src/lib/network/packet/src/lib.rs

// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! Parsing and serialization of (network) packets.
//!
//! `packet` is a library to help with the parsing and serialization of nested
//! packets. Network packets are the most common use case, but it supports any
//! packet structure with headers, footers, and nesting.
//!
//! # Model
//!
//! The core components of `packet` are the various buffer traits (`XxxBuffer`
//! and `XxxBufferMut`). A buffer is a byte buffer with a prefix, a body, and a
//! suffix. The size of the buffer is referred to as its "capacity", and the
//! size of the body is referred to as its "length". Depending on which traits
//! are implemented, the body of the buffer may be able to shrink or grow as
//! allowed by the capacity as packets are parsed or serialized.
//!
//! ## Parsing
//!
//! When parsing packets, the body of the buffer stores the next packet to be
//! parsed. When a packet is parsed from the buffer, any headers, footers, and
//! padding are "consumed" from the buffer. Thus, after a packet has been
//! parsed, the body of the buffer is equal to the body of the packet, and the
//! next call to `parse` will pick up where the previous call left off, parsing
//! the next encapsulated packet.
//!
//! Packet objects - the Rust objects which are the result of a successful
//! parsing operation - are advised to simply keep references into the buffer
//! for the header, footer, and body. This avoids any unnecessary copying.
//!
//! For example, consider the following packet structure, in which a TCP segment
//! is encapsulated in an IPv4 packet, which is encapsulated in an Ethernet
//! frame. In this example, we omit the Ethernet Frame Check Sequence (FCS)
//! footer. If there were any footers, they would be treated the same as
//! headers, except that they would be consumed from the end and working towards
//! the beginning, as opposed to headers, which are consumed from the beginning
//! and working towards the end.
//!
//! Also note that, in order to satisfy Ethernet's minimum body size
//! requirement, padding is added after the IPv4 packet. The IPv4 packet and
//! padding together are considered the body of the Ethernet frame. If we were
//! to include the Ethernet FCS footer in this example, it would go after the
//! padding.
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|-------------------|--------------------|-----|
//!   Ethernet header      IPv4 header         TCP segment      Padding
//! ```
//!
//! At first, the buffer's body would be equal to the bytes of the Ethernet
//! frame (although depending on how the buffer was initialized, it might have
//! extra capacity in addition to the body):
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|-------------------|--------------------|-----|
//!   Ethernet header      IPv4 header         TCP segment      Padding
//!
//! |----------------------------------------------------------------|
//!                             Buffer Body
//! ```
//!
//! First, the Ethernet frame is parsed. This results in a hypothetical
//! `EthernetFrame` object (this library does not provide any concrete parsing
//! implementations) with references into the buffer, and updates the body of
//! the buffer to be equal to the body of the Ethernet frame:
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|----------------------------------------------|
//!   Ethernet header                  Ethernet body
//!          |                                 |
//!          +--------------------------+      |
//!                                     |      |
//!                   EthernetFrame { header, body }
//!
//! |-----------------|----------------------------------------------|
//!    buffer prefix                   buffer body
//! ```
//!
//! The `EthernetFrame` object mutably borrows the buffer. So long as it exists,
//! the buffer cannot be used directly (although the `EthernetFrame` object may
//! be used to access or modify the contents of the buffer). In order to parse
//! the body of the Ethernet frame, we have to drop the `EthernetFrame` object
//! so that we can call methods on the buffer again. \[1\]
//!
//! After dropping the `EthernetFrame` object, the IPv4 packet is parsed. Recall
//! that the Ethernet body contains both the IPv4 packet and some padding. Since
//! IPv4 packets encode their own length, the IPv4 packet parser is able to
//! detect that some of the bytes it's operating on are padding bytes. It is the
//! parser's responsibility to consume and discard these bytes so that they are
//! not erroneously treated as part of the IPv4 packet's body in subsequent
//! parsings.
//!
//! This parsing results in a hypothetical `Ipv4Packet` object with references
//! into the buffer, and updates the body of the buffer to be equal to the body
//! of the IPv4 packet:
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|-------------------|--------------------|-----|
//!                        IPv4 header          IPv4 body
//!                             |                   |
//!                             +-----------+       |
//!                                         |       |
//!                          Ipv4Packet { header, body }
//!
//! |-------------------------------------|--------------------|-----|
//!              buffer prefix                 buffer body       buffer suffix
//! ```
//!
//! We can continue this process as long as we like, repeatedly parsing
//! subsequent packet bodies until there are no more packets to parse.
//!
//! \[1\] It is also possible to treat the `EthernetFrame`'s `body` field as a
//! buffer and parse from it directly. However, this has the disadvantage that
//! if parsing is spread across multiple functions, the functions which parse
//! the inner packets only see part of the buffer, and so if they wish to later
//! re-use the buffer for serializing new packets (see the "Serialization"
//! section of this documentation), they are limited to doing so in a smaller
//! buffer, making it more likely that a new buffer will need to be allocated.
//!
//! ## Serialization
//!
//! In this section, we will illustrate serialization using the same packet
//! structure that was used to illustrate parsing - a TCP segment in an IPv4
//! packet in an Ethernet frame.
//!
//! Serialization comprises two tasks:
//! - First, given a buffer with sufficient capacity, and part of the packet
//!   already serialized, serialize the next layer of the packet. For example,
//!   given a buffer with a TCP segment already serialized in it, serialize the
//!   IPv4 header, resulting in an IPv4 packet containing a TCP segment.
//! - Second, given a description of a nested sequence of packets, figure out
//!   the constraints that a buffer must satisfy in order to be able to fit the
//!   entire sequence, and allocate a buffer which satisfies those constraints.
//!   This buffer is then used to serialize one layer at a time, as described in
//!   the previous bullet.
//!
//! ### Serializing into a buffer
//!
//! The [`PacketBuilder`] trait is implemented by types which are capable of
//! serializing a new layer of a packet into an existing buffer. For example, we
//! might define an `Ipv4PacketBuilder` type, which describes the source IP
//! address, destination IP address, and any other metadata required to generate
//! the header of an IPv4 packet. Importantly, a `PacketBuilder` does *not*
//! define any encapsulated packets. In order to construct a TCP segment in an
//! IPv4 packet, we would need a separate `TcpSegmentBuilder` to describe the
//! TCP segment.
//!
//! A `PacketBuilder` exposes the number of bytes it requires for headers,
//! footers, and minimum and maximum body lengths via the `constraints` method.
//! It serializes via the `serialize` method.
//!
//! In order to serialize a `PacketBuilder`, a [`SerializeTarget`] must first be
//! constructed. A `SerializeTarget` is a view into a buffer used for
//! serialization, and it is initialized with the proper number of bytes for the
//! header, footer, and body. The number of bytes required for these is
//! discovered through calls to the `PacketBuilder`'s `constraints` method.
//!
//! The `PacketBuilder`'s `serialize` method serializes the headers and footers
//! of the packet into the buffer. It expects that the `SerializeTarget` is
//! initialized with a body equal to the body which will be encapsulated. For
//! example, imagine that we are trying to serialize a TCP segment in an IPv4
//! packet in an Ethernet frame, and that, so far, we have only serialized the
//! TCP segment:
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-------------------------------------|--------------------|-----|
//!                                             TCP segment
//!
//! |-------------------------------------|--------------------|-----|
//!              buffer prefix                 buffer body       buffer suffix
//! ```
//!
//! Note that the buffer's body is currently equal to the TCP segment, and the
//! contents of the body are already initialized to the segment's contents.
//!
//! Given an `Ipv4PacketBuilder`, we call the appropriate methods to discover
//! that it requires 20 bytes for its header. Thus, we modify the buffer by
//! extending the body by 20 bytes, and constructing a `SerializeTarget` whose
//! header references the newly-added 20 bytes, and whose body references the
//! old contents of the body, corresponding to the TCP segment.
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|-------------------|--------------------|-----|
//!                        IPv4 header          IPv4 body
//!                             |                   |
//!                             +-----------+       |
//!                                         |       |
//!                      SerializeTarget { header, body }
//!
//! |-----------------|----------------------------------------|-----|
//!    buffer prefix                 buffer body                 buffer suffix
//! ```
//!
//! We then pass the `SerializeTarget` to a call to the `Ipv4PacketBuilder`'s
//! `serialize` method, and it serializes the IPv4 header in the space provided.
//! When the call to `serialize` returns, the `SerializeTarget` and
//! `Ipv4PacketBuilder` have been discarded, and the buffer's body is now equal
//! to the bytes of the IPv4 packet.
//!
//! ```text
//! |-------------------------------------|++++++++++++++++++++|-----| TCP segment
//! |-----------------|++++++++++++++++++++++++++++++++++++++++|-----| IPv4 packet
//! |++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++| Ethernet frame
//!
//! |-----------------|----------------------------------------|-----|
//!                                  IPv4 packet
//!
//! |-----------------|----------------------------------------|-----|
//!    buffer prefix                 buffer body                 buffer suffix
//! ```
//!
//! Now, we are ready to repeat the same process with the Ethernet layer of the
//! packet.
//!
//! ### Constructing a buffer for serialization
//!
//! Now that we know how, given a buffer with a subset of a packet serialized
//! into it, we can serialize the next layer of the packet, we need to figure
//! out how to construct such a buffer in the first place.
//!
//! The primary challenge here is that we need to be able to commit to what
//! we're going to serialize before we actually serialize it. For example,
//! consider sending a TCP segment to the network. From the perspective of the
//! TCP module of our code, we don't know how large the buffer needs to be
//! because don't know what packet layers our TCP segment will be encapsulated
//! inside of. If the IP layer decides to route our segment over an Ethernet
//! link, then we'll need to have a buffer large enough for a TCP segment in an
//! IPv4 packet in an Ethernet segment. If, on the other hand, the IP layer
//! decides to route our segment through a GRE tunnel, then we'll need to have a
//! buffer large enough for a TCP segment in an IPv4 packet in a GRE packet in
//! an IP packet in an Ethernet segment.
//!
//! We accomplish this commit-before-serializing via the [`Serializer`] trait. A
//! `Serializer` describes a packet which can be serialized in the future, but
//! which has not yet been serialized. Unlike a `PacketBuilder`, a `Serializer`
//! describes all layers of a packet up to a certain point. For example, a
//! `Serializer` might describe a TCP segment, or it might describe a TCP
//! segment in an IP packet, or it might describe a TCP segment in an IP packet
//! in an Ethernet frame, etc.
//!
//! #### Constructing a `Serializer`
//!
//! `Serializer`s are recursive - a `Serializer` combined with a `PacketBuilder`
//! yields a new `Serializer` which describes encapsulating the original
//! `Serializer` in a new packet layer. For example, a `Serializer` describing a
//! TCP segment combined with an `Ipv4PacketBuilder` yields a `Serializer` which
//! describes a TCP segment in an IPv4 packet. Concretely, given a `Serializer`,
//! `s`, and a `PacketBuilder`, `b`, a new `Serializer` can be constructed by
//! calling `s.encapsulate(b)`. The [`Serializer::encapsulate`] method consumes
//! both the `Serializer` and the `PacketBuilder` by value, and returns a new
//! `Serializer`.
//!
//! Note that, while `Serializer`s are passed around by value, they are only as
//! large in memory as the `PacketBuilder`s they're constructed from, and those
//! should, in most cases, be quite small. If size is a concern, the
//! `PacketBuilder` trait can be implemented for a reference type (e.g.,
//! `&Ipv4PacketBuilder`), and references passed around instead of values.
//!
//! #### Constructing a buffer from a `Serializer`
//!
//! If `Serializer`s are constructed by starting at the innermost packet layer
//! and working outwards, adding packet layers, then in order to turn a
//! `Serializer` into a buffer, they are consumed by starting at the outermost
//! packet layer and working inwards.
//!
//! In order to construct a buffer, the [`Serializer::serialize`] method is
//! provided. It takes a [`NestedPacketBuilder`], which describes one or more
//! encapsulating packet layers. For example, when serializing a TCP segment in
//! an IP packet in an Ethernet frame, the `serialize` call on the IP packet
//! `Serializer` would be given a `NestedPacketBuilder` describing the Ethernet
//! frame. This call would then compute a new `NestedPacketBuilder` describing
//! the combined IP packet and Ethernet frame, and would pass this to a call to
//! `serialize` on the TCP segment `Serializer`.
//!
//! When the innermost call to `serialize` is reached, it is that call's
//! responsibility to produce a buffer which satisfies the constraints passed to
//! it, and to initialize that buffer's body with the contents of its packet.
//! For example, the TCP segment `Serializer` from the preceding example would
//! need to produce a buffer with 38 bytes of prefix for the IP and Ethernet
//! headers, and whose body was initialized to the bytes of the TCP segment.
//!
//! We can now see how `Serializer`s and `PacketBuilder`s compose - the buffer
//! returned from a call to `serialize` satisfies the requirements of the
//! `PacketBuilder::serialize` method - its body is initialized to the packet to
//! be encapsulated, and enough prefix and suffix space exist to serialize this
//! layer's header and footer. For example, the call to `Serializer::serialize`
//! on the TCP segment serializer would return a buffer with 38 bytes of prefix
//! and a body initialized to the bytes of the TCP segment. The call to
//! `Serializer::serialize` on the IP packet would then pass this buffer to a
//! call to `PacketBuilder::serialize` on its `Ipv4PacketBuilder`, resulting in
//! a buffer with 18 bytes of prefix and a body initialized to the bytes of the
//! entire IP packet. This buffer would then be suitable to return from the call
//! to `Serializer::serialize`, allowing the Ethernet layer to continue
//! operating on the buffer, and so on.
//!
//! Note in particular that, throughout this entire process of constructing
//! `Serializer`s and `PacketBuilder`s and then consuming them, a buffer is only
//! allocated once, and each byte of the packet is only serialized once. No
//! temporary buffers or copying between buffers are required.
//!
//! #### Reusing buffers
//!
//! Another important property of the `Serializer` trait is that it can be
//! implemented by buffers. Since buffers contain prefixes, bodies, and
//! suffixes, and since the `Serializer::serialize` method consumes the
//! `Serializer` by value and returns a buffer by value, a buffer is itself a
//! valid `Serializer`. When `serialize` is called, so long as it already
//! satisfies the constraints requested, it can simply return itself by value.
//! If the constraints are not satisfied, it may need to produce a different
//! buffer through some user-defined mechanism (see the [`BufferProvider`] trait
//! for details).
//!
//! This allows existing buffers to be reused in many cases. For example,
//! consider receiving a packet in a buffer, and then responding to that packet
//! with a new packet. The buffer that the original packet was stored in can be
//! used to serialize the new packet, avoiding any unnecessary allocation.

