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    use super::{BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
    use crate::mem::MaybeUninit;

    /// Copies the entire contents of a reader into a writer.
    ///
    /// This function will continuously read data from `reader` and then
    /// write it into `writer` in a streaming fashion until `reader`
    /// returns EOF.
    ///
    /// On success, the total number of bytes that were copied from
    /// `reader` to `writer` is returned.
    ///
    /// If you’re wanting to copy the contents of one file to another and you’re
    /// working with filesystem paths, see the [`fs::copy`] function.
    ///
    /// [`fs::copy`]: crate::fs::copy
    ///
    /// # Errors
    ///
    /// This function will return an error immediately if any call to [`read`] or
    /// [`write`] returns an error. All instances of [`ErrorKind::Interrupted`] are
    /// handled by this function and the underlying operation is retried.
    ///
    /// [`read`]: Read::read
    /// [`write`]: Write::write
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io;
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut reader: &[u8] = b"hello";
    ///     let mut writer: Vec<u8> = vec![];
    ///
    ///     io::copy(&mut reader, &mut writer)?;
    ///
    ///     assert_eq!(&b"hello"[..], &writer[..]);
    ///     Ok(())
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
    where
        R: Read,
        W: Write,
    {
        cfg_if::cfg_if! {
            if #[cfg(any(target_os = "linux", target_os = "android"))] {
                crate::sys::kernel_copy::copy_spec(reader, writer)
            } else {
                generic_copy(reader, writer)
            }
        }
    }

    /// The userspace read-write-loop implementation of `io::copy` that is used when
    /// OS-specific specializations for copy offloading are not available or not applicable.
    pub(crate) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
    where
        R: Read,
        W: Write,
    {
        BufferedCopySpec::copy_to(reader, writer)
    }

    /// Specialization of the read-write loop that either uses a stack buffer
    /// or reuses the internal buffer of a BufWriter
    trait BufferedCopySpec: Write {
        fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64>;
    }

    impl<W: Write + ?Sized> BufferedCopySpec for W {
        default fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
            stack_buffer_copy(reader, writer)
        }
    }

    impl<I: Write> BufferedCopySpec for BufWriter<I> {
        fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
            if writer.capacity() < DEFAULT_BUF_SIZE {
                return stack_buffer_copy(reader, writer);
            }

            // FIXME: #42788
            //
            //   - This creates a (mut) reference to a slice of
            //     _uninitialized_ integers, which is **undefined behavior**
            //
            //   - Only the standard library gets to soundly "ignore" this,
            //     based on its privileged knowledge of unstable rustc
            //     internals;
            unsafe {
                let spare_cap = writer.buffer_mut().spare_capacity_mut();
                reader.initializer().initialize(MaybeUninit::slice_assume_init_mut(spare_cap));
            }

            let mut len = 0;

            loop {
                let buf = writer.buffer_mut();
                let spare_cap = buf.spare_capacity_mut();

                if spare_cap.len() >= DEFAULT_BUF_SIZE {
                    match reader.read(unsafe { MaybeUninit::slice_assume_init_mut(spare_cap) }) {
                        Ok(0) => return Ok(len), // EOF reached
                        Ok(bytes_read) => {
                            assert!(bytes_read <= spare_cap.len());
                            // Safety: The initializer contract guarantees that either it or `read`
                            // will have initialized these bytes. And we just checked that the number
                            // of bytes is within the buffer capacity.
                            unsafe { buf.set_len(buf.len() + bytes_read) };
                            len += bytes_read as u64;
                            // Read again if the buffer still has enough capacity, as BufWriter itself would do
                            // This will occur if the reader returns short reads
                            continue;
                        }
                        Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                        Err(e) => return Err(e),
                    }
                }

                writer.flush_buf()?;
            }
        }
    }

    fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
        reader: &mut R,
        writer: &mut W,
    ) -> Result<u64> {
        let mut buf = MaybeUninit::<[u8; DEFAULT_BUF_SIZE]>::uninit();
        // FIXME: #42788
        //
        //   - This creates a (mut) reference to a slice of
        //     _uninitialized_ integers, which is **undefined behavior**
        //
        //   - Only the standard library gets to soundly "ignore" this,
        //     based on its privileged knowledge of unstable rustc
        //     internals;
        unsafe {
            reader.initializer().initialize(buf.assume_init_mut());
        }

        let mut written = 0;
        loop {
            let len = match reader.read(unsafe { buf.assume_init_mut() }) {
                Ok(0) => return Ok(written),
                Ok(len) => len,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => return Err(e),
            };
            writer.write_all(unsafe { &buf.assume_init_ref()[..len] })?;
            written += len as u64;
        }
    }