Fix existing benchmarks

The #[cfg(bench)] attribute does not exist and is always false. Lets
define a feature "bench" which can be used to enable benchmarks when
building with nightly.

Cargo.toml

@@ -29,6 +29,7 @@ alloc = []
 std = []
 clock = ["time", "std"]
 wasmbind = ["wasm-bindgen", "js-sys"]
+bench = ["std"]
 
 [dependencies]
 time = { version = "0.1.39", optional = true }

src/lib.rs

@@ -383,7 +383,7 @@
 
 #![doc(html_root_url = "https://docs.rs/chrono/latest/")]
 
-#![cfg_attr(bench, feature(test))] // lib stability features as per RFC #507
+#![cfg_attr(feature = "bench", feature(test))] // lib stability features as per RFC #507
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 
@@ -427,6 +427,8 @@ extern crate doc_comment;
 extern crate wasm_bindgen;
 #[cfg(all(target_arch = "wasm32", feature="wasmbind"))]
 extern crate js_sys;
+#[cfg(feature = "bench")]
+extern crate test;
 
 #[cfg(test)]
 doctest!("../README.md");

src/naive/internals.rs

@@ -470,7 +470,6 @@ impl fmt::Debug for Mdf {
 #[cfg(test)]
 mod tests {
     #[cfg(test)] extern crate num_iter;
-    #[cfg(bench)] extern crate test;
 
     use Weekday;
     use super::{Of, Mdf};
@@ -517,7 +516,7 @@ mod tests {
         assert_eq!(GF.nisoweeks(), 52);
     }
 
-    #[cfg(bench)]
+    #[cfg(feature = "bench")]
     #[bench]
     fn bench_year_flags_from_year(bh: &mut test::Bencher) {
         bh.iter(|| {