Unify alloc and std cargo features

This change provides a common convention for using allocator-dependent
features with:

    #![cfg(feature = "alloc")]

When available, `Vec` is imported consistently as `prelude::Vec`, which
means modules that need access to `Vec` can simply do:

    use prelude::*;

and if an allocator is available, `Vec` will be in the crate prelude.

This allows all `alloc` vs `std` gating to be handled in `lib.rs`,
`build.rs`, and `prelude.rs` so the rest of the codebase doesn't have to
do any gating whatsoever.

.travis.yml

@@ -15,16 +15,18 @@ env:
   - TEST_COMMAND=test EXTRA_FLAGS='' FEATURES='serde'
   # Tests building without std. We have to select a backend, so we select the one
   # most likely to be useful in an embedded environment.
-  - TEST_COMMAND=build EXTRA_FLAGS=--no-default-features FEATURES='u32_backend'
+  - TEST_COMMAND=build EXTRA_FLAGS='--no-default-features' FEATURES='u32_backend'
+  # Tests no_std+alloc usage using the most embedded-friendly backend
+  - TEST_COMMAND=test EXTRA_FLAGS='--lib --no-default-features' FEATURES='alloc u32_backend'
 
 matrix:
   exclude:
     # Test the avx2 backend only on nightly
     - rust: stable
       env: TEST_COMMAND=test EXTRA_FLAGS='--no-default-features' FEATURES='std avx2_backend'
-    # Test no_std only on nightly.
+    # Test no_std+alloc only on nightly
     - rust: stable
-      env: TEST_COMMAND=build EXTRA_FLAGS=--no-default-features FEATURES='u32_backend'
+      env: TEST_COMMAND=test EXTRA_FLAGS='--lib --no-default-features' FEATURES='alloc u32_backend'
 
 script:
   - cargo $TEST_COMMAND --features="$FEATURES" $EXTRA_FLAGS

Cargo.toml

@@ -61,7 +61,7 @@ serde = { version = "1.0", optional = true }
 [features]
 nightly = ["subtle/nightly", "clear_on_drop/nightly"]
 default = ["std", "u64_backend"]
-std = ["subtle/std", "rand/std"]
+std = ["alloc", "subtle/std", "rand/std"]
 alloc = []
 yolocrypto = []
 

build.rs

@@ -1,9 +1,12 @@
+#![cfg_attr(all(feature = "alloc", not(feature = "std")), feature(alloc))]
 #![cfg_attr(feature = "nightly", feature(cfg_target_feature))]
 #![cfg_attr(all(feature = "nightly", feature = "avx2_backend"), feature(stdsimd))]
 #![allow(unused_variables)]
 #![allow(non_snake_case)]
 #![allow(dead_code)]
 
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+extern crate alloc;
 extern crate byteorder;
 extern crate clear_on_drop;
 extern crate core;
@@ -53,6 +56,8 @@ mod field;
 mod curve_models;
 #[path = "src/backend/mod.rs"]
 mod backend;
+#[path = "src/prelude.rs"]
+mod prelude;
 #[path = "src/scalar_mul/mod.rs"]
 mod scalar_mul;
 

src/backend/avx2/scalar_mul/straus.rs

@@ -20,6 +20,9 @@ use scalar::Scalar;
 use scalar_mul::window::{LookupTable, NafLookupTable5};
 use traits::{Identity, MultiscalarMul, VartimeMultiscalarMul};
 
+#[allow(unused_imports)]
+use prelude::*;
+
 /// Multiscalar multiplication using interleaved window / Straus'
 /// method.  See the `Straus` struct in the serial backend for more
 /// details.
@@ -30,7 +33,7 @@ use traits::{Identity, MultiscalarMul, VartimeMultiscalarMul};
 /// point representation on the fly.
 pub struct Straus {}
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl MultiscalarMul for Straus {
     type Point = EdwardsPoint;
 
@@ -68,7 +71,7 @@ impl MultiscalarMul for Straus {
     }
 }
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl VartimeMultiscalarMul for Straus {
     type Point = EdwardsPoint;
 

src/backend/mod.rs

@@ -21,6 +21,12 @@
 //! `32bit` since identifiers can't start with letters, and the backends
 //! do use `u32`/`u64`, so this seems like a least-bad option.
 
