compio-buf 0.8.2-rc.1

#[cfg(feature = "allocator_api")]
use std::alloc::Allocator;
use std::{error::Error, fmt::Display, mem::MaybeUninit, ops::RangeBounds, rc::Rc, sync::Arc};

use crate::*;

/// A trait for immutable buffers.
///
/// The `IoBuf` trait is implemented by buffer types that can be passed to
/// immutable completion-based IO operations, like writing its content to a
/// file. This trait will only take initialized bytes of a buffer into account.
pub trait IoBuf: 'static {
    /// Get the slice of initialized bytes.
    fn as_init(&self) -> &[u8];

    /// Length of initialized bytes in the buffer.
    fn buf_len(&self) -> usize {
        self.as_init().len()
    }

    /// Raw pointer to the buffer.
    fn buf_ptr(&self) -> *const u8 {
        self.as_init().as_ptr()
    }

    /// Check if the buffer is empty.
    fn is_empty(&self) -> bool {
        self.buf_len() == 0
    }

    /// Returns a view of the buffer with the specified range.
    ///
    /// This method is similar to Rust's slicing (`&buf[..]`), but takes
    /// ownership of the buffer.
    ///
    /// # Examples
    ///
    /// ```
    /// use compio_buf::IoBuf;
    ///
    /// let buf = b"hello world";
    /// assert_eq!(buf.slice(6..).as_init(), b"world");
    /// ```
    ///
    /// # Panics
    /// Panics if:
    /// * begin > buf_len()
    /// * end < begin
    fn slice(self, range: impl std::ops::RangeBounds<usize>) -> Slice<Self>
    where
        Self: Sized,
    {
        use std::ops::Bound;

        let begin = match range.start_bound() {
            Bound::Included(&n) => n,
            Bound::Excluded(&n) => n + 1,
            Bound::Unbounded => 0,
        };

        let end = match range.end_bound() {
            Bound::Included(&n) => Some(n.checked_add(1).expect("out of range")),
            Bound::Excluded(&n) => Some(n),
            Bound::Unbounded => None,
        };

        assert!(begin <= self.buf_len());

        if let Some(end) = end {
            assert!(begin <= end);
        }

        // SAFETY: begin <= self.buf_len()
        unsafe { Slice::new(self, begin, end) }
    }

    /// Create a [`Reader`] from this buffer, which implements
    /// [`std::io::Read`].
    fn into_reader(self) -> Reader<Self>
    where
        Self: Sized,
    {
        Reader::new(self)
    }

    /// Create a [`ReaderRef`] from a reference of the buffer, which
    /// implements [`std::io::Read`].
    fn as_reader(&self) -> ReaderRef<'_, Self> {
        ReaderRef::new(self)
    }
}

impl<B: IoBuf + ?Sized> IoBuf for &'static B {
    fn as_init(&self) -> &[u8] {
        (**self).as_init()
    }
}

impl<B: IoBuf + ?Sized> IoBuf for &'static mut B {
    fn as_init(&self) -> &[u8] {
        (**self).as_init()
    }
}

impl<B: IoBuf + ?Sized, #[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBuf
    for t_alloc!(Box, B, A)
{
    fn as_init(&self) -> &[u8] {
        (**self).as_init()
    }
}

impl<B: IoBuf + ?Sized, #[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBuf
    for t_alloc!(Rc, B, A)
{
    fn as_init(&self) -> &[u8] {
        (**self).as_init()
    }
}

impl IoBuf for [u8] {
    fn as_init(&self) -> &[u8] {
        self
    }
}

impl<const N: usize> IoBuf for [u8; N] {
    fn as_init(&self) -> &[u8] {
        self
    }
}

impl<#[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBuf for t_alloc!(Vec, u8, A) {
    fn as_init(&self) -> &[u8] {
        self
    }
}

impl IoBuf for str {
    fn as_init(&self) -> &[u8] {
        self.as_bytes()
    }
}

impl IoBuf for String {
    fn as_init(&self) -> &[u8] {
        self.as_bytes()
    }
}

impl<B: IoBuf + ?Sized, #[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBuf
    for t_alloc!(Arc, B, A)
{
    fn as_init(&self) -> &[u8] {
        (**self).as_init()
    }
}

#[cfg(feature = "bytes")]
impl IoBuf for bytes::Bytes {
    fn as_init(&self) -> &[u8] {
        self
    }
}

#[cfg(feature = "bytes")]
impl IoBuf for bytes::BytesMut {
    fn as_init(&self) -> &[u8] {
        self
    }
}