/// Emits method impls for [`FragmentedBuffer`] which assume that the type is
/// a contiguous buffer which implements [`AsRef`].
macro_rules! fragmented_buffer_method_impls {
    () => {
        fn len(&self) -> usize {
            self.as_ref().len()
        }

        fn with_bytes<R, F>(&self, f: F) -> R
        where
            F: for<'macro_a, 'macro_b> FnOnce(FragmentedBytes<'macro_a, 'macro_b>) -> R,
        {
            let mut bs = [AsRef::<[u8]>::as_ref(self)];
            f(FragmentedBytes::new(&mut bs))
        }

        fn to_flattened_vec(&self) -> Vec<u8> {
            self.as_ref().to_vec()
        }
    };
}

/// Emits method impls for [`FragmentedBufferMut`] which assume that the type is
/// a contiguous buffer which implements [`AsMut`].
macro_rules! fragmented_buffer_mut_method_impls {
    () => {
        fn with_bytes_mut<R, F>(&mut self, f: F) -> R
        where
            F: for<'macro_a, 'macro_b> FnOnce(FragmentedBytesMut<'macro_a, 'macro_b>) -> R,
        {
            let mut bs = [AsMut::<[u8]>::as_mut(self)];
            f(FragmentedBytesMut::new(&mut bs))
        }

        fn zero_range<R>(&mut self, range: R)
        where
            R: RangeBounds<usize>,
        {
            let len = FragmentedBuffer::len(self);
            let range = crate::canonicalize_range(len, &range);
            crate::zero(&mut self.as_mut()[range.start..range.end]);
        }

        fn copy_within<R: RangeBounds<usize>>(&mut self, src: R, dest: usize) {
            self.as_mut().copy_within(src, dest);
        }
    };
}

mod fragmented;
pub mod records;
pub mod serialize;
mod util;

pub use crate::fragmented::*;
pub use crate::serialize::*;
pub use crate::util::*;

use std::cmp;
use std::convert::Infallible as Never;
use std::mem;
use std::ops::{Bound, Range, RangeBounds};

use zerocopy::{AsBytes, ByteSlice, ByteSliceMut, FromBytes, NoCell, Ref, Unaligned};

/// A buffer that may be fragmented in multiple parts which are discontiguous in
/// memory.
pub trait FragmentedBuffer {
    /// Gets the total length, in bytes, of this `FragmentedBuffer`.
    fn len(&self) -> usize;

    /// Returns `true` if this `FragmentedBuffer` is empty.
    fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Invokes a callback on a view into this buffer's contents as
    /// [`FragmentedBytes`].
    fn with_bytes<R, F>(&self, f: F) -> R
    where
        F: for<'a, 'b> FnOnce(FragmentedBytes<'a, 'b>) -> R;

    /// Returns a flattened version of this buffer, copying its contents into a
    /// [`Vec`].
    fn to_flattened_vec(&self) -> Vec<u8> {
        self.with_bytes(|b| b.to_flattened_vec())
    }
}

/// A [`FragmentedBuffer`] with mutable access to its contents.
pub trait FragmentedBufferMut: FragmentedBuffer {
    /// Invokes a callback on a mutable view into this buffer's contents as
    /// [`FragmentedBytesMut`].
    fn with_bytes_mut<R, F>(&mut self, f: F) -> R
    where
        F: for<'a, 'b> FnOnce(FragmentedBytesMut<'a, 'b>) -> R;

    /// Sets all bytes in `range` to zero.
    ///
    /// # Panics
    ///
    /// Panics if the provided `range` is not within the bounds of this
    /// `FragmentedBufferMut`, or if the range is nonsensical (the end precedes
    /// the start).
    fn zero_range<R>(&mut self, range: R)
    where
        R: RangeBounds<usize>,
    {
        let len = self.len();
        let range = canonicalize_range(len, &range);
        self.with_bytes_mut(|mut b| {
            zero_iter(b.iter_mut().skip(range.start).take(range.end - range.start))
        })
    }

    /// Copies elements from one part of the `FragmentedBufferMut` to another
    /// part of itself.
    ///
    /// `src` is the range within `self` to copy from. `dst` is the starting
    /// index of the range within `self` to copy to, which will have the same
    /// length as `src`. The two ranges may overlap. The ends of the two ranges
    /// must be less than or equal to `self.len()`.
    ///
    /// # Panics
    ///
    /// Panics if either the source or destination range is out of bounds, or if
    /// `src` is nonsensical (its end precedes its start).
    fn copy_within<R: RangeBounds<usize>>(&mut self, src: R, dst: usize) {
        self.with_bytes_mut(|mut b| b.copy_within(src, dst));
    }

    /// Copies all the bytes from another `FragmentedBuffer` `other` into
    /// `self`.
    ///
    /// # Panics
    ///
    /// Panics if `self.len() != other.len()`.
    fn copy_from<B: FragmentedBuffer>(&mut self, other: &B) {
        self.with_bytes_mut(|dst| {
            other.with_bytes(|src| {
                let dst = dst.try_into_contiguous();
                let src = src.try_into_contiguous();
                match (dst, src) {
                    (Ok(dst), Ok(src)) => {
                        dst.copy_from_slice(src);
                    }
                    (Ok(dst), Err(src)) => {
                        src.copy_into_slice(dst);
                    }
                    (Err(mut dst), Ok(src)) => {
                        dst.copy_from_slice(src);
                    }
                    (Err(mut dst), Err(src)) => {
                        dst.copy_from(&src);
                    }
                }
            });
        });
    }
}

/// A buffer that is contiguous in memory.
///
/// If the implementing type is a buffer which exposes a prefix and a suffix,
/// the [`AsRef`] implementation provides access only to the body. If [`AsMut`]
/// is also implemented, it must provide access to the same bytes as [`AsRef`].
pub trait ContiguousBuffer: FragmentedBuffer + AsRef<[u8]> {}

/// A mutable buffer that is contiguous in memory.
///
/// If the implementing type is a buffer which exposes a prefix and a suffix,
/// the [`AsMut`] implementation provides access only to the body.
///
/// `ContiguousBufferMut` is shorthand for `ContiguousBuffer +
/// FragmentedBufferMut + AsMut<[u8]>`.
pub trait ContiguousBufferMut: ContiguousBuffer + FragmentedBufferMut + AsMut<[u8]> {}
impl<B: ContiguousBuffer + FragmentedBufferMut + AsMut<[u8]>> ContiguousBufferMut for B {}

/// A buffer that can reduce its size.
///
/// A `ShrinkBuffer` is a buffer that can be reduced in size without the
/// guarantee that the prefix or suffix will be retained. This is typically
/// sufficient for parsing, but not for serialization.
///
/// # Notable implementations
///
/// `ShrinkBuffer` is implemented for byte slices - `&[u8]` and `&mut [u8]`.
/// These types do not implement [`GrowBuffer`]; once bytes are consumed from
/// their bodies, those bytes are discarded and cannot be recovered.
pub trait ShrinkBuffer: FragmentedBuffer {
    /// Shrinks the front of the body towards the end of the buffer.
    ///
    /// `shrink_front` consumes the `n` left-most bytes of the body, and adds
    /// them to the prefix.
    ///
    /// # Panics
    ///
    /// Panics if `n` is larger than the body.
    fn shrink_front(&mut self, n: usize);

    /// Shrinks the buffer to be no larger than `len` bytes, consuming from the
    /// front.
    ///
    /// `shrink_front_to` consumes as many of the left-most bytes of the body as
    /// necessary to ensure that the buffer is no longer than `len` bytes. It
    /// adds any bytes consumed to the prefix. If the body is already not longer
    /// than `len` bytes, `shrink_front_to` does nothing.
    fn shrink_front_to(&mut self, len: usize) {
        let old_len = self.len();
        let new_len = cmp::min(old_len, len);
        self.shrink_front(old_len - new_len);
    }

    /// Shrinks the back of the body towards the beginning of the buffer.
    ///
    /// `shrink_back` consumes the `n` right-most bytes of the body, and adds
    /// them to the suffix.
    ///
    /// # Panics
    ///
    /// Panics if `n` is larger than the body.
    fn shrink_back(&mut self, n: usize);

    /// Shrinks the buffer to be no larger than `len` bytes, consuming from the
    /// back.
    ///
    /// `shrink_back_to` consumes as many of the right-most bytes of the body as
    /// necessary to ensure that the buffer is no longer than `len` bytes.
    /// It adds any bytes consumed to the suffix. If the body is already no
    /// longer than `len` bytes, `shrink_back_to` does nothing.
    fn shrink_back_to(&mut self, len: usize) {
        let old_len = self.len();
        let new_len = cmp::min(old_len, len);
        self.shrink_back(old_len - new_len);
    }

    /// Shrinks the body.
    ///
    /// `shrink` shrinks the body to be equal to `range` of the previous body.
    /// Any bytes preceding the range are added to the prefix, and any bytes
    /// following the range are added to the suffix.
    ///
    /// # Panics
    ///
    /// Panics if `range` is out of bounds of the body, or if the range
    /// is nonsensical (the end precedes the start).
    fn shrink<R: RangeBounds<usize>>(&mut self, range: R) {
        let len = self.len();
        let range = canonicalize_range(len, &range);
        self.shrink_front(range.start);
        self.shrink_back(len - range.end);
    }
}

/// A byte buffer used for parsing.
///
/// A `ParseBuffer` is a [`ContiguousBuffer`] that can shrink in size.
///
/// While a `ParseBuffer` allows the ranges covered by its prefix, body, and
/// suffix to be modified, it only provides immutable access to their contents.
/// For mutable access, see [`ParseBufferMut`].
///
/// # Notable implementations
///
/// `ParseBuffer` is implemented for byte slices - `&[u8]` and `&mut [u8]`.
/// These types do not implement [`GrowBuffer`]; once bytes are consumed from
/// their bodies, those bytes are discarded and cannot be recovered.
pub trait ParseBuffer: ShrinkBuffer + ContiguousBuffer {
    /// Parses a packet from the body.
    ///
    /// `parse` parses a packet from the body by invoking [`P::parse`] on a
    /// [`BufferView`] into this buffer. Any bytes consumed from the
    /// `BufferView` are also consumed from the body, and added to the prefix or
    /// suffix. After `parse` has returned, the buffer's body will contain only
    /// those bytes which were not consumed by the call to `P::parse`.
    ///
    /// See the [`BufferView`] and [`ParsablePacket`] documentation for more
    /// details.
    ///
    /// [`P::parse`]: ParsablePacket::parse
    fn parse<'a, P: ParsablePacket<&'a [u8], ()>>(&'a mut self) -> Result<P, P::Error> {
        self.parse_with(())
    }

    /// Parses a packet with arguments.
    ///
    /// `parse_with` is like [`parse`], but it accepts arguments to pass to
    /// [`P::parse`].
    ///
    /// [`parse`]: ParseBuffer::parse
    /// [`P::parse`]: ParsablePacket::parse
    fn parse_with<'a, ParseArgs, P: ParsablePacket<&'a [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error>;
}

