Add test for unused_borrows lint

Author: ecstatic-morse

src/test/ui/lint/unused-borrows.rs

@@ -0,0 +1,33 @@
+#![deny(unused_must_use)]
+
+fn foo(_: i32) -> bool { todo!() }
+
+fn bar() -> &'static i32 {
+    &42;
+    //~^ unused
+
+    &mut foo(42);
+    //~^ unused
+
+    &&42;
+    //~^ unused
+
+    &&mut 42;
+    //~^ unused
+
+    &mut &42;
+    //~^ unused
+
+    let _result = foo(4)
+        && foo(2); // Misplaced semi-colon (perhaps due to reordering of lines)
+        && foo(42);
+        //~^ unused
+
+    let _ = &42; // ok
+
+    &42 // ok
+}
+
+fn main() {
+    let _ = bar();
+}

src/test/ui/lint/unused-borrows.stderr

@@ -0,0 +1,44 @@
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:6:5
+   |
+LL |     &42;
+   |     ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-borrows.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:9:5
+   |
+LL |     &mut foo(42);
+   |     ^^^^^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:12:5
+   |
+LL |     &&42;
+   |     ^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:15:5
+   |
+LL |     &&mut 42;
+   |     ^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:18:5
+   |
+LL |     &mut &42;
+   |     ^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:23:9
+   |
+LL |         && foo(42);
+   |         ^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+