gate benchmark feature on nightly toolchain

Closes #12653

src/ast.rs

@@ -2889,7 +2889,7 @@ fn test_is_same_node() {
 }
 
 // Run with cargo +nightly bench --features=benchmark
-#[cfg(feature = "benchmark")]
+#[cfg(all(nightly, feature = "benchmark"))]
 #[cfg(test)]
 mod bench {
     extern crate test;

src/common.rs

@@ -707,7 +707,7 @@ macro_rules! check_decode {
     }
 }
 
-#[cfg(feature = "benchmark")]
+#[cfg(all(nightly, feature = "benchmark"))]
 #[cfg(test)]
 mod bench {
     extern crate test;

src/lib.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(feature = "benchmark", feature(test))]
+#![cfg_attr(all(nightly, feature = "benchmark"), feature(test))]
 #![allow(non_camel_case_types)]
 
 pub const BUILD_VERSION: &str = env!("FISH_BUILD_VERSION");