/// A [`ParseBuffer`] which provides mutable access to its contents.
///
/// While a [`ParseBuffer`] allows the ranges covered by its prefix, body, and
/// suffix to be modified, it only provides immutable access to their contents.
/// A `ParseBufferMut`, on the other hand, provides mutable access to the
/// contents of its prefix, body, and suffix.
///
/// # Notable implementations
///
/// `ParseBufferMut` is implemented for mutable byte slices - `&mut [u8]`.
/// Mutable byte slices do not implement [`GrowBuffer`] or [`GrowBufferMut`];
/// once bytes are consumed from their bodies, those bytes are discarded and
/// cannot be recovered.
pub trait ParseBufferMut: ParseBuffer + ContiguousBufferMut {
    /// Parses a mutable packet from the body.
    ///
    /// `parse_mut` is like [`ParseBuffer::parse`], but instead of calling
    /// [`P::parse`] on a [`BufferView`], it calls [`P::parse_mut`] on a
    /// [`BufferViewMut`]. The effect is that the parsed packet can contain
    /// mutable references to the buffer. This can be useful if you want to
    /// modify parsed packets in-place.
    ///
    /// Depending on the implementation of [`P::parse_mut`], the contents
    /// of the buffer may be modified during parsing.
    ///
    /// See the [`BufferViewMut`] and [`ParsablePacket`] documentation for more
    /// details.
    ///
    /// [`P::parse`]: ParsablePacket::parse
    /// [`P::parse_mut`]: ParsablePacket::parse_mut
    fn parse_mut<'a, P: ParsablePacket<&'a mut [u8], ()>>(&'a mut self) -> Result<P, P::Error> {
        self.parse_with_mut(())
    }

    /// Parses a mutable packet with arguments.
    ///
    /// `parse_with_mut` is like [`parse_mut`], but it accepts arguments to pass
    /// to [`P::parse_mut`].
    ///
    /// [`parse_mut`]: ParseBufferMut::parse_mut
    /// [`P::parse_mut`]: ParsablePacket::parse_mut
    fn parse_with_mut<'a, ParseArgs, P: ParsablePacket<&'a mut [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error>;
}

/// A buffer that can grow its body by taking space from its prefix and suffix.
///
/// A `GrowBuffer` is a byte buffer with a prefix, a body, and a suffix. The
/// size of the buffer is referred to as its "capacity", and the size of the
/// body is referred to as its "length". The body of the buffer can shrink or
/// grow as allowed by the capacity as packets are parsed or serialized.
///
/// A `GrowBuffer` guarantees never to discard bytes from the prefix or suffix,
/// which is an important requirement for serialization. \[1\] For parsing, this
/// guarantee is not needed. The subset of methods which do not require this
/// guarantee are defined in the [`ShrinkBuffer`] trait, which does not have
/// this requirement.
///
/// While a `GrowBuffer` allows the ranges covered by its prefix, body, and
/// suffix to be modified, it only provides immutable access to their contents.
/// For mutable access, see [`GrowBufferMut`].
///
/// If a type implements `GrowBuffer`, then its implementations of the methods
/// on [`FragmentedBuffer`] provide access only to the buffer's body. In
/// particular, [`len`] returns the body's length, [`with_bytes`] provides
/// access to the body, and [`to_flattened_vec`] returns a copy of the body.
///
/// \[1\] If `GrowBuffer`s could shrink their prefix or suffix, then it would
/// not be possible to guarantee that a call to [`undo_parse`] wouldn't panic.
/// `undo_parse` is used when retaining previously-parsed packets for
/// serialization, which is useful in scenarios such as packet forwarding.
///
/// [`len`]: FragmentedBuffer::len
/// [`with_bytes`]: FragmentedBuffer::with_bytes
/// [`to_flattened_vec`]: FragmentedBuffer::to_flattened_vec
/// [`undo_parse`]: GrowBuffer::undo_parse
pub trait GrowBuffer: FragmentedBuffer {
    /// Gets a view into the parts of this `GrowBuffer`.
    ///
    /// Calls `f`, passing the prefix, body, and suffix as arguments (in that
    /// order).
    fn with_parts<O, F>(&self, f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a [u8], FragmentedBytes<'a, 'b>, &'a [u8]) -> O;

    /// The capacity of the buffer.
    ///
    /// `b.capacity()` is equivalent to `b.prefix_len() + b.len() +
    /// b.suffix_len()`.
    fn capacity(&self) -> usize {
        self.with_parts(|prefix, body, suffix| prefix.len() + body.len() + suffix.len())
    }

    /// The length of the prefix.
    fn prefix_len(&self) -> usize {
        self.with_parts(|prefix, _body, _suffix| prefix.len())
    }

    /// The length of the suffix.
    fn suffix_len(&self) -> usize {
        self.with_parts(|_prefix, _body, suffix| suffix.len())
    }

    /// Grows the front of the body towards Growf the buffer.
    ///
    /// `grow_front` consumes the right-most `n` bytes of the prefix, and adds
    /// them to the body.
    ///
    /// # Panics
    ///
    /// Panics if `n` is larger than the prefix.
    fn grow_front(&mut self, n: usize);

    /// Grows the back of the body towards the end of the buffer.
    ///
    /// `grow_back` consumes the left-most `n` bytes of the suffix, and adds
    /// them to the body.
    ///
    /// # Panics
    ///
    /// Panics if `n` is larger than the suffix.
    fn grow_back(&mut self, n: usize);

    /// Resets the body to be equal to the entire buffer.
    ///
    /// `reset` consumes the entire prefix and suffix, adding them to the body.
    fn reset(&mut self) {
        self.grow_front(self.prefix_len());
        self.grow_back(self.suffix_len());
    }

    /// Undoes the effects of a previous parse in preparation for serialization.
    ///
    /// `undo_parse` undoes the effects of having previously parsed a packet by
    /// consuming the appropriate number of bytes from the prefix and suffix.
    /// After a call to `undo_parse`, the buffer's body will contain the bytes
    /// of the previously-parsed packet, including any headers or footers. This
    /// allows a previously-parsed packet to be used in serialization.
    ///
    /// `undo_parse` takes a [`ParseMetadata`], which can be obtained from
    /// [`ParsablePacket::parse_metadata`].
    ///
    /// `undo_parse` must always be called starting with the most recently
    /// parsed packet, followed by the second most recently parsed packet, and
    /// so on. Otherwise, it may panic, and in any case, almost certainly won't
    /// produce the desired buffer contents.
    ///
    /// # Padding
    ///
    /// If, during parsing, a packet encountered post-packet padding that was
    /// discarded (see the documentation on [`ParsablePacket::parse`]), calling
    /// `undo_parse` on the `ParseMetadata` from that packet will not undo the
    /// effects of consuming and discarding that padding. The reason for this is
    /// that the padding is not considered part of the packet itself (the body
    /// it was parsed from can be thought of comprising the packet and
    /// post-packet padding back-to-back).
    ///
    /// Calling `undo_parse` on the next encapsulating packet (the one whose
    /// body contained the padding) will undo those effects.
    ///
    /// # Panics
    ///
    /// `undo_parse` may panic if called in the wrong order. See the first
    /// section of this documentation for details.
    fn undo_parse(&mut self, meta: ParseMetadata) {
        if self.len() < meta.body_len {
            // There were padding bytes which were stripped when parsing the
            // encapsulated packet. We need to add them back in order to restore
            // the original packet.
            let len = self.len();
            self.grow_back(meta.body_len - len);
        }
        self.grow_front(meta.header_len);
        self.grow_back(meta.footer_len);
    }
}

/// A [`GrowBuffer`] which provides mutable access to its contents.
///
/// While a [`GrowBuffer`] allows the ranges covered by its prefix, body, and
/// suffix to be modified, it only provides immutable access to their contents.
/// A `GrowBufferMut`, on the other hand, provides mutable access to the
/// contents of its prefix, body, and suffix.
pub trait GrowBufferMut: GrowBuffer + FragmentedBufferMut {
    /// Gets a mutable view into the parts of this `GrowBufferMut`.
    ///
    /// Calls `f`, passing the prefix, body, and suffix as arguments (in that
    /// order).
    fn with_parts_mut<O, F>(&mut self, f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a mut [u8], FragmentedBytesMut<'a, 'b>, &'a mut [u8]) -> O;

    /// Gets a mutable view into the header of the previously parsed packet.
    ///
    /// Calls `f`, passing the subset of the prefix that forms the header of the
    /// previously parsed packet, as specified by `meta`.
    ///
    /// # Panics
    ///
    /// Panics if `meta.header_len` is greater than the size of `self`'s prefix.
    fn with_header_mut_with_meta<O, F>(&mut self, meta: ParseMetadata, f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a mut [u8]) -> O,
    {
        self.with_parts_mut(|prefix, _body, _suffix| {
            assert!(
                prefix.len() >= meta.header_len,
                "prefix ({} bytes) too small to contain header ({} bytes)",
                prefix.len(),
                meta.header_len
            );
            let offset = prefix.len() - meta.header_len;
            let header = &mut prefix[offset..];
            f(header)
        })
    }

    /// Extends the front of the body towards the beginning of the buffer,
    /// zeroing the new bytes.
    ///
    /// `grow_front_zero` calls [`GrowBuffer::grow_front`] and sets the
    /// newly-added bytes to 0. This can be useful when serializing to ensure
    /// that the contents of packets previously stored in the buffer are not
    /// leaked.
    fn grow_front_zero(&mut self, n: usize) {
        self.grow_front(n);
        self.zero_range(..n);
    }

    /// Extends the back of the body towards the end of the buffer, zeroing the
    /// new bytes.
    ///
    /// `grow_back_zero` calls [`GrowBuffer::grow_back`] and sets the
    /// newly-added bytes to 0. This can be useful when serializing to ensure
    /// that the contents of packets previously stored in the buffer are not
    /// leaked.
    fn grow_back_zero(&mut self, n: usize) {
        let old_len = self.len();
        self.grow_back(n);
        self.zero_range(old_len..);
    }

    /// Resets the body to be equal to the entire buffer, zeroing the new bytes.
    ///
    /// Like [`GrowBuffer::reset`], `reset_zero` consumes the entire prefix and
    /// suffix, adding them to the body. It sets these bytes to 0. This can be
    /// useful when serializing to ensure that the contents of packets
    /// previously stored in the buffer are not leaked.
    fn reset_zero(&mut self) {
        self.grow_front_zero(self.prefix_len());
        self.grow_back_zero(self.suffix_len());
    }

    /// Serializes a packet in the buffer.
    ///
    /// *This method is usually called by this crate during the serialization of
    /// a [`Serializer`], not directly by the user.*
    ///
    /// `serialize` serializes the packet described by `builder` into the
    /// buffer. The body of the buffer is used as the body of the packet, and
    /// the prefix and suffix of the buffer are used to serialize the packet's
    /// header and footer.
    ///
    /// If `builder` has a minimum body size which is larger than the current
    /// body, then `serialize` first grows the body to the right (towards the
    /// end of the buffer) with padding bytes in order to meet the minimum body
    /// size. This is transparent to the `builder` - it always just sees a body
    /// which meets the minimum body size requirement.
    ///
    /// The added padding is zeroed in order to avoid leaking the contents of
    /// packets previously stored in the buffer.
    ///
    /// # Panics
    ///
    /// `serialize` panics if there are not enough prefix or suffix bytes to
    /// serialize the packet. In particular, `b.serialize(builder)` with `c =
    /// builder.constraints()` panics if either of the following hold:
    /// - `b.prefix_len() < c.header_len()`
    /// - `b.len() + b.suffix_len() < c.min_body_bytes() + c.footer_len()`
    #[doc(hidden)]
    fn serialize<B: PacketBuilder>(&mut self, builder: B) {
        let c = builder.constraints();
        if self.len() < c.min_body_len() {
            // The body isn't large enough to satisfy the minimum body length
            // requirement, so we add padding.

