Migrate ff to rand_core 0.3 (used by rand 0.5)

ff/src/lib.rs

@@ -1,7 +1,7 @@
 #![allow(unused_imports)]
 
 extern crate byteorder;
-extern crate rand;
+extern crate rand_core;
 
 #[cfg(feature = "derive")]
 #[macro_use]
@@ -10,14 +10,18 @@ extern crate ff_derive;
 #[cfg(feature = "derive")]
 pub use ff_derive::*;
 
+use rand_core::RngCore;
 use std::error::Error;
 use std::fmt;
 use std::io::{self, Read, Write};
 
 /// This trait represents an element of a field.
 pub trait Field:
-    Sized + Eq + Copy + Clone + Send + Sync + fmt::Debug + fmt::Display + 'static + rand::Rand
+    Sized + Eq + Copy + Clone + Send + Sync + fmt::Debug + fmt::Display + 'static
 {
+    /// Returns an element chosen uniformly at random using a user-provided RNG.
+    fn random<R: RngCore>(rng: &mut R) -> Self;
+
     /// Returns the zero element of the field, the additive identity.
     fn zero() -> Self;
 
@@ -100,7 +104,6 @@ pub trait PrimeFieldRepr:
     + fmt::Debug
     + fmt::Display
     + 'static
-    + rand::Rand
     + AsRef<[u64]>
     + AsMut<[u64]>
     + From<u64>