Add run-pass test for expansion info

serde_macros/tests/run-pass/identity-op.rs

@@ -0,0 +1,12 @@
+#![feature(custom_derive, plugin)]
+#![plugin(serde_macros, clippy)]
+
+#![deny(identity_op)]
+
+// The derived implementation uses 0+1 to add up the number of fields
+// serialized, which Clippy warns about. If the expansion info is registered
+// correctly, the Clippy lint is not triggered.
+#[derive(Serialize)]
+struct A { b: u8 }
+
+fn main() {}