fix compile in stable.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "eval"
-version = "0.3.0"
+version = "0.3.1"
 description = "Expression evaluator"
 keywords = ["expression", "evaluate", "evaluator", "expr", "template"]
 authors = ["fengcen <fengcen.love@gmail.com>"]
@@ -18,3 +18,6 @@ path = "src/lib.rs"
 serde = "^0.8"
 serde_json = "^0.8"
 quick-error = "^1.1"
+
+[features]
+unstable = []

README.md

@@ -1,7 +1,7 @@
 
 eval
-===
-[![docs](https://docs.rs/eval/badge.svg?version=0.3.0 "docs")](https://docs.rs/eval)
+====
+[![docs](https://docs.rs/eval/badge.svg?version=0.3.1 "docs")](https://docs.rs/eval)
 
 Eval is a powerful expression evaluator.
 

src/lib.rs

@@ -92,8 +92,7 @@
 //!
 #![recursion_limit="100"]
 #![deny(missing_docs)]
-#![feature(proc_macro, test)]
-extern crate test;
+#![cfg_attr(feature = "unstable", feature(test))]
 
 #[macro_use]
 extern crate quick_error;
@@ -135,7 +134,6 @@ type Compiled = Box<Fn(&[Context], &Functions) -> Result<Value, Error>>;
 
 #[cfg(test)]
 mod tests {
-    use test;
     use serde_json::to_value;
     use error::Error;
     use Expr;
@@ -542,7 +540,13 @@ mod tests {
 
         assert_eq!(tree.parse_node(), Err(Error::CommaNotWithFunction));
     }
+}
 
+#[cfg(all(feature = "unstable", test))]
+mod benches {
+    extern crate test;
+    use eval;
+    use tree::Tree;
 
     #[bench]
     fn bench_deep_brackets(b: &mut test::Bencher) {