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

README.rst

@@ -60,18 +60,20 @@ installed.
 Building with `no_std`
 ----------------------
 
-To build cretonne without libstd, enable the `no_std` feature on `lib/cretonne`,
+To build cretonne without libstd, disable the `std` feature on `lib/cretonne`,
-`lib/frontend`, `lib/native`, and `lib/wasm`.
+`lib/frontend`, `lib/native`, and `lib/wasm`, which is otherwise enabled by
+default, and enable the `no_std` feature.
 
 For example, to build `cretonne`:
 
     cd lib/cretonne
-    cargo build --features no_std
+    cargo build --no-default-features --features no_std
 
 Or, when using `cretonne` as a dependency (in Cargo.toml):
 
     [dependency.cretonne]
-    path = "..."
+    ...
+    default-features = false
     features = ["no_std"]
 
 `no_std` is currently "best effort". We won't try to break it, and we'll

cranelift/test-no_std.sh

@@ -21,6 +21,7 @@ for LIB in $LIBS
 do
     banner "Rust unit tests in $LIB"
     cd "lib/$LIB"
+    cargo test --no-default-features --features no_std
     cargo test --features no_std
     cd "$topdir"
 done

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.
+# The "std" feature enables use of libstd. The "no_std" feature enables use
-# Enabling this disables use of `stdlib`.
+# 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};

lib/frontend/Cargo.toml

@@ -12,7 +12,9 @@ readme = "README.md"
 name = "cton_frontend"
 
 [dependencies]
-cretonne = { path = "../cretonne", version = "0.1.0" }
+cretonne = { path = "../cretonne", version = "0.1.0", default-features = false }
 
 [features]
+default = ["std"]
+std = ["cretonne/std"]
 no_std = ["cretonne/no_std"]

lib/frontend/src/lib.rs

@@ -142,14 +142,14 @@
 //! }
 //! ```
 
-#![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))]
 
 extern crate cretonne;
 
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 extern crate alloc;
 
 pub use frontend::{ILBuilder, FunctionBuilder};
@@ -157,7 +157,7 @@ pub use frontend::{ILBuilder, FunctionBuilder};
 mod frontend;
 mod ssa;
 
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 mod std {
     pub use alloc::vec;
     pub use core::*;

lib/native/Cargo.toml

@@ -11,10 +11,12 @@ readme = "README.md"
 name = "cton_native"
 
 [dependencies]
-cretonne = { path = "../cretonne", version = "0.1.0" }
+cretonne = { path = "../cretonne", version = "0.1.0", default-features = false }
 
 [target.'cfg(any(target_arch = "x86", target_arch = "x86_64"))'.dependencies]
 raw-cpuid = "3.0.0"
 
 [features]
+default = ["std"]
+std = ["cretonne/std"]
 no_std = ["cretonne/no_std"]

lib/native/src/lib.rs

@@ -1,8 +1,9 @@
 //! Performs autodetection of the host for the purposes of running
 //! Cretonne to generate code to run on the same machine.
-#![cfg_attr(feature = "no_std", no_std)]
 #![deny(missing_docs)]
 
+#![cfg_attr(not(feature = "std"), no_std)]
+
 extern crate cretonne;
 
 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]

lib/wasm/Cargo.toml

@@ -12,12 +12,13 @@ name = "cton_wasm"
 
 [dependencies]
 wasmparser = "0.14.1"
-cretonne = { path = "../cretonne", version = "0.1.0" }
+cretonne = { path = "../cretonne", version = "0.1.0", default_features = false }
-cretonne-frontend = { path = "../frontend", version = "0.1.0" }
+cretonne-frontend = { path = "../frontend", version = "0.1.0", default_features = false }
 
 [dependencies.hashmap_core]
 version = "0.1.1"
 optional = true
+
 [dependencies.error_core]
 version = "0.1.0"
 optional = true
@@ -26,6 +27,6 @@ optional = true
 tempdir = "0.3.5"
 
 [features]
-# Currently, the only feature is the `no_std` feature.
+default = ["std"]
-# Enabling this disables use of `stdlib`.
+std = ["cretonne/std", "cretonne-frontend/std"]
 no_std = ["hashmap_core", "error_core", "cretonne/no_std", "cretonne-frontend/no_std"]

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;