#[cfg(feature = "read_buf")]
impl IoBuf for std::io::BorrowedBuf<'static> {
    fn as_init(&self) -> &[u8] {
        self.filled()
    }
}

#[cfg(feature = "arrayvec")]
impl<const N: usize> IoBuf for arrayvec::ArrayVec<u8, N> {
    fn as_init(&self) -> &[u8] {
        self
    }
}

#[cfg(feature = "smallvec")]
impl<const N: usize> IoBuf for smallvec::SmallVec<[u8; N]>
where
    [u8; N]: smallvec::Array<Item = u8>,
{
    fn as_init(&self) -> &[u8] {
        self
    }
}

#[cfg(feature = "memmap2")]
impl IoBuf for memmap2::Mmap {
    fn as_init(&self) -> &[u8] {
        self
    }
}

#[cfg(feature = "memmap2")]
impl IoBuf for memmap2::MmapMut {
    fn as_init(&self) -> &[u8] {
        self
    }
}

/// An error indicating that reserving capacity for a buffer failed.
#[must_use]
#[derive(Debug)]
pub enum ReserveError {
    /// Reservation is not supported.
    NotSupported,

    /// Reservation failed.
    ///
    /// This is usually caused by out-of-memory.
    ReserveFailed(Box<dyn Error + Send + Sync>),
}

impl ReserveError {
    /// Check if the error is [`NotSupported`].
    ///
    /// [`NotSupported`]: ReserveError::NotSupported
    pub fn is_not_supported(&self) -> bool {
        matches!(self, ReserveError::NotSupported)
    }
}

impl Display for ReserveError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ReserveError::NotSupported => write!(f, "reservation is not supported"),
            ReserveError::ReserveFailed(src) => write!(f, "reservation failed: {src}"),
        }
    }
}

impl Error for ReserveError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            ReserveError::ReserveFailed(src) => Some(src.as_ref()),
            _ => None,
        }
    }
}

impl From<ReserveError> for std::io::Error {
    fn from(value: ReserveError) -> Self {
        match value {
            ReserveError::NotSupported => {
                std::io::Error::new(std::io::ErrorKind::Unsupported, "reservation not supported")
            }
            ReserveError::ReserveFailed(src) => {
                std::io::Error::new(std::io::ErrorKind::OutOfMemory, src)
            }
        }
    }
}

/// An error indicating that reserving exact capacity for a buffer failed.
#[must_use]
#[derive(Debug)]
pub enum ReserveExactError {
    /// Reservation is not supported.
    NotSupported,

    /// Reservation failed.
    ///
    /// This is usually caused by out-of-memory.
    ReserveFailed(Box<dyn Error + Send + Sync>),

    /// Reserved size does not match the expected size.
    ExactSizeMismatch {
        /// Expected size to reserve
        expected: usize,

        /// Actual size reserved
        reserved: usize,
    },
}

impl ReserveExactError {
    /// Check if the error is [`NotSupported`]
    ///
    /// [`NotSupported`]: ReserveExactError::NotSupported
    pub fn is_not_supported(&self) -> bool {
        matches!(self, ReserveExactError::NotSupported)
    }
}

impl Display for ReserveExactError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            ReserveExactError::NotSupported => write!(f, "reservation is not supported"),
            ReserveExactError::ReserveFailed(src) => write!(f, "reservation failed: {src}"),
            ReserveExactError::ExactSizeMismatch { reserved, expected } => {
                write!(
                    f,
                    "reserved size mismatch: expected {}, reserved {}",
                    expected, reserved
                )
            }
        }
    }
}

impl From<ReserveError> for ReserveExactError {
    fn from(err: ReserveError) -> Self {
        match err {
            ReserveError::NotSupported => ReserveExactError::NotSupported,
            ReserveError::ReserveFailed(src) => ReserveExactError::ReserveFailed(src),
        }
    }
}

impl Error for ReserveExactError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            ReserveExactError::ReserveFailed(src) => Some(src.as_ref()),
            _ => None,
        }
    }
}

impl From<ReserveExactError> for std::io::Error {
    fn from(value: ReserveExactError) -> Self {
        match value {
            ReserveExactError::NotSupported => {
                std::io::Error::new(std::io::ErrorKind::Unsupported, "reservation not supported")
            }
            ReserveExactError::ReserveFailed(src) => {
                std::io::Error::new(std::io::ErrorKind::OutOfMemory, src)
            }
            ReserveExactError::ExactSizeMismatch { expected, reserved } => std::io::Error::other(
                format!("reserved size mismatch: expected {expected}, reserved {reserved}",),
            ),
        }
    }
}

