Add test for mut arg: &Ty meant to be arg: &mut Ty

This is a mistake I've seen newcomers make where they want to express an "out" argument.

Author: estebank

tests/ui/fn/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs

@@ -0,0 +1,20 @@
+#![deny(unused_assignments, unused_variables)]
+struct Object;
+
+fn change_object(mut object: &Object) {
+    let object2 = Object;
+    object = object2; //~ ERROR mismatched types
+}
+
+fn change_object2(mut object: &Object) { //~ ERROR variable `object` is assigned to, but never used
+    let object2 = Object;
+    object = &object2;
+    //~^ ERROR `object2` does not live long enough
+    //~| ERROR value assigned to `object` is never read
+}
+
+fn main() {
+    let object = Object;
+    change_object(&object);
+    change_object2(&object);
+}

tests/ui/fn/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.stderr

@@ -0,0 +1,60 @@
+error[E0308]: mismatched types
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:6:14
+   |
+LL | fn change_object(mut object: &Object) {
+   |                              ------- expected due to this parameter type
+LL |     let object2 = Object;
+LL |     object = object2;
+   |              ^^^^^^^ expected `&Object`, found `Object`
+   |
+help: consider borrowing here
+   |
+LL |     object = &object2;
+   |              +
+
+error: value assigned to `object` is never read
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:11:5
+   |
+LL |     object = &object2;
+   |     ^^^^^^
+   |
+   = help: maybe it is overwritten before being read?
+note: the lint level is defined here
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:1:9
+   |
+LL | #![deny(unused_assignments, unused_variables)]
+   |         ^^^^^^^^^^^^^^^^^^
+
+error: variable `object` is assigned to, but never used
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:9:23
+   |
+LL | fn change_object2(mut object: &Object) {
+   |                       ^^^^^^
+   |
+   = note: consider using `_object` instead
+note: the lint level is defined here
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:1:29
+   |
+LL | #![deny(unused_assignments, unused_variables)]
+   |                             ^^^^^^^^^^^^^^^^
+
+error[E0597]: `object2` does not live long enough
+  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:11:14
+   |
+LL | fn change_object2(mut object: &Object) {
+   |                               - let's call the lifetime of this reference `'1`
+LL |     let object2 = Object;
+   |         ------- binding `object2` declared here
+LL |     object = &object2;
+   |     ---------^^^^^^^^
+   |     |        |
+   |     |        borrowed value does not live long enough
+   |     assignment requires that `object2` is borrowed for `'1`
+...
+LL | }
+   | - `object2` dropped here while still borrowed
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0308, E0597.
+For more information about an error, try `rustc --explain E0308`.