Update test for needless-borrow FP for mutable ref

tests/ui/needless_borrow.rs

@@ -1,17 +1,16 @@
 // run-rustfix
 
-#![allow(clippy::needless_borrowed_reference)]
-
-fn x(y: &i32) -> i32 {
-    *y
-}
-
 #[warn(clippy::all, clippy::needless_borrow)]
 #[allow(unused_variables)]
 fn main() {
     let a = 5;
-    let b = x(&a);
-    let c = x(&&a);
+    let _ = x(&a); // no warning
+    let _ = x(&&a); // warn
+
+    let mut b = 5;
+    mut_ref(&mut b); // no warning
+    mut_ref(&mut &mut b); // warn
+
     let s = &String::from("hi");
     let s_ident = f(&s); // should not error, because `&String` implements Copy, but `String` does not
     let g_val = g(&Vec::new()); // should not error, because `&Vec<T>` derefs to `&[T]`
@@ -29,6 +28,15 @@ fn main() {
     };
 }
 
+#[allow(clippy::needless_borrowed_reference)]
+fn x(y: &i32) -> i32 {
+    *y
+}
+
+fn mut_ref(y: &mut i32) {
+    *y = 5;
+}
+
 fn f<T: Copy>(y: &T) -> T {
     *y
 }