            // SECURITY: Use _zero to ensure we zero padding bytes to prevent
            // leaking information from packets previously stored in this
            // buffer.
            let len = self.len();
            self.grow_back_zero(c.min_body_len() - len);
        }

        // These aren't necessary for correctness (grow_xxx_zero will panic
        // under the same conditions that these assertions will fail), but they
        // provide nicer error messages for debugging.
        debug_assert!(
            self.prefix_len() >= c.header_len(),
            "prefix ({} bytes) too small to serialize header ({} bytes)",
            self.prefix_len(),
            c.header_len()
        );
        debug_assert!(
            self.suffix_len() >= c.footer_len(),
            "suffix ({} bytes) too small to serialize footer ({} bytes)",
            self.suffix_len(),
            c.footer_len()
        );

        self.with_parts_mut(|prefix, body, suffix| {
            let header = prefix.len() - c.header_len();
            let header = &mut prefix[header..];
            let footer = &mut suffix[..c.footer_len()];
            // SECURITY: zero here is technically unnecessary since it's
            // PacketBuilder::serialize's responsibility to zero/initialize the
            // header and footer, but we do it anyway to hedge against
            // non-compliant PacketBuilder::serialize implementations. If this
            // becomes a performance issue, we can revisit it, but the optimizer
            // will probably take care of it for us.
            zero(header);
            zero(footer);
            builder.serialize(&mut SerializeTarget { header, footer }, body);
        });

        self.grow_front(c.header_len());
        self.grow_back(c.footer_len());
    }
}

/// A byte buffer that can be serialized into multiple times.
///
/// `ReusableBuffer` is a shorthand for `GrowBufferMut + ShrinkBuffer`. A
/// `ReusableBuffer` can be serialized into multiple times - the
/// [`ShrinkBuffer`] implementation allows the buffer's capacity to be reclaimed
/// for a new serialization pass.
pub trait ReusableBuffer: GrowBufferMut + ShrinkBuffer {}
impl<B> ReusableBuffer for B where B: GrowBufferMut + ShrinkBuffer {}

/// A byte buffer used for parsing that can grow back to its original size.
///
/// `Buffer` owns its backing memory and so implies `GrowBuffer + ParseBuffer`.
/// A `Buffer` can be used for parsing (via [`ParseBuffer`]) and then grow back
/// to its original size (via [`GrowBuffer`]). Since it owns the backing memory,
/// it also provides the ability to provide both a parsed and un-parsed view
/// into a packet via [`Buffer::parse_with_view`].
pub trait Buffer: GrowBuffer + ParseBuffer {
    /// Like [`ParseBuffer::parse_with`] but additionally provides an
    /// un-structured view into the parsed data on successful parsing.
    fn parse_with_view<'a, ParseArgs, P: ParsablePacket<&'a [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<(P, &'a [u8]), P::Error>;
}

/// A byte buffer used for parsing and serialization.
///
/// `BufferMut` is a shorthand for `GrowBufferMut + ParseBufferMut`. A
/// `BufferMut` can be used for parsing (via [`ParseBufferMut`]) and
/// serialization (via [`GrowBufferMut`]).
pub trait BufferMut: GrowBufferMut + ParseBufferMut + Buffer {}
impl<B> BufferMut for B where B: GrowBufferMut + ParseBufferMut + Buffer {}

/// An empty buffer.
///
/// An `EmptyBuf` is a buffer with 0 bytes of length or capacity. It implements
/// all of the buffer traits (`XxxBuffer` and `XxxBufferMut`) and both buffer
/// view traits ([`BufferView`] and [`BufferViewMut`]).
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct EmptyBuf;

impl AsRef<[u8]> for EmptyBuf {
    #[inline]
    fn as_ref(&self) -> &[u8] {
        &[]
    }
}
impl AsMut<[u8]> for EmptyBuf {
    #[inline]
    fn as_mut(&mut self) -> &mut [u8] {
        &mut []
    }
}
impl FragmentedBuffer for EmptyBuf {
    fragmented_buffer_method_impls!();
}
impl FragmentedBufferMut for EmptyBuf {
    fragmented_buffer_mut_method_impls!();
}
impl ContiguousBuffer for EmptyBuf {}
impl ShrinkBuffer for EmptyBuf {
    #[inline]
    fn shrink_front(&mut self, n: usize) {
        assert_eq!(n, 0);
    }
    #[inline]
    fn shrink_back(&mut self, n: usize) {
        assert_eq!(n, 0);
    }
}
impl ParseBuffer for EmptyBuf {
    #[inline]
    fn parse_with<'a, ParseArgs, P: ParsablePacket<&'a [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error> {
        P::parse(EmptyBuf, args)
    }
}
impl ParseBufferMut for EmptyBuf {
    #[inline]
    fn parse_with_mut<'a, ParseArgs, P: ParsablePacket<&'a mut [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error> {
        P::parse_mut(EmptyBuf, args)
    }
}
impl GrowBuffer for EmptyBuf {
    #[inline]
    fn with_parts<O, F>(&self, f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a [u8], FragmentedBytes<'a, 'b>, &'a [u8]) -> O,
    {
        f(&[], FragmentedBytes::new_empty(), &[])
    }
    #[inline]
    fn grow_front(&mut self, n: usize) {
        assert_eq!(n, 0);
    }
    #[inline]
    fn grow_back(&mut self, n: usize) {
        assert_eq!(n, 0);
    }
}
impl GrowBufferMut for EmptyBuf {
    fn with_parts_mut<O, F>(&mut self, f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a mut [u8], FragmentedBytesMut<'a, 'b>, &'a mut [u8]) -> O,
    {
        f(&mut [], FragmentedBytesMut::new_empty(), &mut [])
    }
}
impl<'a> BufferView<&'a [u8]> for EmptyBuf {
    #[inline]
    fn len(&self) -> usize {
        0
    }
    #[inline]
    fn take_front(&mut self, n: usize) -> Option<&'a [u8]> {
        if n > 0 {
            return None;
        }
        Some(&[])
    }
    #[inline]
    fn take_back(&mut self, n: usize) -> Option<&'a [u8]> {
        if n > 0 {
            return None;
        }
        Some(&[])
    }
    #[inline]
    fn into_rest(self) -> &'a [u8] {
        &[]
    }
}
impl<'a> BufferView<&'a mut [u8]> for EmptyBuf {
    #[inline]
    fn len(&self) -> usize {
        0
    }
    #[inline]
    fn take_front(&mut self, n: usize) -> Option<&'a mut [u8]> {
        if n > 0 {
            return None;
        }
        Some(&mut [])
    }
    #[inline]
    fn take_back(&mut self, n: usize) -> Option<&'a mut [u8]> {
        if n > 0 {
            return None;
        }
        Some(&mut [])
    }
    #[inline]
    fn into_rest(self) -> &'a mut [u8] {
        &mut []
    }
}
impl<'a> BufferViewMut<&'a mut [u8]> for EmptyBuf {}
impl Buffer for EmptyBuf {
    fn parse_with_view<'a, ParseArgs, P: ParsablePacket<&'a [u8], ParseArgs>>(
        &'a mut self,
        args: ParseArgs,
    ) -> Result<(P, &'a [u8]), P::Error> {
        self.parse_with(args).map(|r| (r, [].as_slice()))
    }
}

impl FragmentedBuffer for Never {
    fn len(&self) -> usize {
        match *self {}
    }

    fn with_bytes<R, F>(&self, _f: F) -> R
    where
        F: for<'a, 'b> FnOnce(FragmentedBytes<'a, 'b>) -> R,
    {
        match *self {}
    }
}
impl FragmentedBufferMut for Never {
    fn with_bytes_mut<R, F>(&mut self, _f: F) -> R
    where
        F: for<'a, 'b> FnOnce(FragmentedBytesMut<'a, 'b>) -> R,
    {
        match *self {}
    }
}
impl ShrinkBuffer for Never {
    fn shrink_front(&mut self, _n: usize) {}
    fn shrink_back(&mut self, _n: usize) {}
}
impl GrowBuffer for Never {
    fn with_parts<O, F>(&self, _f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a [u8], FragmentedBytes<'a, 'b>, &'a [u8]) -> O,
    {
        match *self {}
    }
    fn grow_front(&mut self, _n: usize) {}
    fn grow_back(&mut self, _n: usize) {}
}
impl GrowBufferMut for Never {
    fn with_parts_mut<O, F>(&mut self, _f: F) -> O
    where
        F: for<'a, 'b> FnOnce(&'a mut [u8], FragmentedBytesMut<'a, 'b>, &'a mut [u8]) -> O,
    {
        match *self {}
    }
}

/// A view into a [`ShrinkBuffer`].
///
/// A `BufferView` borrows a `ShrinkBuffer`, and provides methods to consume
/// bytes from the buffer's body. It is primarily intended to be used for
/// parsing, although it provides methods which are useful for serialization as
/// well.
///
/// A `BufferView` only provides immutable access to the contents of the buffer.
/// For mutable access, see [`BufferViewMut`].
///
/// # Notable implementations
///
/// `BufferView` is implemented for mutable references to byte slices (`&mut
/// &[u8]` and `&mut &mut [u8]`).
pub trait BufferView<B: ByteSlice>: Sized + AsRef<[u8]> {
    /// The length of the buffer's body.
    fn len(&self) -> usize {
        self.as_ref().len()
    }

    /// Is the buffer's body empty?
    fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// Takes `n` bytes from the front of the buffer's body.
    ///
    /// `take_front` consumes `n` bytes from the front of the buffer's body.
    /// After a successful call to `take_front(n)`, the body is `n` bytes
    /// shorter and, if `Self: GrowBuffer`, the prefix is `n` bytes longer. If
    /// the body is not at least `n` bytes in length, `take_front` returns
    /// `None`.
    fn take_front(&mut self, n: usize) -> Option<B>;

    /// Takes `n` bytes from the back of the buffer's body.
    ///
    /// `take_back` consumes `n` bytes from the back of the buffer's body. After
    /// a successful call to `take_back(n)`, the body is `n` bytes shorter and,
    /// if `Self: GrowBuffer`, the suffix is `n` bytes longer. If the body is
    /// not at least `n` bytes in length, `take_back` returns `None`.
    fn take_back(&mut self, n: usize) -> Option<B>;

    /// Takes the rest of the buffer's body from the front.
    ///
    /// `take_rest_front` consumes the rest of the bytes from the buffer's body.
    /// After a call to `take_rest_front`, the body is empty and, if `Self:
    /// GrowBuffer`, the bytes which were previously in the body are now in the
    /// prefix.
    fn take_rest_front(&mut self) -> B {
        let len = self.len();
        self.take_front(len).unwrap()
    }

    /// Takes the rest of the buffer's body from the back.
    ///
    /// `take_rest_back` consumes the rest of the bytes from the buffer's body.
    /// After a call to `take_rest_back`, the body is empty and, if `Self:
    /// GrowBuffer`, the bytes which were previously in the body are now in the
    /// suffix.
    fn take_rest_back(&mut self) -> B {
        let len = self.len();
        self.take_back(len).unwrap()
    }

    /// Takes a single byte of the buffer's body from the front.
    ///
    /// `take_byte_front` consumes a single byte from the from of the buffer's
    /// body. It's equivalent to calling `take_front(1)` and copying out the
    /// single byte on successful return.
    fn take_byte_front(&mut self) -> Option<u8> {
        self.take_front(1).map(|x| x[0])
    }

    /// Takes a single byte of the buffer's body from the back.
    ///
    /// `take_byte_back` consumes a single byte from the fron of the buffer's
    /// body. It's equivalent to calling `take_back(1)` and copying out the
    /// single byte on successful return.
    fn take_byte_back(&mut self) -> Option<u8> {
        self.take_back(1).map(|x| x[0])
    }

    /// Converts this view into a reference to the buffer's body.
    ///
    /// `into_rest` consumes this `BufferView` by value, and returns a reference
    /// to the buffer's body. Unlike `take_rest`, the body is not consumed - it
    /// is left unchanged.
    fn into_rest(self) -> B;

    /// Peeks at an object at the front of the buffer's body.
    ///
    /// `peek_obj_front` peeks at `size_of::<T>()` bytes at the front of the
    /// buffer's body, and interprets them as a `T`. Unlike `take_obj_front`,
    /// `peek_obj_front` does not modify the body. If the body is not at least
    /// `size_of::<T>()` bytes in length, `peek_obj_front` returns `None`.
    fn peek_obj_front<T>(&mut self) -> Option<&T>
    where
        T: FromBytes + NoCell + Unaligned,
    {
        Some(Ref::<_, T>::new_unaligned_from_prefix((&*self).as_ref())?.0.into_ref())
    }

    /// Takes an object from the front of the buffer's body.
    ///
    /// `take_obj_front` consumes `size_of::<T>()` bytes from the front of the
    /// buffer's body, and interprets them as a `T`. After a successful call to
    /// `take_obj_front::<T>()`, the body is `size_of::<T>()` bytes shorter and,
    /// if `Self: GrowBuffer`, the prefix is `size_of::<T>()` bytes longer. If
    /// the body is not at least `size_of::<T>()` bytes in length,
    /// `take_obj_front` returns `None`.
    fn take_obj_front<T>(&mut self) -> Option<Ref<B, T>>
    where
        T: Unaligned,
    {
        let bytes = self.take_front(mem::size_of::<T>())?;
        // new_unaligned only returns None if there aren't enough bytes
        Some(Ref::new_unaligned(bytes).unwrap())
    }

    /// Takes a slice of objects from the front of the buffer's body.
    ///
    /// `take_slice_front` consumes `n * size_of::<T>()` bytes from the front of
    /// the buffer's body, and interprets them as a `[T]` with `n` elements.
    /// After a successful call to `take_slice_front::<T>()`, the body is `n *
    /// size_of::<T>()` bytes shorter and, if `Self: GrowBuffer`, the prefix is
    /// `n * size_of::<T>()` bytes longer. If the body is not at least `n *
    /// size_of::<T>()` bytes in length, `take_slice_front` returns `None`.
    ///
    /// # Panics
    ///
    /// Panics if `T` is a zero-sized type.
    fn take_slice_front<T>(&mut self, n: usize) -> Option<Ref<B, [T]>>
    where
        T: Unaligned,
    {
        let bytes = self.take_front(n * mem::size_of::<T>())?;
        // `new_slice_unaligned` will return `None` only if `bytes.len()` is
        // not a multiple of `mem::size_of::<T>()`.
        Some(Ref::new_slice_unaligned(bytes).unwrap())
    }

