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    // This lint claims ugly casting is somehow safer than transmute, but there's
    // no evidence that is the case. Shush.
    #![allow(clippy::transmute_ptr_to_ptr)]

    use std::fmt;
    use std::mem::{self, MaybeUninit};

    /// A wrapper around a byte buffer that is incrementally filled and initialized.
    ///
    /// This type is a sort of "double cursor". It tracks three regions in the
    /// buffer: a region at the beginning of the buffer that has been logically
    /// filled with data, a region that has been initialized at some point but not
    /// yet logically filled, and a region at the end that may be uninitialized.
    /// The filled region is guaranteed to be a subset of the initialized region.
    ///
    /// In summary, the contents of the buffer can be visualized as:
    ///
    /// ```not_rust
    /// [             capacity              ]
    /// [ filled |         unfilled         ]
    /// [    initialized    | uninitialized ]
    /// ```
    ///
    /// It is undefined behavior to de-initialize any bytes from the uninitialized
    /// region, since it is merely unknown whether this region is uninitialized or
    /// not, and if part of it turns out to be initialized, it must stay initialized.
    pub struct ReadBuf<'a> {
        buf: &'a mut [MaybeUninit<u8>],
        filled: usize,
        initialized: usize,
    }

    impl<'a> ReadBuf<'a> {
        /// Creates a new `ReadBuf` from a fully initialized buffer.
        #[inline]
        pub fn new(buf: &'a mut [u8]) -> ReadBuf<'a> {
            let initialized = buf.len();
            let buf = unsafe { mem::transmute::<&mut [u8], &mut [MaybeUninit<u8>]>(buf) };
            ReadBuf {
                buf,
                filled: 0,
                initialized,
            }
        }

        /// Creates a new `ReadBuf` from a fully uninitialized buffer.
        ///
        /// Use `assume_init` if part of the buffer is known to be already inintialized.
        #[inline]
        pub fn uninit(buf: &'a mut [MaybeUninit<u8>]) -> ReadBuf<'a> {
            ReadBuf {
                buf,
                filled: 0,
                initialized: 0,
            }
        }

        /// Returns the total capacity of the buffer.
        #[inline]
        pub fn capacity(&self) -> usize {
            self.buf.len()
        }

        /// Returns a shared reference to the filled portion of the buffer.
        #[inline]
        pub fn filled(&self) -> &[u8] {
            let slice = &self.buf[..self.filled];
            // safety: filled describes how far into the buffer that the
            // user has filled with bytes, so it's been initialized.
            // TODO: This could use `MaybeUninit::slice_get_ref` when it is stable.
            unsafe { mem::transmute::<&[MaybeUninit<u8>], &[u8]>(slice) }
        }

        /// Returns a mutable reference to the filled portion of the buffer.
        #[inline]
        pub fn filled_mut(&mut self) -> &mut [u8] {
            let slice = &mut self.buf[..self.filled];
            // safety: filled describes how far into the buffer that the
            // user has filled with bytes, so it's been initialized.
            // TODO: This could use `MaybeUninit::slice_get_mut` when it is stable.
            unsafe { mem::transmute::<&mut [MaybeUninit<u8>], &mut [u8]>(slice) }
        }

        /// Returns a new `ReadBuf` comprised of the unfilled section up to `n`.
        #[inline]
        pub fn take(&mut self, n: usize) -> ReadBuf<'_> {
            let max = std::cmp::min(self.remaining(), n);
            // Saftey: We don't set any of the `unfilled_mut` with `MaybeUninit::uninit`.
            unsafe { ReadBuf::uninit(&mut self.unfilled_mut()[..max]) }
        }

        /// Returns a shared reference to the initialized portion of the buffer.
        ///
        /// This includes the filled portion.
        #[inline]
        pub fn initialized(&self) -> &[u8] {
            let slice = &self.buf[..self.initialized];
            // safety: initialized describes how far into the buffer that the
            // user has at some point initialized with bytes.
            // TODO: This could use `MaybeUninit::slice_get_ref` when it is stable.
            unsafe { mem::transmute::<&[MaybeUninit<u8>], &[u8]>(slice) }
        }

        /// Returns a mutable reference to the initialized portion of the buffer.
        ///
        /// This includes the filled portion.
        #[inline]
        pub fn initialized_mut(&mut self) -> &mut [u8] {
            let slice = &mut self.buf[..self.initialized];
            // safety: initialized describes how far into the buffer that the
            // user has at some point initialized with bytes.
            // TODO: This could use `MaybeUninit::slice_get_mut` when it is stable.
            unsafe { mem::transmute::<&mut [MaybeUninit<u8>], &mut [u8]>(slice) }
        }

