Added tests for bugs fixed.

src/test/compile-fail/lint-unused-mut-variables.rs

@@ -47,6 +47,20 @@ fn main() {
     let x = |mut y: isize| 10; //~ ERROR: variable does not need to be mutable
     fn what(mut foo: isize) {} //~ ERROR: variable does not need to be mutable
 
+    let mut a = &mut 5; //~ ERROR: variable does not need to be mutable
+    *a = 4;
+
+    let mut a = 5;
+    let mut b = (&mut a,);
+    *b.0 = 4; //~^ ERROR: variable does not need to be mutable
+
+    fn mut_ref_arg(mut arg : &mut [u8]) -> &mut [u8] {
+        &mut arg[..] //~^ ERROR: variable does not need to be mutable
+    }
+
+    let mut v : &mut Vec<()> = &mut vec![]; //~ ERROR: variable does not need to be mutable
+    v.push(());
+
     // positive cases
     let mut a = 2;
     a = 3;