    /// Peeks at an object at the back of the buffer's body.
    ///
    /// `peek_obj_back` peeks at `size_of::<T>()` bytes at the back of the
    /// buffer's body, and interprets them as a `T`. Unlike `take_obj_back`,
    /// `peek_obj_back` does not modify the body. If the body is not at least
    /// `size_of::<T>()` bytes in length, `peek_obj_back` returns `None`.
    fn peek_obj_back<T>(&mut self) -> Option<&T>
    where
        T: FromBytes + NoCell + Unaligned,
    {
        Some(Ref::<_, T>::new_unaligned_from_suffix((&*self).as_ref())?.1.into_ref())
    }

    /// Takes an object from the back of the buffer's body.
    ///
    /// `take_obj_back` consumes `size_of::<T>()` bytes from the back of the
    /// buffer's body, and interprets them as a `T`. After a successful call to
    /// `take_obj_back::<T>()`, the body is `size_of::<T>()` bytes shorter and,
    /// if `Self: GrowBuffer`, the suffix is `size_of::<T>()` bytes longer. If
    /// the body is not at least `size_of::<T>()` bytes in length,
    /// `take_obj_back` returns `None`.
    fn take_obj_back<T>(&mut self) -> Option<Ref<B, T>>
    where
        T: Unaligned,
    {
        let bytes = self.take_back(mem::size_of::<T>())?;
        // new_unaligned only returns None if there aren't enough bytes
        Some(Ref::new_unaligned(bytes).unwrap())
    }

    /// Takes a slice of objects from the back of the buffer's body.
    ///
    /// `take_slice_back` consumes `n * size_of::<T>()` bytes from the back of
    /// the buffer's body, and interprets them as a `[T]` with `n` elements.
    /// After a successful call to `take_slice_back::<T>()`, the body is `n *
    /// size_of::<T>()` bytes shorter and, if `Self: GrowBuffer`, the suffix is
    /// `n * size_of::<T>()` bytes longer. If the body is not at least `n *
    /// size_of::<T>()` bytes in length, `take_slice_back` returns `None`.
    ///
    /// # Panics
    ///
    /// Panics if `T` is a zero-sized type.
    fn take_slice_back<T>(&mut self, n: usize) -> Option<Ref<B, [T]>>
    where
        T: Unaligned,
    {
        let bytes = self.take_back(n * mem::size_of::<T>())?;
        // `new_slice_unaligned` will return `None` only if `bytes.len()` is
        // not a multiple of `mem::size_of::<T>()`.
        Some(Ref::new_slice_unaligned(bytes).unwrap())
    }
}

/// A mutable view into a `Buffer`.
///
/// A `BufferViewMut` is a [`BufferView`] which provides mutable access to the
/// contents of the buffer.
///
/// # Notable implementations
///
/// `BufferViewMut` is implemented for `&mut &mut [u8]`.
pub trait BufferViewMut<B: ByteSliceMut>: BufferView<B> + AsMut<[u8]> {
    /// Takes `n` bytes from the front of the buffer's body and zeroes them.
    ///
    /// `take_front_zero` is like [`BufferView::take_front`], except that it
    /// zeroes the bytes before returning them. This can be useful when
    /// serializing to ensure that the contents of packets previously stored in
    /// the buffer are not leaked.
    fn take_front_zero(&mut self, n: usize) -> Option<B> {
        self.take_front(n).map(|mut buf| {
            zero(buf.deref_mut());
            buf
        })
    }

    /// Takes `n` bytes from the back of the buffer's body and zeroes them.
    ///
    /// `take_back_zero` is like [`BufferView::take_back`], except that it
    /// zeroes the bytes before returning them. This can be useful when
    /// serializing to ensure that the contents of packets previously stored in
    /// the buffer are not leaked.
    fn take_back_zero(&mut self, n: usize) -> Option<B> {
        self.take_back(n).map(|mut buf| {
            zero(buf.deref_mut());
            buf
        })
    }

    /// Takes the rest of the buffer's body from the front and zeroes it.
    ///
    /// `take_rest_front_zero` is like [`BufferView::take_rest_front`], except
    /// that it zeroes the bytes before returning them. This can be useful when
    /// serializing to ensure that the contents of packets previously stored in
    /// the buffer are not leaked.
    fn take_rest_front_zero(mut self) -> B {
        let len = self.len();
        self.take_front_zero(len).unwrap()
    }

    /// Takes the rest of the buffer's body from the back and zeroes it.
    ///
    /// `take_rest_back_zero` is like [`BufferView::take_rest_back`], except
    /// that it zeroes the bytes before returning them. This can be useful when
    /// serializing to ensure that the contents of packets previously stored in
    /// the buffer are not leaked.
    fn take_rest_back_zero(mut self) -> B {
        let len = self.len();
        self.take_front_zero(len).unwrap()
    }

    /// Converts this view into a reference to the buffer's body, and zeroes it.
    ///
    /// `into_rest_zero` is like [`BufferView::into_rest`], except that it
    /// zeroes the bytes before returning them. This can be useful when
    /// serializing to ensure that the contents of packets previously stored in
    /// the buffer are not leaked.
    fn into_rest_zero(self) -> B {
        let mut bytes = self.into_rest();
        zero(&mut bytes);
        bytes
    }

    /// Takes an object from the front of the buffer's body and zeroes it.
    ///
    /// `take_obj_front_zero` is like [`BufferView::take_obj_front`], except
    /// that it zeroes the bytes before converting them to a `T`. This can be
    /// useful when serializing to ensure that the contents of packets
    /// previously stored in the buffer are not leaked.
    fn take_obj_front_zero<T>(&mut self) -> Option<Ref<B, T>>
    where
        T: Unaligned,
    {
        let bytes = self.take_front(mem::size_of::<T>())?;
        // new_unaligned_zeroed only returns None if there aren't enough bytes
        Some(Ref::new_unaligned_zeroed(bytes).unwrap())
    }

    /// Takes an object from the back of the buffer's body and zeroes it.
    ///
    /// `take_obj_back_zero` is like [`BufferView::take_obj_back`], except that
    /// it zeroes the bytes before converting them to a `T`. This can be useful
    /// when serializing to ensure that the contents of packets previously
    /// stored in the buffer are not leaked.
    fn take_obj_back_zero<T>(&mut self) -> Option<Ref<B, T>>
    where
        T: Unaligned,
    {
        let bytes = self.take_back(mem::size_of::<T>())?;
        // new_unaligned_zeroed only returns None if there aren't enough bytes
        Some(Ref::new_unaligned_zeroed(bytes).unwrap())
    }

    /// Writes an object to the front of the buffer's body, consuming the bytes.
    ///
    /// `write_obj_front` consumes `size_of_val(obj)` bytes from the front of
    /// the buffer's body, and overwrites them with `obj`. After a successful
    /// call to `write_obj_front(obj)`, the body is `size_of_val(obj)` bytes
    /// shorter and, if `Self: GrowBuffer`, the prefix is `size_of_val(obj)`
    /// bytes longer. If the body is not at least `size_of_val(obj)` bytes in
    /// length, `write_obj_front` returns `None`.
    fn write_obj_front<T>(&mut self, obj: &T) -> Option<()>
    where
        T: ?Sized + AsBytes + NoCell,
    {
        let mut bytes = self.take_front(mem::size_of_val(obj))?;
        bytes.copy_from_slice(obj.as_bytes());
        Some(())
    }

    /// Writes an object to the back of the buffer's body, consuming the bytes.
    ///
    /// `write_obj_back` consumes `size_of_val(obj)` bytes from the back of the
    /// buffer's body, and overwrites them with `obj`. After a successful call
    /// to `write_obj_back(obj)`, the body is `size_of_val(obj)` bytes shorter
    /// and, if `Self: GrowBuffer`, the suffix is `size_of_val(obj)` bytes
    /// longer. If the body is not at least `size_of_val(obj)` bytes in length,
    /// `write_obj_back` returns `None`.
    fn write_obj_back<T>(&mut self, obj: &T) -> Option<()>
    where
        T: ?Sized + AsBytes + NoCell,
    {
        let mut bytes = self.take_back(mem::size_of_val(obj))?;
        bytes.copy_from_slice(obj.as_bytes());
        Some(())
    }
}

// NOTE on undo_parse algorithm: It's important that ParseMetadata only describe
// the packet itself, and not any padding. This is because the user might call
// undo_parse on a packet only once, and then serialize that packet inside of
// another packet with a lower minimum body length requirement than the one it
// was encapsulated in during parsing. In this case, if we were to include
// padding, we would spuriously serialize an unnecessarily large body. Omitting
// the padding is required for this reason. It is acceptable because, using the
// body_len field of the encapsulating packet's ParseMetadata, it is possible
// for undo_parse to reconstruct how many padding bytes there were if it needs
// to.
//
// undo_parse also needs to differentiate between bytes which were consumed from
// the beginning and end of the buffer. For normal packets this is easy -
// headers are consumed from the beginning, and footers from the end. For inner
// packets, which do not have a header/footer distinction (at least from the
// perspective of this crate), we arbitrarily decide that all bytes are consumed
// from the beginning. So long as ParsablePacket implementations obey this
// requirement, undo_parse will work properly. In order to support this,
// ParseMetadata::from_inner_packet constructs a ParseMetadata in which the only
// non-zero field is header_len.

/// Metadata about a previously-parsed packet used to undo its parsing.
///
/// See [`GrowBuffer::undo_parse`] for more details.
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct ParseMetadata {
    header_len: usize,
    body_len: usize,
    footer_len: usize,
}

impl ParseMetadata {
    /// Constructs a new `ParseMetadata` from information about a packet.
    pub fn from_packet(header_len: usize, body_len: usize, footer_len: usize) -> ParseMetadata {
        ParseMetadata { header_len, body_len, footer_len }
    }

    /// Constructs a new `ParseMetadata` from information about an inner packet.
    ///
    /// Since inner packets do not have a header/body/footer distinction (at
    /// least from the perspective of the utilities in this crate), we
    /// arbitrarily produce a `ParseMetadata` with a header length and no body
    /// or footer lengths. Thus, `from_inner_packet(len)` is equivalent to
    /// `from_packet(len, 0, 0)`.
    pub fn from_inner_packet(len: usize) -> ParseMetadata {
        ParseMetadata { header_len: len, body_len: 0, footer_len: 0 }
    }

    /// Gets the header length.
    ///
    /// `header_len` returns the length of the header of the packet described by
    /// this `ParseMetadata`.
    pub fn header_len(&self) -> usize {
        self.header_len
    }

    /// Gets the body length.
    ///
    /// `body_len` returns the length of the body of the packet described by
    /// this `ParseMetadata`.
    pub fn body_len(&self) -> usize {
        self.body_len
    }

    /// Gets the footer length.
    ///
    /// `footer_len` returns the length of the footer of the packet described by
    /// this `ParseMetadata`.
    pub fn footer_len(&self) -> usize {
        self.footer_len
    }
}

/// A packet which can be parsed from a buffer.
///
/// A `ParsablePacket` is a packet which can be parsed from the body of a
/// buffer. For performance reasons, it is recommended that as much of the
/// packet object as possible be stored as references into the body in order to
/// avoid copying.
pub trait ParsablePacket<B: ByteSlice, ParseArgs>: Sized {
    /// The type of errors returned from [`parse`] and [`parse_mut`].
    ///
    /// [`parse`]: ParsablePacket::parse
    /// [`parse_mut`]: ParsablePacket::parse_mut
    type Error;

    /// Parses a packet from a buffer.
    ///
    /// Given a view into a buffer, `parse` parses a packet by consuming bytes
    /// from the buffer's body. This works slightly differently for normal
    /// packets and inner packets (those which do not contain other packets).
    ///
    /// ## Packets
    ///
    /// When parsing a packet which contains another packet, the outer packet's
    /// header and footer should be consumed from the beginning and end of the
    /// buffer's body respectively. The packet's body should be constructed from
    /// a reference to the buffer's body (i.e., [`BufferView::into_rest`]), but
    /// the buffer's body should not be consumed. This allows the next
    /// encapsulated packet to be parsed from the remaining buffer body. See the
    /// crate documentation for more details.
    ///
    /// ## Inner Packets
    ///
    /// When parsing packets which do not contain other packets, the entire
    /// packet's contents should be consumed from the beginning of the buffer's
    /// body. The buffer's body should be empty after `parse` has returned.
    ///
    /// # Padding
    ///
    /// There may be post-packet padding (coming after the entire packet,
    /// including any footer) which was added in order to satisfy the minimum
    /// body length requirement of an encapsulating packet. If the packet
    /// currently being parsed describes its own length (and thus, it's possible
    /// to determine whether there's any padding), `parse` is required to
    /// consume any post-packet padding from the buffer's suffix. If this
    /// invariant is not upheld, future calls to [`ParseBuffer::parse`] or
    /// [`GrowBuffer::undo_parse`] may behave incorrectly.
    ///
    /// Pre-packet padding is not supported; if a protocol supports such
    /// padding, it must be handled in a way that is transparent to this API. In
    /// particular, that means that the [`parse_metadata`] method must treat that
    /// padding as part of the packet.
    ///
    /// [`parse_metadata`]: ParsablePacket::parse_metadata
    fn parse<BV: BufferView<B>>(buffer: BV, args: ParseArgs) -> Result<Self, Self::Error>;