#[cfg(feature = "smallvec")]
mod smallvec_err {
    use std::{error::Error, fmt::Display};

    use smallvec::CollectionAllocErr;

    #[derive(Debug)]
    pub(super) struct SmallVecErr(pub CollectionAllocErr);

    impl Display for SmallVecErr {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            write!(f, "SmallVec allocation error: {}", self.0)
        }
    }

    impl Error for SmallVecErr {}
}

/// A trait for mutable buffers.
///
/// The `IoBufMut` trait is implemented by buffer types that can be passed to
/// mutable completion-based IO operations, like reading content from a file and
/// write to the buffer. This trait will take all space of a buffer into
/// account, including uninitialized bytes.
pub trait IoBufMut: IoBuf + SetLen {
    /// Get the full mutable slice of the buffer, including both initialized
    /// and uninitialized bytes.
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>];

    /// Initialize all bytes in the buffer and return them.
    ///
    /// Bytes in the already-initialized prefix (`0..buf_len()`) are preserved.
    /// Only the uninitialized tail (`buf_len()..buf_capacity()`) is
    /// zero-initialized.
    fn ensure_init(&mut self) -> &mut [u8] {
        let len = (*self).buf_len();
        let slice = self.as_uninit();
        slice[len..].fill(MaybeUninit::new(0));
        unsafe { slice.assume_init_mut() }
    }

    /// Total capacity of the buffer, including both initialized and
    /// uninitialized bytes.
    fn buf_capacity(&mut self) -> usize {
        self.as_uninit().len()
    }

    /// Get the raw mutable pointer to the buffer.
    fn buf_mut_ptr(&mut self) -> *mut MaybeUninit<u8> {
        self.as_uninit().as_mut_ptr()
    }

    /// Get the mutable slice of initialized bytes. The content is the same as
    /// [`IoBuf::as_init`], but mutable.
    fn as_mut_slice(&mut self) -> &mut [u8] {
        let len = (*self).buf_len();
        let ptr = (*self).buf_mut_ptr();
        // SAFETY:
        // - lifetime of the returned slice is bounded by &mut self
        // - bytes within `len` are guaranteed to be initialized
        // - the pointer is derived from
        unsafe { std::slice::from_raw_parts_mut(ptr as *mut u8, len) }
    }

    /// Extend the buffer by copying bytes from `src`.
    ///
    /// The buffer will reserve additional capacity if necessary, and return an
    /// error when reservation failed.
    ///
    /// Notice that this may move the memory of the buffer, so it's UB to
    /// call this after the buffer is being pinned.
    // FIXME: Change to `slice::write_copy_of_slice` when stabilized
    fn extend_from_slice(&mut self, src: &[u8]) -> Result<(), ReserveError> {
        let len = src.len();
        let init = (*self).buf_len();
        self.reserve(len)?;
        let ptr = self.buf_mut_ptr().wrapping_add(init);

        unsafe {
            // SAFETY:
            // - we have reserved enough capacity so the ptr and len stays in one allocation
            // - src is valid for len bytes
            // - ptr is valid for len bytes
            // - &mut self guarantees that src cannot overlap with dst
            std::ptr::copy_nonoverlapping(src.as_ptr() as _, ptr, len);

            // SAFETY: the bytes in range [init, init + len) are initialized now
            self.advance_to(init + len);
        }

        Ok(())
    }

    /// Like [`slice::copy_within`], copy a range of bytes within the buffer to
    /// another location in the same buffer. This will count in both initialized
    /// and uninitialized bytes.
    ///
    /// # Panics
    ///
    /// This method will panic if the source or destination range is out of
    /// bounds.
    ///
    /// [`slice::copy_within`]: https://doc.rust-lang.org/std/primitive.slice.html#method.copy_within
    fn copy_within<R>(&mut self, src: R, dest: usize)
    where
        R: RangeBounds<usize>,
    {
        self.as_uninit().copy_within(src, dest);
    }

    /// Reserve additional capacity for the buffer.
    ///
    /// By default, this checks if the spare capacity is enough to fit in
    /// `len`-bytes. If it does, returns `Ok(())`, and otherwise returns
    /// [`Err(ReserveError::NotSupported)`]. Types that support dynamic
    /// resizing (like `Vec<u8>`) will override this method to actually
    /// reserve capacity. The return value indicates whether the reservation
    /// succeeded. See [`ReserveError`] for details.
    ///
    /// Notice that this may move the memory of the buffer, so it's UB to
    /// call this after the buffer is being pinned.
    ///
    /// [`Err(ReserveError::NotSupported)`]: ReserveError::NotSupported
    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        let init = (*self).buf_len();
        if len <= self.buf_capacity() - init {
            return Ok(());
        }
        Err(ReserveError::NotSupported)
    }