+#[cfg(not(any(feature = "u32_backend", feature = "u64_backend", feature = "avx2_backend")))]
+compile_error!(
+    "no curve25519-dalek backend cargo feature enabled! \
+     please enable one of: u32_backend, u64_backend, avx2_backend"
+);
+
 #[cfg(feature = "u32_backend")]
 pub mod u32;
 

src/edwards.rs

@@ -115,6 +115,9 @@ use curve_models::CompletedPoint;
 use curve_models::AffineNielsPoint;
 use curve_models::ProjectiveNielsPoint;
 
+#[allow(unused_imports)]
+use prelude::*;
+
 use scalar_mul::window::LookupTable;
 
 use traits::{Identity, IsIdentity};
@@ -540,7 +543,7 @@ impl<'a, 'b> Mul<&'b EdwardsPoint> for &'a Scalar {
 // These use the iterator's size hint and the target settings to
 // forward to a specific backend implementation.
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl MultiscalarMul for EdwardsPoint {
     type Point = EdwardsPoint;
     
@@ -569,7 +572,7 @@ impl MultiscalarMul for EdwardsPoint {
     }
 }
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl VartimeMultiscalarMul for EdwardsPoint {
     type Point = EdwardsPoint;
     

src/field.rs

@@ -135,7 +135,7 @@ impl FieldElement {
     /// Given a slice of public `FieldElements`, replace each with its inverse.
     ///
     /// All input `FieldElements` **MUST** be nonzero.
-    #[cfg(any(feature = "alloc", feature = "std"))]
+    #[cfg(feature = "alloc")]
     pub fn batch_invert(inputs: &mut [FieldElement]) {
         // Montgomery’s Trick and Fast Implementation of Masked AES
         // Genelle, Prouff and Quisquater

src/lib.rs

@@ -8,10 +8,9 @@
 // - Isis Agora Lovecruft <isis@patternsinthevoid.net>
 // - Henry de Valence <hdevalence@hdevalence.ca>
 
-#![cfg_attr(not(feature = "std"), no_std)]
-
-#![cfg_attr(feature = "alloc", feature(alloc))]
+#![no_std]
 
+#![cfg_attr(all(feature = "alloc", not(feature = "std")), feature(alloc))]
 #![cfg_attr(feature = "nightly", feature(cfg_target_feature))]
 #![cfg_attr(feature = "nightly", feature(external_doc))]
 #![cfg_attr(all(feature = "nightly", feature = "avx2_backend"), feature(stdsimd))]
@@ -32,11 +31,14 @@
 // External dependencies:
 //------------------------------------------------------------------------
 
-#[cfg(feature = "std")]
-extern crate core;
-#[cfg(feature = "alloc")]
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+#[macro_use]
 extern crate alloc;
 
+#[cfg(feature = "std")]
+#[macro_use]
+extern crate std;
+
 extern crate rand;
 extern crate clear_on_drop;
 extern crate byteorder;
@@ -94,5 +96,8 @@ pub(crate) mod backend;
 // Internal curve models which are not part of the public API.
 pub(crate) mod curve_models;
 
+// Crate-local prelude (for alloc-dependent features like `Vec`)
+pub(crate) mod prelude;
+
 // Implementations of scalar mul algorithms live here
 pub(crate) mod scalar_mul;

src/montgomery.rs

@@ -290,6 +290,7 @@ mod test {
     use constants;
     use super::*;
 
+    #[cfg(feature = "rand")]
     use rand::rngs::OsRng;
 
