Auto merge of #1019 - est31:master, r=alexcrichton

Simplify the stdbuild section

Found this when encountering the code in the rustc submodule and changing the allow for the warnings to deny.

* `no_std` is stable so it does not have to be listed in the `feature` attribute
* `no_std` as an attribute for the crate is already implied by the `#![cfg_attr(not(feature = "use_std"), no_std)]` below
* `staged_api` as an attribute gives a warning. That also matches my knowledge.

Author: bors

src/lib.rs

@@ -86,10 +86,8 @@
 ))]
 
 // Attributes needed when building as part of the standard library
-#![cfg_attr(feature = "stdbuild", feature(no_std, staged_api, custom_attribute, cfg_target_vendor))]
+#![cfg_attr(feature = "stdbuild", feature(staged_api, custom_attribute, cfg_target_vendor))]
 #![cfg_attr(feature = "stdbuild", feature(link_cfg, repr_packed))]
-#![cfg_attr(feature = "stdbuild", no_std)]
-#![cfg_attr(feature = "stdbuild", staged_api)]
 #![cfg_attr(feature = "stdbuild", allow(warnings))]
 #![cfg_attr(feature = "stdbuild", unstable(feature = "libc",
                                reason = "use `libc` from crates.io",