    /// Parses a packet from a mutable buffer.
    ///
    /// `parse_mut` is like [`parse`], except that it operates on a mutable
    /// buffer view.
    ///
    /// [`parse`]: ParsablePacket::parse
    fn parse_mut<BV: BufferViewMut<B>>(buffer: BV, args: ParseArgs) -> Result<Self, Self::Error>
    where
        B: ByteSliceMut,
    {
        Self::parse(buffer, args)
    }

    /// Gets metadata about this packet required by [`GrowBuffer::undo_parse`].
    ///
    /// The returned [`ParseMetadata`] records the number of header and footer
    /// bytes consumed by this packet during parsing, and the number of bytes
    /// left in the body (not consumed from the buffer). For packets which
    /// encapsulate other packets, the header length must be equal to the number
    /// of bytes consumed from the prefix, and the footer length must be equal
    /// to the number of bytes consumed from the suffix. For inner packets, use
    /// [`ParseMetadata::from_inner_packet`].
    ///
    /// There is one exception: if any post-packet padding was consumed from the
    /// suffix, this should not be included, as it is not considered part of the
    /// packet. For example, consider a packet with 8 bytes of footer followed
    /// by 8 bytes of post-packet padding. Parsing this packet would consume 16
    /// bytes from the suffix, but calling `parse_metadata` on the resulting
    /// object would return a `ParseMetadata` with only 8 bytes of footer.
    fn parse_metadata(&self) -> ParseMetadata;
}

fn zero_iter<'a, I: Iterator<Item = &'a mut u8>>(bytes: I) {
    for byte in bytes {
        *byte = 0;
    }
}

fn zero(bytes: &mut [u8]) {
    for byte in bytes.iter_mut() {
        *byte = 0;
    }
}
impl<'a> FragmentedBuffer for &'a [u8] {
    fragmented_buffer_method_impls!();
}
impl<'a> ContiguousBuffer for &'a [u8] {}
impl<'a> ShrinkBuffer for &'a [u8] {
    fn shrink_front(&mut self, n: usize) {
        let _: &[u8] = take_front(self, n);
    }
    fn shrink_back(&mut self, n: usize) {
        let _: &[u8] = take_back(self, n);
    }
}
impl<'a> ParseBuffer for &'a [u8] {
    fn parse_with<'b, ParseArgs, P: ParsablePacket<&'b [u8], ParseArgs>>(
        &'b mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error> {
        // A `&'b mut &'a [u8]` wrapper which implements `BufferView<&'b [u8]>`
        // instead of `BufferView<&'a [u8]>`. This is needed thanks to fact that
        // `P: ParsablePacket` has the lifetime `'b`, not `'a`.
        struct ByteSlice<'a, 'b>(&'b mut &'a [u8]);

        impl<'a, 'b> AsRef<[u8]> for ByteSlice<'a, 'b> {
            fn as_ref(&self) -> &[u8] {
                &self.0
            }
        }

        impl<'b, 'a: 'b> BufferView<&'b [u8]> for ByteSlice<'a, 'b> {
            fn len(&self) -> usize {
                <[u8]>::len(self.0)
            }
            fn take_front(&mut self, n: usize) -> Option<&'b [u8]> {
                if self.0.len() < n {
                    return None;
                }
                Some(take_front(self.0, n))
            }
            fn take_back(&mut self, n: usize) -> Option<&'b [u8]> {
                if self.0.len() < n {
                    return None;
                }
                Some(take_back(self.0, n))
            }
            fn into_rest(self) -> &'b [u8] {
                self.0
            }
        }

        P::parse(ByteSlice(self), args)
    }
}
impl<'a> FragmentedBuffer for &'a mut [u8] {
    fragmented_buffer_method_impls!();
}
impl<'a> FragmentedBufferMut for &'a mut [u8] {
    fragmented_buffer_mut_method_impls!();
}
impl<'a> ContiguousBuffer for &'a mut [u8] {}
impl<'a> ShrinkBuffer for &'a mut [u8] {
    fn shrink_front(&mut self, n: usize) {
        let _: &[u8] = take_front_mut(self, n);
    }
    fn shrink_back(&mut self, n: usize) {
        let _: &[u8] = take_back_mut(self, n);
    }
}
impl<'a> ParseBuffer for &'a mut [u8] {
    fn parse_with<'b, ParseArgs, P: ParsablePacket<&'b [u8], ParseArgs>>(
        &'b mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error> {
        P::parse(self, args)
    }
}

impl<'a> ParseBufferMut for &'a mut [u8] {
    fn parse_with_mut<'b, ParseArgs, P: ParsablePacket<&'b mut [u8], ParseArgs>>(
        &'b mut self,
        args: ParseArgs,
    ) -> Result<P, P::Error> {
        P::parse_mut(self, args)
    }
}

impl<'b, 'a: 'b> BufferView<&'a [u8]> for &'b mut &'a [u8] {
    fn len(&self) -> usize {
        <[u8]>::len(self)
    }
    fn take_front(&mut self, n: usize) -> Option<&'a [u8]> {
        if self.len() < n {
            return None;
        }
        Some(take_front(self, n))
    }
    fn take_back(&mut self, n: usize) -> Option<&'a [u8]> {
        if self.len() < n {
            return None;
        }
        Some(take_back(self, n))
    }
    fn into_rest(self) -> &'a [u8] {
        self
    }
}

impl<'b, 'a: 'b> BufferView<&'b [u8]> for &'b mut &'a mut [u8] {
    fn len(&self) -> usize {
        <[u8]>::len(self)
    }
    fn take_front(&mut self, n: usize) -> Option<&'b [u8]> {
        if <[u8]>::len(self) < n {
            return None;
        }
        Some(take_front_mut(self, n))
    }
    fn take_back(&mut self, n: usize) -> Option<&'b [u8]> {
        if <[u8]>::len(self) < n {
            return None;
        }
        Some(take_back_mut(self, n))
    }
    fn into_rest(self) -> &'b [u8] {
        self
    }
}

impl<'b, 'a: 'b> BufferView<&'b mut [u8]> for &'b mut &'a mut [u8] {
    fn len(&self) -> usize {
        <[u8]>::len(self)
    }
    fn take_front(&mut self, n: usize) -> Option<&'b mut [u8]> {
        if <[u8]>::len(self) < n {
            return None;
        }
        Some(take_front_mut(self, n))
    }
    fn take_back(&mut self, n: usize) -> Option<&'b mut [u8]> {
        if <[u8]>::len(self) < n {
            return None;
        }
        Some(take_back_mut(self, n))
    }
    fn into_rest(self) -> &'b mut [u8] {
        self
    }
}

impl<'b, 'a: 'b> BufferViewMut<&'b mut [u8]> for &'b mut &'a mut [u8] {}

/// A [`BufferViewMut`] into a `&mut [u8]`.
///
/// This type is useful for instantiating a mutable view into a slice that can
/// be used for parsing, where any parsing that is done only affects this view
/// and therefore need not be "undone" later.
///
/// Note that `BufferViewMut<&mut [u8]>` is also implemented for &mut &mut [u8]
/// (a mutable reference to a mutable byte slice), but this can be problematic
/// if you need to materialize an *owned* type that implements `BufferViewMut`,
/// in order to pass it to a function, for example, so that it does not hold a
/// reference to a temporary value.
pub struct SliceBufViewMut<'a>(&'a mut [u8]);

impl<'a> SliceBufViewMut<'a> {
    pub fn new(buf: &'a mut [u8]) -> Self {
        Self(buf)
    }
}

impl<'a> BufferView<&'a mut [u8]> for SliceBufViewMut<'a> {
    fn take_front(&mut self, n: usize) -> Option<&'a mut [u8]> {
        let Self(buf) = self;
        if <[u8]>::len(buf) < n {
            return None;
        }
        Some(take_front_mut(buf, n))
    }

    fn take_back(&mut self, n: usize) -> Option<&'a mut [u8]> {
        let Self(buf) = self;
        if <[u8]>::len(buf) < n {
            return None;
        }
        Some(take_back_mut(buf, n))
    }

    fn into_rest(self) -> &'a mut [u8] {
        self.0
    }
}

impl<'a> BufferViewMut<&'a mut [u8]> for SliceBufViewMut<'a> {}

impl<'a> AsRef<[u8]> for SliceBufViewMut<'a> {
    fn as_ref(&self) -> &[u8] {
        self.0
    }
}

impl<'a> AsMut<[u8]> for SliceBufViewMut<'a> {
    fn as_mut(&mut self) -> &mut [u8] {
        self.0
    }
}

fn take_front<'a>(bytes: &mut &'a [u8], n: usize) -> &'a [u8] {
    let (prefix, rest) = mem::replace(bytes, &[]).split_at(n);
    *bytes = rest;
    prefix
}

fn take_back<'a>(bytes: &mut &'a [u8], n: usize) -> &'a [u8] {
    let split = bytes.len() - n;
    let (rest, suffix) = mem::replace(bytes, &[]).split_at(split);
    *bytes = rest;
    suffix
}

fn take_front_mut<'a>(bytes: &mut &'a mut [u8], n: usize) -> &'a mut [u8] {
    let (prefix, rest) = mem::replace(bytes, &mut []).split_at_mut(n);
    *bytes = rest;
    prefix
}

fn take_back_mut<'a>(bytes: &mut &'a mut [u8], n: usize) -> &'a mut [u8] {
    let split = <[u8]>::len(bytes) - n;
    let (rest, suffix) = mem::replace(bytes, &mut []).split_at_mut(split);
    *bytes = rest;
    suffix
}

// Returns the inclusive-exclusive equivalent of the bound, verifying that it is
// in range of `len`, and panicking if it is not or if the range is nonsensical.
fn canonicalize_range<R: RangeBounds<usize>>(len: usize, range: &R) -> Range<usize> {
    let lower = canonicalize_lower_bound(range.start_bound());
    let upper = canonicalize_upper_bound(len, range.end_bound()).expect("range out of bounds");
    assert!(lower <= upper, "invalid range: upper bound precedes lower bound");
    lower..upper
}

// Returns the inclusive equivalent of the bound.
fn canonicalize_lower_bound(bound: Bound<&usize>) -> usize {
    match bound {
        Bound::Included(x) => *x,
        Bound::Excluded(x) => *x + 1,
        Bound::Unbounded => 0,
    }
}

// Returns the exclusive equivalent of the bound, verifying that it is in range
// of `len`.
fn canonicalize_upper_bound(len: usize, bound: Bound<&usize>) -> Option<usize> {
    let bound = match bound {
        Bound::Included(x) => *x + 1,
        Bound::Excluded(x) => *x,
        Bound::Unbounded => len,
    };
    if bound > len {
        return None;
    }
    Some(bound)
}

mod sealed {
    pub trait Sealed {}
}

#[cfg(test)]
mod tests {
    use super::*;

    // Call test_buffer, test_buffer_view, and test_buffer_view_post for each of
    // the Buffer types. Call test_parse_buffer and test_buffer_view for each of
    // the ParseBuffer types.

