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    // Copyright 2015-2016 Brian Smith.
    //
    // Permission to use, copy, modify, and/or distribute this software for any
    // purpose with or without fee is hereby granted, provided that the above
    // copyright notice and this permission notice appear in all copies.
    //
    // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
    // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
    // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
    // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
    // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    //! Polyfills for functionality that will (hopefully) be added to Rust's
    //! standard library soon.

    use core;

    // A better name for the `&*` idiom for removing the mutability from a
    // reference.
    #[cfg(feature = "use_heap")]
    #[inline(always)]
    pub fn ref_from_mut_ref<'a, T: ?Sized>(x: &'a mut T) -> &'a T { x }

    #[inline(always)]
    pub fn u64_from_usize(x: usize) -> u64 { x as u64 }

    /// `core::num::Wrapping` doesn't support `rotate_left`.
    /// There is no usable trait for `rotate_left`, so this polyfill just
    /// hard-codes u32. https://github.com/rust-lang/rust/issues/32463
    #[inline(always)]
    pub fn wrapping_rotate_left_u32(x: core::num::Wrapping<u32>, n: u32)
                                    -> core::num::Wrapping<u32> {
        core::num::Wrapping(x.0.rotate_left(n))
    }

    pub mod slice {
        use core;

        #[inline(always)]
        pub fn u32_from_be_u8(buffer: &[u8; 4]) -> u32 {
            u32::from(buffer[0]) << 24 |
            u32::from(buffer[1]) << 16 |
            u32::from(buffer[2]) << 8 |
            u32::from(buffer[3])
        }

        #[inline(always)]
        pub fn u32_from_le_u8(buffer: &[u8; 4]) -> u32 {
            u32::from(buffer[0]) |
            u32::from(buffer[1]) << 8 |
            u32::from(buffer[2]) << 16 |
            u32::from(buffer[3]) << 24
        }

        #[inline(always)]
        pub fn be_u8_from_u32(value: u32) -> [u8; 4] {
            [((value >> 24) & 0xff) as u8,
             ((value >> 16) & 0xff) as u8,
             ((value >> 8) & 0xff) as u8,
             (value & 0xff) as u8]
        }

        // https://github.com/rust-lang/rust/issues/27750
        // https://internals.rust-lang.org/t/stabilizing-basic-functions-on-arrays-and-slices/2868
        #[inline(always)]
        pub fn fill(dest: &mut [u8], value: u8) {
            for d in dest {
                *d = value;
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        pub fn u64_as_u8(src: &[u64]) -> &[u8] {
            unsafe {
                core::slice::from_raw_parts(src.as_ptr() as *const u8,
                                            src.len() * 8)
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        pub fn u64_as_u8_mut(src: &mut [u64]) -> &mut [u8] {
            unsafe {
                core::slice::from_raw_parts_mut(src.as_mut_ptr() as *mut u8,
                                                src.len() * 8)
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        #[allow(dead_code)] // Only used on 32-bit builds currently
        pub fn u32_as_u8<'a>(src: &'a [u32]) -> &'a [u8] {
            unsafe {
                core::slice::from_raw_parts(src.as_ptr() as *mut u8, src.len() * 4)
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        #[allow(dead_code)] // Only used on 32-bit builds currently
        pub fn u32_as_u8_mut<'a>(src: &'a mut [u32]) -> &'a mut [u8] {
            unsafe {
                core::slice::from_raw_parts_mut(src.as_mut_ptr() as *mut u8,
                                                src.len() * 4)
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        pub fn u64_as_u32(src: &[u64]) -> &[u32] {
            unsafe {
                core::slice::from_raw_parts(src.as_ptr() as *const u32,
                                            src.len() * 2)
            }
        }

        // https://internals.rust-lang.org/t/safe-trasnsmute-for-slices-e-g-u64-u32-particularly-simd-types/2871
        #[inline(always)]
        pub fn u64_as_u32_mut(src: &mut [u64]) -> &mut [u32] {
            unsafe {
                core::slice::from_raw_parts_mut(src.as_mut_ptr() as *mut u32,
                                                src.len() * 2)
            }
        }

        #[inline(always)]
        pub fn as_wrapping_mut<T>(src: &mut [T]) -> &mut [core::num::Wrapping<T>] {
            unsafe {
                core::slice::from_raw_parts_mut(
                    src.as_mut_ptr() as *mut core::num::Wrapping<T>, src.len())
            }
        }
    }

    /// Returns a reference to the elements of `$slice` as an array, verifying that
    /// the slice is of length `$len`.
    macro_rules! slice_as_array_ref {
        ($slice:expr, $len:expr) => {
            {
                use error;

                fn slice_as_array_ref<T>(slice: &[T])
                                         -> Result<&[T; $len], error::Unspecified> {
                    if slice.len() != $len {
                        return Err(error::Unspecified);
                    }
                    Ok(unsafe {
                        &*(slice.as_ptr() as *const [T; $len])
                    })
                }
                slice_as_array_ref($slice)
            }
        }
    }

    /// Returns a reference to elements of `$slice` as a mutable array, verifying
    /// that the slice is of length `$len`.
    macro_rules! slice_as_array_ref_mut {
        ($slice:expr, $len:expr) => {
            {
                use error;

                fn slice_as_array_ref<T>(slice: &mut [T])
                                         -> Result<&mut [T; $len],
                                                   error::Unspecified> {
                    if slice.len() != $len {
                        return Err(error::Unspecified);
                    }
                    Ok(unsafe {
                        &mut *(slice.as_mut_ptr() as *mut [T; $len])
                    })
                }
                slice_as_array_ref($slice)
            }
        }
    }

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