Add test for unnecessary mut passed lint

tests/compile-fail/mut_reference.rs

@@ -0,0 +1,43 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+fn takes_an_immutable_reference(a: &i32) {
+}
+
+fn takes_a_mutable_reference(a: &mut i32) {
+}
+
+struct MyStruct;
+
+impl MyStruct {
+    fn takes_an_immutable_reference(&self, a: &i32) {
+    }
+
+    fn takes_a_mutable_reference(&self, a: &mut i32) {
+    }
+}
+
+#[deny(unnecessary_mut_passed)]
+fn main() {
+    // Functions
+    takes_an_immutable_reference(&mut 42); //~ERROR The function/method "takes_an_immutable_reference" doesn't need a mutable reference
+    
+    // Methods
+    let my_struct = MyStruct;
+    my_struct.takes_an_immutable_reference(&mut 42); //~ERROR The function/method "takes_an_immutable_reference" doesn't need a mutable reference
+    
+
+    // No error
+    
+    // Functions
+    takes_an_immutable_reference(&42);
+    takes_a_mutable_reference(&mut 42);
+    let mut a = &mut 42;
+    takes_an_immutable_reference(a);
+    
+    // Methods
+    my_struct.takes_an_immutable_reference(&42);
+    my_struct.takes_a_mutable_reference(&mut 42);
+    my_struct.takes_an_immutable_reference(a);
+    
+}