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diff --git a/rand/rand_core/src/impls.rs b/rand/rand_core/src/impls.rs
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+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Helper functions for implementing `RngCore` functions.
+//!
+//! For cross-platform reproducibility, these functions all use Little Endian:
+//! least-significant part first. For example, `next_u64_via_u32` takes `u32`
+//! values `x, y`, then outputs `(y << 32) | x`. To implement `next_u32`
+//! from `next_u64` in little-endian order, one should use `next_u64() as u32`.
+//!
+//! Byte-swapping (like the std `to_le` functions) is only needed to convert
+//! to/from byte sequences, and since its purpose is reproducibility,
+//! non-reproducible sources (e.g. `OsRng`) need not bother with it.
+
+use core::intrinsics::transmute;
+use core::ptr::copy_nonoverlapping;
+use core::slice;
+use core::cmp::min;
+use core::mem::size_of;
+use RngCore;
+
+
+/// Implement `next_u64` via `next_u32`, little-endian order.
+pub fn next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
+    // Use LE; we explicitly generate one value before the next.
+    let x = u64::from(rng.next_u32());
+    let y = u64::from(rng.next_u32());
+    (y << 32) | x
+}
+
+/// Implement `fill_bytes` via `next_u64` and `next_u32`, little-endian order.
+///
+/// The fastest way to fill a slice is usually to work as long as possible with
+/// integers. That is why this method mostly uses `next_u64`, and only when
+/// there are 4 or less bytes remaining at the end of the slice it uses
+/// `next_u32` once.
+pub fn fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
+    let mut left = dest;
+    while left.len() >= 8 {
+        let (l, r) = {left}.split_at_mut(8);
+        left = r;
+        let chunk: [u8; 8] = unsafe {
+            transmute(rng.next_u64().to_le())
+        };
+        l.copy_from_slice(&chunk);
+    }
+    let n = left.len();
+    if n > 4 {
+        let chunk: [u8; 8] = unsafe {
+            transmute(rng.next_u64().to_le())
+        };
+        left.copy_from_slice(&chunk[..n]);
+    } else if n > 0 {
+        let chunk: [u8; 4] = unsafe {
+            transmute(rng.next_u32().to_le())
+        };
+        left.copy_from_slice(&chunk[..n]);
+    }
+}
+
+macro_rules! impl_uint_from_fill {
+    ($rng:expr, $ty:ty, $N:expr) => ({
+        debug_assert!($N == size_of::<$ty>());
+
+        let mut int: $ty = 0;
+        unsafe {
+            let ptr = &mut int as *mut $ty as *mut u8;
+            let slice = slice::from_raw_parts_mut(ptr, $N);
+            $rng.fill_bytes(slice);
+        }
+        int
+    });
+}
+
+macro_rules! fill_via_chunks {
+    ($src:expr, $dst:expr, $ty:ty, $size:expr) => ({
+        let chunk_size_u8 = min($src.len() * $size, $dst.len());
+        let chunk_size = (chunk_size_u8 + $size - 1) / $size;
+        if cfg!(target_endian="little") {
+            unsafe {
+                copy_nonoverlapping(
+                    $src.as_ptr() as *const u8,
+                    $dst.as_mut_ptr(),
+                    chunk_size_u8);
+            }
+        } else {
+            for (&n, chunk) in $src.iter().zip($dst.chunks_mut($size)) {
+                let tmp = n.to_le();
+                let src_ptr = &tmp as *const $ty as *const u8;
+                unsafe {
+                    copy_nonoverlapping(src_ptr,
+                                        chunk.as_mut_ptr(),
+                                        chunk.len());
+                }
+            }
+        }
+
+        (chunk_size, chunk_size_u8)
+    });
+}
+
+/// Implement `fill_bytes` by reading chunks from the output buffer of a block
+/// based RNG.
+///
+/// The return values are `(consumed_u32, filled_u8)`.
+///
+/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
+/// the length of `dest`.
+/// `consumed_u32` is the number of words consumed from `src`, which is the same
+/// as `filled_u8 / 4` rounded up.
+///
+/// # Example
+/// (from `IsaacRng`)
+///
+/// ```ignore
+/// fn fill_bytes(&mut self, dest: &mut [u8]) {
+///     let mut read_len = 0;
+///     while read_len < dest.len() {
+///         if self.index >= self.rsl.len() {
+///             self.isaac();
+///         }
+///
+///         let (consumed_u32, filled_u8) =
+///             impls::fill_via_u32_chunks(&mut self.rsl[self.index..],
+///                                        &mut dest[read_len..]);
+///
+///         self.index += consumed_u32;
+///         read_len += filled_u8;
+///     }
+/// }
+/// ```
+pub fn fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize) {
+    fill_via_chunks!(src, dest, u32, 4)
+}
+
+/// Implement `fill_bytes` by reading chunks from the output buffer of a block
+/// based RNG.
+///
+/// The return values are `(consumed_u64, filled_u8)`.
+/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
+/// the length of `dest`.
+/// `consumed_u64` is the number of words consumed from `src`, which is the same
+/// as `filled_u8 / 8` rounded up.
+///
+/// See `fill_via_u32_chunks` for an example.
+pub fn fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize) {
+    fill_via_chunks!(src, dest, u64, 8)
+}
+
+/// Implement `next_u32` via `fill_bytes`, little-endian order.
+pub fn next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32 {
+    impl_uint_from_fill!(rng, u32, 4)
+}
+
+/// Implement `next_u64` via `fill_bytes`, little-endian order.
+pub fn next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
+    impl_uint_from_fill!(rng, u64, 8)
+}
+
+// TODO: implement tests for the above