Specify the cfg in which 'tri!' is used

serde/src/lib.rs

@@ -221,7 +221,7 @@ mod lib {
 // performed implicitly by the `?` operator or the standard library's `try!`
 // macro. This simplified macro gives a 5.5% improvement in compile time
 // compared to standard `try!`, and 9% improvement compared to `?`.
-#[allow(unused_macros)]
+#[cfg(not(no_serde_derive))]
 macro_rules! tri {
     ($expr:expr) => {
         match $expr {