a temporary with access to the borrow is created here ...
+LL |             #[cfg(borrowck)] { x = true; }
+   |                                ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |         };
+   |          - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:87:36
+   |
+LL |                 super let b = identity(&DropMe(&mut x));
+   |                                         --------------
+   |                                         |      |
+   |                                         |      `x` is borrowed here
+   |                                         a temporary with access to the borrow is created here ...
+...
+LL |                 #[cfg(borrowck)] { x = true; }
+   |                                    ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |         ));
+   |           - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:107:28
+   |
+LL |                 super let b = DropMe(&mut x);
+   |                                      ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `b` is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:125:28
+   |
+LL |             super let b = DropMe(&mut x);
+   |                                  ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `b` is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:143:28
+   |
+LL |             super let b = DropMe(&mut x);
+   |                                  ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `b` is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:159:28
+   |
+LL |             b = DropMe(&mut x);
+   |                        ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+LL |         drop(a);
+   |              - borrow later used here
+
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/super-let.rs:172:33
+   |
+LL |         #[cfg(borrowck)] { a = &String::from("asdf"); };
+   |                                 ^^^^^^^^^^^^^^^^^^^^- temporary value is freed at the end of this statement
+   |                                 |
+   |                                 creates a temporary value which is freed while still in use
+...
+LL |         let _ = a;
+   |                 - borrow later used here
+   |
+   = note: consider using a `let` binding to create a longer lived value
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:206:28
+   |
+LL |                     super let d = &DropMe(&mut x);
+   |                                    --------------
+   |                                    |      |
+   |                                    |      `x` is borrowed here
+   |                                    a temporary with access to the borrow is created here ...
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:227:32
+   |
+LL |                     super let d = identity(&DropMe(&mut x));
+   |                                             --------------
+   |                                             |      |
+   |                                             |      `x` is borrowed here
+   |                                             a temporary with access to the borrow is created here ...
+...
+LL |             #[cfg(borrowck)] { x = true; }
+   |                                ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |         };
+   |          - ... and the borrow might be used here, when that temporary is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:246:28
+   |
+LL |             super let b = DropMe(&mut x);
+   |                                  ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `b` is dropped and runs the `Drop` code for type `DropMe`
+
+error[E0506]: cannot assign to `x` because it is borrowed
+  --> $DIR/super-let.rs:263:28
+   |
+LL |             let dropme = Some(DropMe(&mut x));
+   |                                      ------ `x` is borrowed here
+...
+LL |         #[cfg(borrowck)] { x = true; }
+   |                            ^^^^^^^^ `x` is assigned to here but it was already borrowed
+...
+LL |     }
+   |     - borrow might be used here, when `x` is dropped and runs the `Drop` code for type `DropMe`
+
+error: aborting due to 13 previous errors
+
+Some errors have detailed explanations: E0506, E0716.
+For more information about an error, try `rustc --explain E0506`.