Add #![no_std] (fixes #16)

Use ::core in lieu of ::std, allowing this crate to be usable in #![no_std]
environments.

Gates features that presently depend on ::std (presently just rand) behind a
"std" cargo feature, which is enabled by default.

Cargo.toml

@@ -14,9 +14,16 @@ exclude = [
     ".gitignore"
 ]
 
-[dependencies]
-rand = "0.3"
-arrayref = "0.3.2"
+[dependencies.rand]
+optional = true
+version = "0.3"
+
+[dependencies.arrayref]
+version = "0.3.2"
+
+[features]
+default = ["std"]
+std = ["rand"]
 
 # The development profile, used for `cargo build`.
 [profile.dev]

src/curve.rs

@@ -77,10 +77,10 @@
 // affine and projective cakes and eat both of them too.
 #![allow(non_snake_case)]
 
-use std::fmt::Debug;
-use std::iter::Iterator;
-use std::ops::{Add, Sub, Neg, Index};
-use std::cmp::{PartialEq, Eq};
+use core::fmt::Debug;
+use core::iter::Iterator;
+use core::ops::{Add, Sub, Neg, Index};
+use core::cmp::{PartialEq, Eq};
 
 use constants;
 use field::FieldElement;
@@ -100,7 +100,7 @@ use util::bytes_equal_ct;
 pub struct CompressedPoint(pub [u8; 32]);
 
 impl Debug for CompressedPoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "CompressedPoint: {:?}", &self.0[..])
     }
 }
@@ -763,35 +763,35 @@ impl ExtendedPoint {
 // ------------------------------------------------------------------------
 
 impl Debug for ExtendedPoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "ExtendedPoint(\n\tX: {:?},\n\tY: {:?},\n\tZ: {:?},\n\tT: {:?}\n)",
                &self.X, &self.Y, &self.Z, &self.T)
     }
 }
 
 impl Debug for ProjectivePoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "ProjectivePoint(\n\tX: {:?},\n\tY: {:?},\n\tZ: {:?}\n)",
                &self.X, &self.Y, &self.Z)
     }
 }
 
 impl Debug for CompletedPoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "CompletedPoint(\n\tX: {:?},\n\tY: {:?},\n\tZ: {:?},\n\tT: {:?}\n)",
                &self.X, &self.Y, &self.Z, &self.T)
     }
 }
 
 impl Debug for PreComputedPoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "PreComputedPoint(\n\ty_plus_x: {:?},\n\ty_minus_x: {:?},\n\txy2d: {:?}\n)",
                &self.y_plus_x, &self.y_minus_x, &self.xy2d)
     }
 }
 
 impl Debug for CachedPoint {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "CachedPoint(\n\tY_plus_X: {:?},\n\tY_minus_X: {:?},\n\tZ: {:?},\n\tT2d: {:?}\n)",
                &self.Y_plus_X, &self.Y_minus_X, &self.Z, &self.T2d)
     }

src/field.rs

@@ -15,14 +15,14 @@
 //! Based on Adam Langley's curve25519-donna and (Golang) ed25519
 //! implementations.
 
-use std::clone::Clone;
-use std::fmt::Debug;
-use std::ops::{Add, AddAssign};
-use std::ops::{Sub, SubAssign};
-use std::ops::{Mul, MulAssign};
-use std::ops::{Index, IndexMut};
-use std::cmp::{Eq, PartialEq};
-use std::ops::Neg;
+use core::clone::Clone;
+use core::fmt::Debug;
+use core::ops::{Add, AddAssign};
+use core::ops::{Sub, SubAssign};
+use core::ops::{Mul, MulAssign};
+use core::ops::{Index, IndexMut};
+use core::cmp::{Eq, PartialEq};
+use core::ops::Neg;
 
 use util::byte_is_nonzero;
 
@@ -65,7 +65,7 @@ impl PartialEq for FieldElement {
 impl Eq for FieldElement {}
 
 impl Debug for FieldElement {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
         write!(f, "FieldElement: {:?}", &self.0[..])
     }
 }

src/lib.rs

@@ -9,6 +9,7 @@
 //  - Isis Agora Lovecruft <[email protected]>
 //  - Henry de Valence <[email protected]>
 
+#![no_std]
 #![allow(unused_features)]
 #![feature(test)]
 #![deny(missing_docs)] // refuse to compile if documentation is missing
@@ -37,6 +38,7 @@ extern crate test;
 #[macro_use]
 extern crate arrayref;
 
+#[cfg(feature = "std")]
 extern crate rand;
 
 // Modules for low-level operations directly on field elements and curve points.

src/scalar.rs

@@ -26,9 +26,10 @@
 //! However, in contrast to `FieldElement`s, `Scalar`s are stored in
 //! memory as bytes, allowing easy access to the bits of the `Scalar`.
 
-use std::clone::Clone;
-use std::ops::{Index, IndexMut};
+use core::clone::Clone;
+use core::ops::{Index, IndexMut};
 
+#[cfg(feature = "std")]
 use rand::Rng;
 
 use field::{load3, load4};
@@ -73,6 +74,7 @@ impl Scalar {
     /// # Returns
     ///
     /// A random scalar within ℤ/lℤ.
+    #[cfg(feature = "std")]
     pub fn random<T: Rng>(csprng: &mut T) -> Self {
         let mut scalar_bytes = [0u8; 64];
         csprng.fill_bytes(&mut scalar_bytes);