     /// Test Montgomery -> Edwards on the X/Ed25519 basepoint
@@ -343,6 +344,7 @@ mod test {
         assert_eq!(u18, u18_unred);
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn montgomery_ladder_matches_edwards_scalarmult() {
         let mut csprng: OsRng = OsRng::new().unwrap();

src/prelude.rs

@@ -0,0 +1,8 @@
+//! Crate-local prelude (for alloc-dependent features like `Vec`)
+
+// TODO: switch to alloc::prelude
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+pub use alloc::vec::Vec;
+
+#[cfg(feature = "std")]
+pub use std::vec::Vec;

src/ristretto.rs

@@ -178,6 +178,9 @@ use subtle::Choice;
 use edwards::EdwardsPoint;
 use edwards::EdwardsBasepointTable;
 
+#[allow(unused_imports)]
+use prelude::*;
+
 use scalar::Scalar;
 
 use curve_models::CompletedPoint;
@@ -418,7 +421,7 @@ impl RistrettoPoint {
     /// }
     /// # }
     /// ```
-    #[cfg(any(feature = "alloc", feature = "std"))]
+    #[cfg(feature = "alloc")]
     pub fn double_and_compress_batch<'a, I>(points: I) -> Vec<CompressedRistretto>
         where I: IntoIterator<Item = &'a RistrettoPoint>
     {
@@ -798,7 +801,7 @@ define_mul_variants!(LHS = Scalar, RHS = RistrettoPoint, Output = RistrettoPoint
 // These use iterator combinators to unwrap the underlying points and
 // forward to the EdwardsPoint implementations.
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl MultiscalarMul for RistrettoPoint {
     type Point = RistrettoPoint;
     
@@ -816,7 +819,7 @@ impl MultiscalarMul for RistrettoPoint {
     }
 }
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl VartimeMultiscalarMul for RistrettoPoint {
     type Point = RistrettoPoint;
     
@@ -955,6 +958,7 @@ impl Debug for RistrettoPoint {
 
 #[cfg(all(test, feature = "stage2_build"))]
 mod test {
+    #[cfg(feature = "rand")]
     use rand::rngs::OsRng;
 
     use scalar::Scalar;
@@ -1090,6 +1094,7 @@ mod test {
         }
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn four_torsion_random() {
         let mut rng = OsRng::new().unwrap();
@@ -1152,6 +1157,7 @@ mod test {
         }
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn random_roundtrip() {
         let mut rng = OsRng::new().unwrap();
@@ -1164,6 +1170,7 @@ mod test {
         }
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn double_and_compress_1024_random_points() {
         let mut rng = OsRng::new().unwrap();
@@ -1178,6 +1185,7 @@ mod test {
         }
     }
 
+    #[cfg(feature = "rand")]
     #[test]
     fn random_is_valid() {
         let mut rng = OsRng::new().unwrap();

src/scalar.rs

@@ -725,7 +725,7 @@ impl Scalar {
     /// assert_eq!(scalars[3], Scalar::from(11u64).invert());
     /// # }
     /// ```
-    #[cfg(any(feature = "alloc", feature = "std"))]
+    #[cfg(feature = "alloc")]
     pub fn batch_invert(inputs: &mut [Scalar]) -> Scalar {
         // This code is essentially identical to the FieldElement
         // implementation, and is documented there.  Unfortunately,

src/scalar_mul/straus.rs

@@ -24,6 +24,9 @@ use traits::MultiscalarMul;
 #[cfg(any(feature = "alloc", feature = "std"))]
 use traits::VartimeMultiscalarMul;
 
+#[allow(unused_imports)]
+use prelude::*;
+
 /// Perform multiscalar multiplication by the interleaved window
 /// method, also known as Straus' method (since it was apparently
 /// [first published][solution] by Straus in 1964, as a solution to [a
@@ -48,7 +51,7 @@ use traits::VartimeMultiscalarMul;
 #[cfg(any(feature = "alloc", feature = "std"))]
 pub struct Straus {}
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl MultiscalarMul for Straus {
     type Point = EdwardsPoint;
 
@@ -145,7 +148,7 @@ impl MultiscalarMul for Straus {
     }
 }
 
-#[cfg(any(feature = "alloc", feature = "std"))]
+#[cfg(feature = "alloc")]
 impl VartimeMultiscalarMul for Straus {
     type Point = EdwardsPoint;