Add test for double_parens lint.

tests/compile-fail/double_parens.rs

@@ -0,0 +1,51 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(double_parens)]
+#![allow(dead_code)]
+
+fn dummy_fn<T>(_: T) {}
+
+struct DummyStruct;
+
+impl DummyStruct {
+    fn dummy_method<T>(self, _: T) {}
+}
+
+fn simple_double_parens() -> i32 {
+    ((0)) //~ERROR Consider removing unnecessary double parentheses
+}
+
+fn fn_double_parens() {
+    dummy_fn((0)); //~ERROR Consider removing unnecessary double parentheses
+}
+
+fn method_double_parens(x: DummyStruct) {
+    x.dummy_method((0)); //~ERROR Consider removing unnecessary double parentheses
+}
+
+fn tuple_double_parens() -> (i32, i32) {
+    ((1, 2)) //~ERROR Consider removing unnecessary double parentheses
+}
+
+fn unit_double_parens() {
+    (()) //~ERROR Consider removing unnecessary double parentheses
+}
+
+fn fn_tuple_ok() {
+    dummy_fn((1, 2));
+}
+
+fn method_tuple_ok(x: DummyStruct) {
+    x.dummy_method((1, 2));
+}
+
+fn fn_unit_ok() {
+    dummy_fn(());
+}
+
+fn method_unit_ok(x: DummyStruct) {
+    x.dummy_method(());
+}
+
+fn main() {}