    #[test]
    fn test_byte_slice_impl_buffer() {
        let mut avoid_leaks = Vec::new();
        test_parse_buffer::<&[u8], _>(|len| {
            let v = ascending(len);
            // Requires that |avoid_leaks| outlives this reference. In this case, we know
            // |test_parse_buffer| does not retain the reference beyond its run.
            let s = unsafe { std::slice::from_raw_parts(v.as_ptr(), v.len()) };
            let () = avoid_leaks.push(v);
            s
        });
        let buf = ascending(10);
        let mut buf: &[u8] = buf.as_ref();
        test_buffer_view::<&[u8], _>(&mut buf);
    }

    #[test]
    fn test_byte_slice_mut_impl_buffer() {
        let mut avoid_leaks = Vec::new();
        test_parse_buffer::<&mut [u8], _>(|len| {
            let mut v = ascending(len);
            // Requires that |avoid_leaks| outlives this reference. In this case, we know
            // |test_parse_buffer| does not retain the reference beyond its run.
            let s = unsafe { std::slice::from_raw_parts_mut(v.as_mut_ptr(), v.len()) };
            let () = avoid_leaks.push(v);
            s
        });
        let mut buf = ascending(10);
        let mut buf: &mut [u8] = buf.as_mut();
        test_buffer_view::<&mut [u8], _>(&mut buf);
    }

    #[test]
    fn test_either_impl_buffer() {
        macro_rules! test_either {
            ($variant:ident) => {
                test_buffer::<Either<Buf<Vec<u8>>, Buf<Vec<u8>>>, _>(|len| {
                    Either::$variant(Buf::new(ascending(len), ..))
                });
                // Test call to `Buf::buffer_view` which returns a
                // `BufferView`.
                let mut buf: Either<Buf<Vec<u8>>, Buf<Vec<u8>>> =
                    Either::$variant(Buf::new(ascending(10), ..));
                test_buffer_view(match &mut buf {
                    Either::$variant(buf) => buf.buffer_view(),
                    _ => unreachable!(),
                });
                test_buffer_view_post(&buf, true);
                // Test call to `Buf::buffer_view_mut` which returns a
                // `BufferViewMut`.
                let mut buf: Either<Buf<Vec<u8>>, Buf<Vec<u8>>> =
                    Either::$variant(Buf::new(ascending(10), ..));
                test_buffer_view_mut(match &mut buf {
                    Either::$variant(buf) => buf.buffer_view_mut(),
                    _ => unreachable!(),
                });
                test_buffer_view_mut_post(&buf, true);
            };
        }

        test_either!(A);
        test_either!(B);
    }

    #[test]
    fn test_slice_buf_view_mut() {
        let mut buf = ascending(10);

        test_buffer_view(SliceBufViewMut::new(&mut buf));
        test_buffer_view_mut(SliceBufViewMut::new(&mut buf));
    }

    #[test]
    fn test_buf_impl_buffer() {
        test_buffer(|len| Buf::new(ascending(len), ..));
        let mut buf = Buf::new(ascending(10), ..);
        test_buffer_view(buf.buffer_view());
        test_buffer_view_post(&buf, true);
    }

    fn ascending(n: u8) -> Vec<u8> {
        (0..n).collect::<Vec<u8>>()
    }

    // This test performs a number of shrinking operations (for ParseBuffer
    // implementations) followed by their equivalent growing operations (for
    // Buffer implementations only), and at each step, verifies various
    // properties of the buffer. The shrinking part of the test is in
    // test_parse_buffer_inner, while test_buffer calls test_parse_buffer_inner
    // and then performs the growing part of the test.

    // When shrinking, we keep two buffers - 'at_once' and 'separately', and for
    // each test case, we do the following:
    // - shrink the 'at_once' buffer with the 'shrink' field
    // - shrink_front the 'separately' buffer with the 'front' field
    // - shrink_back the 'separately' buffer with the 'back' field
    //
    // When growing, we only keep one buffer from the shrinking phase, and for
    // each test case, we do the following:
    // - grow_front the buffer with the 'front' field
    // - grow_back the buffer with the 'back' field
    //
    // After each action, we verify that the len and contents are as expected.
    // For Buffers, we also verify the cap, prefix, and suffix.
    struct TestCase {
        shrink: Range<usize>,
        front: usize, // shrink or grow the front of the body
        back: usize,  // shrink or grow the back of the body
        cap: usize,
        len: usize,
        pfx: usize,
        sfx: usize,
        contents: &'static [u8],
    }
    #[rustfmt::skip]
    const TEST_CASES: &[TestCase] = &[
        TestCase { shrink: 0..10, front: 0, back: 0, cap: 10, len: 10, pfx: 0, sfx: 0, contents: &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], },
        TestCase { shrink: 2..10, front: 2, back: 0, cap: 10, len: 8,  pfx: 2, sfx: 0, contents: &[2, 3, 4, 5, 6, 7, 8, 9], },
        TestCase { shrink: 0..8,  front: 0, back: 0, cap: 10, len: 8,  pfx: 2, sfx: 0, contents: &[2, 3, 4, 5, 6, 7, 8, 9], },
        TestCase { shrink: 0..6,  front: 0, back: 2, cap: 10, len: 6,  pfx: 2, sfx: 2, contents: &[2, 3, 4, 5, 6, 7], },
        TestCase { shrink: 2..4,  front: 2, back: 2, cap: 10, len: 2,  pfx: 4, sfx: 4, contents: &[4, 5], },
    ];

    // Test a ParseBuffer implementation. 'new_buf' is a function which
    // constructs a buffer of length n, and initializes its contents to [0, 1,
    // 2, ..., n -1].
    fn test_parse_buffer<B: ParseBuffer, N: FnMut(u8) -> B>(new_buf: N) {
        let _: B = test_parse_buffer_inner(new_buf, |buf, _, len, _, _, contents| {
            assert_eq!(buf.len(), len);
            assert_eq!(buf.as_ref(), contents);
        });
    }

    // Code common to test_parse_buffer and test_buffer. 'assert' is a function
    // which takes a buffer, and verifies that its capacity, length, prefix,
    // suffix, and contents are equal to the arguments (in that order). For
    // ParseBuffers, the capacity, prefix, and suffix arguments are irrelevant,
    // and ignored.
    //
    // When the test is done, test_parse_buffer_inner returns one of the buffers
    // it used for testing so that test_buffer can do further testing on it. Its
    // prefix, body, and suffix will be [0, 1, 2, 3], [4, 5], and [6, 7, 8, 9]
    // respectively.
    fn test_parse_buffer_inner<
        B: ParseBuffer,
        N: FnMut(u8) -> B,
        A: Fn(&B, usize, usize, usize, usize, &[u8]),
    >(
        mut new_buf: N,
        assert: A,
    ) -> B {
        let mut at_once = new_buf(10);
        let mut separately = new_buf(10);
        for tc in TEST_CASES {
            at_once.shrink(tc.shrink.clone());
            separately.shrink_front(tc.front);
            separately.shrink_back(tc.back);
            assert(&at_once, tc.cap, tc.len, tc.pfx, tc.sfx, tc.contents);
            assert(&separately, tc.cap, tc.len, tc.pfx, tc.sfx, tc.contents);
        }
        at_once
    }

    // Test a Buffer implementation. 'new_buf' is a function which constructs a
    // buffer of length and capacity n, and initializes its contents to [0, 1,
    // 2, ..., n - 1].
    fn test_buffer<B: Buffer, F: Fn(u8) -> B>(new_buf: F) {
        fn assert<B: Buffer>(
            buf: &B,
            cap: usize,
            len: usize,
            pfx: usize,
            sfx: usize,
            contents: &[u8],
        ) {
            assert_eq!(buf.len(), len);
            assert_eq!(buf.capacity(), cap);
            assert_eq!(buf.prefix_len(), pfx);
            assert_eq!(buf.suffix_len(), sfx);
            assert_eq!(buf.as_ref(), contents);
        }

        let mut buf = test_parse_buffer_inner(new_buf, assert);
        buf.reset();
        assert(&buf, 10, 10, 0, 0, &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][..]);
        buf.shrink_front(4);
        buf.shrink_back(4);
        assert(&buf, 10, 2, 4, 4, &[4, 5][..]);

        for tc in TEST_CASES.iter().rev() {
            assert(&buf, tc.cap, tc.len, tc.pfx, tc.sfx, tc.contents);
            buf.grow_front(tc.front);
            buf.grow_back(tc.back);
        }
    }

    // Test a BufferView implementation. Call with a view into a buffer with no
    // extra capacity whose body contains [0, 1, ..., 9]. After the call
    // returns, call test_buffer_view_post on the buffer.
    fn test_buffer_view<B: ByteSlice, BV: BufferView<B>>(mut view: BV) {
        assert_eq!(view.len(), 10);
        assert_eq!(view.take_front(1).unwrap().as_ref(), &[0][..]);
        assert_eq!(view.len(), 9);
        assert_eq!(view.take_back(1).unwrap().as_ref(), &[9][..]);
        assert_eq!(view.len(), 8);
        assert_eq!(view.peek_obj_front::<[u8; 2]>().unwrap(), &[1, 2]);
        assert_eq!(view.take_obj_front::<[u8; 2]>().unwrap().as_ref(), [1, 2]);
        assert_eq!(view.len(), 6);
        assert_eq!(view.peek_obj_back::<[u8; 2]>().unwrap(), &[7, 8]);
        assert_eq!(view.take_obj_back::<[u8; 2]>().unwrap().as_ref(), [7, 8]);
        assert_eq!(view.len(), 4);
        assert!(view.take_front(5).is_none());
        assert_eq!(view.len(), 4);
        assert!(view.take_back(5).is_none());
        assert_eq!(view.len(), 4);
        assert_eq!(view.into_rest().as_ref(), &[3, 4, 5, 6][..]);
    }

    // Test a BufferViewMut implementation. Call with a mutable view into a buffer
    // with no extra capacity whose body contains [0, 1, ..., 9]. After the call
    // returns, call test_buffer_view_post on the buffer.
    fn test_buffer_view_mut<B: ByteSliceMut, BV: BufferViewMut<B>>(mut view: BV) {
        assert_eq!(view.len(), 10);
        assert_eq!(view.as_mut()[0], 0);
        assert_eq!(view.take_front_zero(1).unwrap().as_ref(), &[0][..]);
        assert_eq!(view.len(), 9);
        assert_eq!(view.as_mut()[0], 1);
        assert_eq!(view.take_front_zero(1).unwrap().as_ref(), &[0][..]);
        assert_eq!(view.len(), 8);
        assert_eq!(view.as_mut()[7], 9);
        assert_eq!(view.take_back_zero(1).unwrap().as_ref(), &[0][..]);
        assert_eq!(view.len(), 7);
        assert_eq!(&view.as_mut()[0..2], &[2, 3][..]);
        assert_eq!(view.peek_obj_front::<[u8; 2]>().unwrap(), &[2, 3]);
        assert_eq!(view.take_obj_front_zero::<[u8; 2]>().unwrap().as_ref(), &[0, 0][..]);
        assert_eq!(view.len(), 5);
        assert_eq!(&view.as_mut()[3..5], &[7, 8][..]);
        assert_eq!(view.peek_obj_back::<[u8; 2]>().unwrap(), &[7, 8]);
        assert_eq!(view.take_obj_back_zero::<[u8; 2]>().unwrap().as_ref(), &[0, 0][..]);
        assert_eq!(view.write_obj_front(&[0u8]), Some(()));
        assert_eq!(view.as_mut(), &[5, 6][..]);
        assert_eq!(view.write_obj_back(&[0u8]), Some(()));
        assert_eq!(view.as_mut(), &[5][..]);
        assert!(view.take_front_zero(2).is_none());
        assert_eq!(view.len(), 1);
        assert!(view.take_back_zero(2).is_none());
        assert_eq!(view.len(), 1);
        assert_eq!(view.as_mut(), &[5][..]);
        assert_eq!(view.into_rest_zero().as_ref(), &[0][..]);
    }

    // Post-verification to test a BufferView implementation. Call after
    // test_buffer_view.
    fn test_buffer_view_post<B: Buffer>(buffer: &B, preserves_cap: bool) {
        assert_eq!(buffer.as_ref(), &[3, 4, 5, 6][..]);
        if preserves_cap {
            assert_eq!(buffer.prefix_len(), 3);
            assert_eq!(buffer.suffix_len(), 3);
        }
    }

    // Post-verification to test a BufferViewMut implementation. Call after
    // test_buffer_view_mut.
    fn test_buffer_view_mut_post<B: Buffer>(buffer: &B, preserves_cap: bool) {
        assert_eq!(buffer.as_ref(), &[0][..]);
        if preserves_cap {
            assert_eq!(buffer.prefix_len(), 5);
            assert_eq!(buffer.suffix_len(), 4);
        }
    }

    #[test]
    fn test_buffer_view_from_buffer() {
        // This test is specifically designed to verify that implementations of
        // ParseBuffer::parse properly construct a BufferView, and that that
        // BufferView properly updates the underlying buffer. It was inspired by
        // the bug with Change-Id Ifeab21fba0f7ba94d1a12756d4e83782002e4e1e.