        /// Returns a mutable reference to the unfilled part of the buffer without ensuring that it has been fully
        /// initialized.
        ///
        /// # Safety
        ///
        /// The caller must not de-initialize portions of the buffer that have already been initialized.
        /// This includes any bytes in the region marked as uninitialized by `ReadBuf`.
        #[inline]
        pub unsafe fn unfilled_mut(&mut self) -> &mut [MaybeUninit<u8>] {
            &mut self.buf[self.filled..]
        }

        /// Returns a mutable reference to the unfilled part of the buffer, ensuring it is fully initialized.
        ///
        /// Since `ReadBuf` tracks the region of the buffer that has been initialized, this is effectively "free" after
        /// the first use.
        #[inline]
        pub fn initialize_unfilled(&mut self) -> &mut [u8] {
            self.initialize_unfilled_to(self.remaining())
        }

        /// Returns a mutable reference to the first `n` bytes of the unfilled part of the buffer, ensuring it is
        /// fully initialized.
        ///
        /// # Panics
        ///
        /// Panics if `self.remaining()` is less than `n`.
        #[inline]
        pub fn initialize_unfilled_to(&mut self, n: usize) -> &mut [u8] {
            assert!(self.remaining() >= n, "n overflows remaining");

            // This can't overflow, otherwise the assert above would have failed.
            let end = self.filled + n;

            if self.initialized < end {
                unsafe {
                    self.buf[self.initialized..end]
                        .as_mut_ptr()
                        .write_bytes(0, end - self.initialized);
                }
                self.initialized = end;
            }

            let slice = &mut self.buf[self.filled..end];
            // safety: just above, we checked that the end of the buf has
            // been initialized to some value.
            unsafe { mem::transmute::<&mut [MaybeUninit<u8>], &mut [u8]>(slice) }
        }

        /// Returns the number of bytes at the end of the slice that have not yet been filled.
        #[inline]
        pub fn remaining(&self) -> usize {
            self.capacity() - self.filled
        }

        /// Clears the buffer, resetting the filled region to empty.
        ///
        /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
        #[inline]
        pub fn clear(&mut self) {
            self.filled = 0;
        }

        /// Advances the size of the filled region of the buffer.
        ///
        /// The number of initialized bytes is not changed.
        ///
        /// # Panics
        ///
        /// Panics if the filled region of the buffer would become larger than the initialized region.
        #[inline]
        pub fn advance(&mut self, n: usize) {
            let new = self.filled.checked_add(n).expect("filled overflow");
            self.set_filled(new);
        }

        /// Sets the size of the filled region of the buffer.
        ///
        /// The number of initialized bytes is not changed.
        ///
        /// Note that this can be used to *shrink* the filled region of the buffer in addition to growing it (for
        /// example, by a `AsyncRead` implementation that compresses data in-place).
        ///
        /// # Panics
        ///
        /// Panics if the filled region of the buffer would become larger than the intialized region.
        #[inline]
        pub fn set_filled(&mut self, n: usize) {
            assert!(
                n <= self.initialized,
                "filled must not become larger than initialized"
            );
            self.filled = n;
        }

        /// Asserts that the first `n` unfilled bytes of the buffer are initialized.
        ///
        /// `ReadBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
        /// bytes than are already known to be initialized.
        ///
        /// # Safety
        ///
        /// The caller must ensure that `n` unfilled bytes of the buffer have already been initialized.
        #[inline]
        pub unsafe fn assume_init(&mut self, n: usize) {
            let new = self.filled + n;
            if new > self.initialized {
                self.initialized = new;
            }
        }

        /// Appends data to the buffer, advancing the written position and possibly also the initialized position.
        ///
        /// # Panics
        ///
        /// Panics if `self.remaining()` is less than `buf.len()`.
        #[inline]
        pub fn put_slice(&mut self, buf: &[u8]) {
            assert!(
                self.remaining() >= buf.len(),
                "buf.len() must fit in remaining()"
            );

            let amt = buf.len();
            // Cannot overflow, asserted above
            let end = self.filled + amt;

            // Safety: the length is asserted above
            unsafe {
                self.buf[self.filled..end]
                    .as_mut_ptr()
                    .cast::<u8>()
                    .copy_from_nonoverlapping(buf.as_ptr(), amt);
            }

            if self.initialized < end {
                self.initialized = end;
            }
            self.filled = end;
        }
    }

    impl fmt::Debug for ReadBuf<'_> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("ReadBuf")
                .field("filled", &self.filled)
                .field("initialized", &self.initialized)
                .field("capacity", &self.capacity())
                .finish()
        }
    }