Add a test showing a false positive on needless_borrow

tests/ui/needless_borrow.rs

@@ -18,6 +18,15 @@ fn main() {
     let vec_val = g(&vec); // should not error, because `&Vec<T>` derefs to `&[T]`
     h(&"foo"); // should not error, because the `&&str` is required, due to `&Trait`
     if let Some(ref cake) = Some(&5) {}
+    let garbl = match 42 {
+        44 => &a,
+        45 => {
+            println!("foo");
+            &&a // FIXME: this should lint, too
+        },
+        46 => &&a,
+        _ => panic!(),
+    };
 }
 
 fn f<T:Copy>(y: &T) -> T {

tests/ui/needless_borrow.stderr

@@ -19,13 +19,21 @@ error: this pattern creates a reference to a reference
    |
    = note: #[deny(needless_borrow)] implied by #[deny(clippy)]
 
-warning: this pattern creates a reference to a reference
+error: this expression borrows a reference that is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:41:31
+  --> $DIR/needless_borrow.rs:27:15
    |
-41 |     let _ = v.iter().filter(|&ref a| a.is_empty());
+27 |         46 => &&a,
+   |               ^^^
+   |
+   = note: #[deny(needless_borrow)] implied by #[deny(clippy)]
+
+warning: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow.rs:50:31
+   |
+50 |     let _ = v.iter().filter(|&ref a| a.is_empty());
    |                               ^^^^^
    |
    = note: #[warn(needless_borrow)] on by default
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors