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

lib/cretonne/Cargo.toml

@@ -17,14 +17,19 @@ name = "cretonne"
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
 # accomodated in `tests`.
+
 [dependencies.hashmap_core]
 version = "0.1.1"
 optional = true
+
 [dependencies.error_core]
 version = "0.1.0"
 optional = true
 
 [features]
-# Currently, the only feature is the `no_std` feature.
-# Enabling this disables use of `stdlib`.
-no_std = ["hashmap_core", "error_core"]
+# The "std" feature enables use of libstd. The "no_std" feature enables use
+# of some minimal std-like replacement libraries. At least one of these two
+# features to be enabled.
+default = ["std"]
+std = []
+no_std = ["hashmap_core", "error_core"]

lib/cretonne/src/lib.rs

@@ -1,16 +1,16 @@
 //! Cretonne code generation library.
-#![cfg_attr(feature = "no_std", no_std)]
 #![deny(missing_docs)]
 
 // Turns on alloc feature if no_std
-#![cfg_attr(feature = "no_std", feature(alloc))]
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
 
 // Include the `hashmap_core` crate if no_std
 #[cfg(feature = "no_std")]
 extern crate hashmap_core;
 #[cfg(feature = "no_std")]
 extern crate error_core;
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 #[macro_use]
 extern crate alloc;
 
@@ -60,7 +60,7 @@ mod unreachable_code;
 mod write;
 
 /// This replaces `std` in builds with no_std.
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 mod std {
     pub use core::*;
     pub use alloc::{boxed, vec, string};