added regression test for #189

tests/compile-fail/identity_op.rs

@@ -10,7 +10,10 @@ fn main() {
     let x = 0;
 
     x + 0;        //~ERROR the operation is ineffective
+    x + (1 - 1);  //~ERROR the operation is ineffective
+    x + 1;
     0 + x;        //~ERROR the operation is ineffective
+    1 + x;
     x - ZERO;     //no error, as we skip lookups (for now)
     x | (0);      //~ERROR the operation is ineffective
     ((ZERO)) | x; //no error, as we skip lookups (for now)
@@ -19,6 +22,8 @@ fn main() {
     1 * x;        //~ERROR the operation is ineffective
     x / ONE;      //no error, as we skip lookups (for now)
 
+    x / 2;        //no false positive
+
     x & NEG_ONE;  //no error, as we skip lookups (for now)
     -1 & x;       //~ERROR the operation is ineffective
 }