Add an explicit std feature so that features are purely additive.

lib/wasm/src/lib.rs

@@ -9,12 +9,12 @@
 //!
 //! The main function of this module is [`translate_module`](fn.translate_module.html).
 
-#![cfg_attr(feature = "no_std", no_std)]
 #![deny(missing_docs)]
 
-#![cfg_attr(feature = "no_std", feature(alloc))]
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
 
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 #[macro_use]
 extern crate alloc;
 
@@ -42,7 +42,7 @@ pub use environ::{FuncEnvironment, ModuleEnvironment, DummyEnvironment, GlobalVa
 pub use translation_utils::{FunctionIndex, GlobalIndex, TableIndex, MemoryIndex, SignatureIndex,
                             Global, GlobalInit, Table, Memory};
 
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 mod std {
     pub use alloc::vec;
     pub use alloc::string;