Add RustcEncodable test

Cargo.toml

@@ -24,6 +24,7 @@ compiletest_rs = "0.0.11"
 regex = "*"
 regex_macros = "*"
 lazy_static = "*"
+rustc-serialize = "0.3"
 
 [features]
 

tests/compile-fail/used_underscore_binding.rs

@@ -12,14 +12,6 @@ fn in_macro(_foo: u32) {
     println!("{}", _foo); //~ ERROR used binding which is prefixed with an underscore
 }
 
-// TODO: This doesn't actually correctly test this. Need to find a #[derive(...)] which sets off
-// the lint if the `in_attributes_expansion` test isn't there
-/// Test that we do not lint for unused underscores in a MacroAttribute expansion
-#[derive(Clone)]
-struct MacroAttributesTest {
-    _foo: u32,
-}
-
 // Struct for testing use of fields prefixed with an underscore
 struct StructFieldTest {
     _underscore_field: u32,
@@ -76,7 +68,6 @@ fn non_variables() {
 
 fn main() {
     let foo = 0u32;
-    let _ = MacroAttributesTest{_foo: 0};
     // tests of unused_underscore lint
     let _ = prefix_underscore(foo);
     in_macro(foo);

tests/used_underscore_binding_macro.rs

@@ -0,0 +1,16 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+extern crate rustc_serialize;
+
+/// Test that we do not lint for unused underscores in a MacroAttribute expansion
+#[deny(used_underscore_binding)]
+#[derive(RustcEncodable)]
+struct MacroAttributesTest {
+    _foo: u32,
+}
+
+#[test]
+fn macro_attributes_test() {
+    let _ = MacroAttributesTest{_foo: 0};
+}