Move feature to doc comment.

Author: finnbear

library/alloc/src/lib.rs

@@ -146,7 +146,6 @@
 #![feature(slice_ptr_len)]
 #![feature(slice_range)]
 #![feature(str_internals)]
-#![feature(string_leak)]
 #![feature(strict_provenance)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]

library/alloc/src/string.rs

@@ -1862,9 +1862,13 @@ impl String {
     /// Simple usage:
     ///
     /// ```
-    /// let x = String::from("bucket");
-    /// let static_ref: &'static mut str = x.leak();
-    /// assert_eq!(static_ref, "bucket");
+    /// #![feature(string_leak)]
+    ///
+    /// pub fn main() {
+    ///     let x = String::from("bucket");
+    ///     let static_ref: &'static mut str = x.leak();
+    ///     assert_eq!(static_ref, "bucket");
+    /// }
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "string_leak", issue = "102929")]