        // This ParsablePacket implementation takes the contents it expects as a
        // parse argument and validates the BufferView[Mut] against it. It consumes
        // one byte from the front and one byte from the back to ensure that that
        // functionality works as well. For a mutable buffer, the implementation also
        // modifies the bytes that were consumed so tests can make sure that the
        // `parse_mut` function was actually called and that the bytes are mutable.
        struct TestParsablePacket {}
        impl<B: ByteSlice> ParsablePacket<B, &[u8]> for TestParsablePacket {
            type Error = ();
            fn parse<BV: BufferView<B>>(
                mut buffer: BV,
                args: &[u8],
            ) -> Result<TestParsablePacket, ()> {
                assert_eq!(buffer.as_ref(), args);
                let _: B = buffer.take_front(1).unwrap();
                let _: B = buffer.take_back(1).unwrap();
                Ok(TestParsablePacket {})
            }

            fn parse_mut<BV: BufferViewMut<B>>(
                mut buffer: BV,
                args: &[u8],
            ) -> Result<TestParsablePacket, ()>
            where
                B: ByteSliceMut,
            {
                assert_eq!(buffer.as_ref(), args);
                buffer.take_front(1).unwrap().as_mut()[0] += 1;
                buffer.take_back(1).unwrap().as_mut()[0] += 2;
                Ok(TestParsablePacket {})
            }

            fn parse_metadata(&self) -> ParseMetadata {
                unimplemented!()
            }
        }

        // immutable byte slices

        let mut buf = &[0, 1, 2, 3, 4, 5, 6, 7][..];
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();

        // test that different temporary values do not affect one another and
        // also that slicing works properly (in that the elements outside of the
        // slice are not exposed in the BufferView[Mut]; this is fairly obvious
        // for slices, but less obvious for Buf, which we test below)
        let buf = &[0, 1, 2, 3, 4, 5, 6, 7][..];
        let TestParsablePacket {} =
            (&buf[1..7]).parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        let TestParsablePacket {} =
            (&buf[1..7]).parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();

        // mutable byte slices

        let mut bytes = [0, 1, 2, 3, 4, 5, 6, 7];
        let mut buf = &mut bytes[..];
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        // test that this also works with parse_with_mut
        let TestParsablePacket {} =
            buf.parse_with_mut::<_, TestParsablePacket>(&[2, 3, 4, 5]).unwrap();
        let TestParsablePacket {} = buf.parse_with_mut::<_, TestParsablePacket>(&[3, 4]).unwrap();
        assert_eq!(bytes, [0, 1, 3, 4, 6, 7, 6, 7]);

        // test that different temporary values do not affect one another and
        // also that slicing works properly (in that the elements outside of the
        // slice are not exposed in the BufferView[Mut]; this is fairly obvious
        // for slices, but less obvious for Buf, which we test below)
        let buf = &mut [0, 1, 2, 3, 4, 5, 6, 7][..];
        let TestParsablePacket {} =
            (&buf[1..7]).parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        let TestParsablePacket {} =
            (&buf[1..7]).parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        let TestParsablePacket {} =
            (&mut buf[1..7]).parse_with_mut::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        let TestParsablePacket {} =
            (&mut buf[1..7]).parse_with_mut::<_, TestParsablePacket>(&[2, 2, 3, 4, 5, 8]).unwrap();
        assert_eq!(buf, &[0, 3, 2, 3, 4, 5, 10, 7][..]);

        // Buf with immutable byte slice

        let mut buf = Buf::new(&[0, 1, 2, 3, 4, 5, 6, 7][..], ..);
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();

        // the same test again, but this time with Buf's range set
        let mut buf = Buf::new(&[0, 1, 2, 3, 4, 5, 6, 7][..], 1..7);
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} = buf.parse_with::<_, TestParsablePacket>(&[2, 3, 4, 5]).unwrap();

        // Buf with mutable byte slice

        let mut bytes = [0, 1, 2, 3, 4, 5, 6, 7];
        let buf = &mut bytes[..];
        let mut buf = Buf::new(&mut buf[..], ..);
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[0, 1, 2, 3, 4, 5, 6, 7]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        // test that this also works with parse_with_mut
        let TestParsablePacket {} =
            buf.parse_with_mut::<_, TestParsablePacket>(&[2, 3, 4, 5]).unwrap();
        let TestParsablePacket {} = buf.parse_with_mut::<_, TestParsablePacket>(&[3, 4]).unwrap();
        assert_eq!(bytes, [0, 1, 3, 4, 6, 7, 6, 7]);
        // the same test again, but this time with Buf's range set
        let mut bytes = [0, 1, 2, 3, 4, 5, 6, 7];
        let buf = &mut bytes[..];
        let mut buf = Buf::new(&mut buf[..], 1..7);
        let TestParsablePacket {} =
            buf.parse_with::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        // test that, after parsing, the bytes consumed are consumed permanently
        let TestParsablePacket {} = buf.parse_with::<_, TestParsablePacket>(&[2, 3, 4, 5]).unwrap();
        assert_eq!(bytes, [0, 1, 2, 3, 4, 5, 6, 7]);
        // test that this also works with parse_with_mut
        let mut bytes = [0, 1, 2, 3, 4, 5, 6, 7];
        let buf = &mut bytes[..];
        let mut buf = Buf::new(&mut buf[..], 1..7);
        let TestParsablePacket {} =
            buf.parse_with_mut::<_, TestParsablePacket>(&[1, 2, 3, 4, 5, 6]).unwrap();
        let TestParsablePacket {} =
            buf.parse_with_mut::<_, TestParsablePacket>(&[2, 3, 4, 5]).unwrap();
        assert_eq!(bytes, [0, 2, 3, 3, 4, 7, 8, 7]);
    }

    #[test]
    fn test_buf_shrink_to() {
        // Tests the shrink_front_to and shrink_back_to methods.
        fn test(buf: &[u8], shrink_to: usize, size_after: usize) {
            let mut buf0 = &buf[..];
            buf0.shrink_front_to(shrink_to);
            assert_eq!(buf0.len(), size_after);
            let mut buf1 = &buf[..];
            buf1.shrink_back_to(shrink_to);
            assert_eq!(buf0.len(), size_after);
        }

        test(&[0, 1, 2, 3], 2, 2);
        test(&[0, 1, 2, 3], 4, 4);
        test(&[0, 1, 2, 3], 8, 4);
    }

    #[test]
    fn test_empty_buf() {
        // Test ParseBuffer impl

        assert_eq!(EmptyBuf.as_ref(), []);
        assert_eq!(EmptyBuf.as_mut(), []);
        EmptyBuf.shrink_front(0);
        EmptyBuf.shrink_back(0);

        struct DummyParsablePacket;
        impl<B: ByteSlice> ParsablePacket<B, ()> for DummyParsablePacket {
            type Error = Never;

            fn parse<BV: BufferView<B>>(buffer: BV, _args: ()) -> Result<Self, Self::Error> {
                assert_eq!(buffer.as_ref(), []);
                Ok(DummyParsablePacket)
            }

            fn parse_mut<BV: BufferViewMut<B>>(
                mut buffer: BV,
                _args: (),
            ) -> Result<Self, Self::Error>
            where
                B: ByteSliceMut,
            {
                assert_eq!(buffer.as_mut(), []);
                Ok(DummyParsablePacket)
            }

            fn parse_metadata(&self) -> ParseMetadata {
                unimplemented!()
            }
        }

        // Test Buffer impl

        assert_eq!(EmptyBuf.prefix_len(), 0);
        assert_eq!(EmptyBuf.suffix_len(), 0);
        EmptyBuf.grow_front(0);
        EmptyBuf.grow_back(0);

        // Test BufferView impl

        assert_eq!(BufferView::<&[u8]>::take_front(&mut EmptyBuf, 0), Some(&[][..]));
        assert_eq!(BufferView::<&[u8]>::take_front(&mut EmptyBuf, 1), None);
        assert_eq!(BufferView::<&[u8]>::take_back(&mut EmptyBuf, 0), Some(&[][..]));
        assert_eq!(BufferView::<&[u8]>::take_back(&mut EmptyBuf, 1), None);
        assert_eq!(BufferView::<&[u8]>::into_rest(EmptyBuf), &[][..]);
    }

    // Each panic test case needs to be in its own function, which results in an
    // explosion of test functions. These macros generates the appropriate
    // function definitions automatically for a given type, reducing the amount
    // of code by a factor of ~4.
    macro_rules! make_parse_buffer_panic_tests {
        (
            $new_empty_buffer:expr,
            $shrink_panics:ident,
            $nonsense_shrink_panics:ident,
        ) => {
            #[test]
            #[should_panic]
            fn $shrink_panics() {
                ($new_empty_buffer).shrink(..1);
            }
            #[test]
            #[should_panic]
            fn $nonsense_shrink_panics() {
                #[allow(clippy::reversed_empty_ranges)] // Intentionally testing with invalid range
                ($new_empty_buffer).shrink(1..0);
            }
        };
    }

    macro_rules! make_panic_tests {
        (
            $new_empty_buffer:expr,
            $shrink_panics:ident,
            $nonsense_shrink_panics:ident,
            $grow_front_panics:ident,
            $grow_back_panics:ident,
        ) => {
            make_parse_buffer_panic_tests!(
                $new_empty_buffer,
                $shrink_panics,
                $nonsense_shrink_panics,
            );
            #[test]
            #[should_panic]
            fn $grow_front_panics() {
                ($new_empty_buffer).grow_front(1);
            }
            #[test]
            #[should_panic]
            fn $grow_back_panics() {
                ($new_empty_buffer).grow_back(1);
            }
        };
    }

    make_parse_buffer_panic_tests!(
        &[][..],
        test_byte_slice_shrink_panics,
        test_byte_slice_nonsense_shrink_panics,
    );
    make_parse_buffer_panic_tests!(
        &mut [][..],
        test_byte_slice_mut_shrink_panics,
        test_byte_slice_mut_nonsense_shrink_panics,
    );
    make_panic_tests!(
        Either::A::<Buf<&[u8]>, Buf<&[u8]>>(Buf::new(&[][..], ..)),
        test_either_slice_panics,
        test_either_nonsense_slice_panics,
        test_either_grow_front_panics,
        test_either_grow_back_panics,
    );
    make_panic_tests!(
        Buf::new(&[][..], ..),
        test_buf_shrink_panics,
        test_buf_nonsense_shrink_panics,
        test_buf_grow_front_panics,
        test_buf_grow_back_panics,
    );
    make_panic_tests!(
        EmptyBuf,
        test_empty_buf_shrink_panics,
        test_empty_buf_nonsense_shrink_panics,
        test_empty_buf_grow_front_panics,
        test_empty_buf_grow_back_panics,
    );

    #[test]
    fn take_rest_front_back() {
        let buf = [1_u8, 2, 3];
        let mut b = &mut &buf[..];
        assert_eq!(b.take_rest_front(), &buf[..]);
        assert_eq!(b.len(), 0);

        let mut b = &mut &buf[..];
        assert_eq!(b.take_rest_back(), &buf[..]);
        assert_eq!(b.len(), 0);
    }

    #[test]
    fn take_byte_front_back() {
        let buf = [1_u8, 2, 3, 4];
        let mut b = &mut &buf[..];
        assert_eq!(b.take_byte_front().unwrap(), 1);
        assert_eq!(b.take_byte_front().unwrap(), 2);
        assert_eq!(b.take_byte_back().unwrap(), 4);
        assert_eq!(b.take_byte_back().unwrap(), 3);
        assert!(b.take_byte_front().is_none());
        assert!(b.take_byte_back().is_none());
    }

    #[test]
    fn with_header_mut_with_meta() {
        #[derive(zerocopy::FromZeros, FromBytes, AsBytes, NoCell, Unaligned)]
        #[repr(C)]
        struct Header {
            two_bytes: [u8; 2],
        }
        struct TestParsablePacket<B> {
            header: Ref<B, Header>,
        }

        #[derive(Debug)]
        struct ParseError;

        impl<B: ByteSlice> ParsablePacket<B, ()> for TestParsablePacket<B> {
            type Error = ParseError;
            fn parse<BV: BufferView<B>>(
                mut buffer: BV,
                _args: (),
            ) -> Result<TestParsablePacket<B>, ParseError> {
                let header = buffer.take_obj_front::<Header>().ok_or(ParseError)?;
                Ok(TestParsablePacket { header })
            }

            fn parse_mut<BV: BufferViewMut<B>>(
                mut buffer: BV,
                _args: (),
            ) -> Result<TestParsablePacket<B>, ParseError>
            where
                B: ByteSliceMut,
            {
                let header = buffer.take_obj_front::<Header>().ok_or(ParseError)?;
                Ok(TestParsablePacket { header })
            }

            fn parse_metadata(&self) -> ParseMetadata {
                let header_len = self.header.bytes().len();
                ParseMetadata::from_packet(header_len, 0, 0)
            }
        }

        // Parse the packet.
        let mut buf = Buf::new(vec![0, 1, 2, 3], ..);
        let outer = buf.parse::<TestParsablePacket<_>>().unwrap();
        assert_eq!(outer.header.two_bytes, [0, 1]);

        // We should be able to access the header we just parsed using its
        // ParseMetadata.
        let meta = outer.parse_metadata();
        buf.with_header_mut_with_meta(meta, |mut header| {
            let TestParsablePacket { mut header } =
                header.parse_mut::<TestParsablePacket<_>>().unwrap();
            assert_eq!(header.two_bytes, [0, 1]);
            header.two_bytes = [2, 3];
        });

        // If we undo the parse and extract the inner buffer, we should be able
        // to see that the header of the outer packet was modified.
        buf.undo_parse(meta);
        assert_eq!(buf.into_inner(), vec![2, 3, 2, 3]);
    }
}