    /// Reserve exactly `len` additional capacity for the buffer.
    ///
    /// By default this falls back to [`IoBufMut::reserve`]. Types that support
    /// dynamic resizing (like `Vec<u8>`) will override this method to
    /// actually reserve capacity. The return value indicates whether the
    /// exact reservation succeeded. See [`ReserveExactError`] for details.
    ///
    /// Notice that this may move the memory of the buffer, so it's UB to
    /// call this after the buffer is being pinned.
    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        self.reserve(len)?;
        Ok(())
    }

    /// Returns an [`Uninit`], which is a [`Slice`] that only exposes
    /// uninitialized bytes.
    ///
    /// It will always point to the uninitialized area of a [`IoBufMut`] even
    /// after reading in some bytes, which is done by [`SetLen`]. This
    /// is useful for writing data into buffer without overwriting any
    /// existing bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use compio_buf::{IoBuf, IoBufMut};
    ///
    /// let mut buf = Vec::from(b"hello world");
    /// buf.reserve_exact(10);
    /// let mut slice = buf.uninit();
    ///
    /// assert_eq!(slice.as_init(), b"");
    /// assert_eq!(slice.buf_capacity(), 10);
    /// ```
    fn uninit(self) -> Uninit<Self>
    where
        Self: Sized,
    {
        Uninit::new(self)
    }

    /// Create a [`Writer`] from this buffer, which implements
    /// [`std::io::Write`].
    fn into_writer(self) -> Writer<Self>
    where
        Self: Sized,
    {
        Writer::new(self)
    }

    /// Create a [`Writer`] from a mutable reference of the buffer, which
    /// implements [`std::io::Write`].
    fn as_writer(&mut self) -> WriterRef<'_, Self> {
        WriterRef::new(self)
    }

    /// Indicate whether the buffer has been filled (uninit portion is empty)
    fn is_filled(&mut self) -> bool {
        let len = (*self).as_init().len();
        let cap = (*self).buf_capacity();
        len == cap
    }
}

impl<B: IoBufMut + ?Sized> IoBufMut for &'static mut B {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        (**self).as_uninit()
    }

    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        (**self).reserve(len)
    }

    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        (**self).reserve_exact(len)
    }
}

impl<B: IoBufMut + ?Sized, #[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBufMut
    for t_alloc!(Box, B, A)
{
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        (**self).as_uninit()
    }

    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        (**self).reserve(len)
    }

    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        (**self).reserve_exact(len)
    }
}

impl<#[cfg(feature = "allocator_api")] A: Allocator + 'static> IoBufMut for t_alloc!(Vec, u8, A) {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        let cap = self.capacity();
        // SAFETY: Vec guarantees that the pointer is valid for `capacity` bytes
        unsafe { std::slice::from_raw_parts_mut(ptr, cap) }
    }

    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        if let Err(e) = Vec::try_reserve(self, len) {
            return Err(ReserveError::ReserveFailed(Box::new(e)));
        }

        Ok(())
    }

    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        if self.capacity() - self.len() >= len {
            return Ok(());
        }

        if let Err(e) = Vec::try_reserve_exact(self, len) {
            return Err(ReserveExactError::ReserveFailed(Box::new(e)));
        }

        if self.capacity() - self.len() != len {
            return Err(ReserveExactError::ExactSizeMismatch {
                reserved: self.capacity() - self.len(),
                expected: len,
            });
        }
        Ok(())
    }
}

impl IoBufMut for [u8] {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        let len = self.len();
        // SAFETY: slice is fully initialized, so treating it as MaybeUninit is safe
        unsafe { std::slice::from_raw_parts_mut(ptr, len) }
    }
}

impl<const N: usize> IoBufMut for [u8; N] {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        // SAFETY: array is fully initialized, so treating it as MaybeUninit is safe
        unsafe { std::slice::from_raw_parts_mut(ptr, N) }
    }
}

#[cfg(feature = "bytes")]
impl IoBufMut for bytes::BytesMut {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        let cap = self.capacity();
        // SAFETY: BytesMut guarantees that the pointer is valid for `capacity` bytes
        unsafe { std::slice::from_raw_parts_mut(ptr, cap) }
    }

    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        bytes::BytesMut::reserve(self, len);
        Ok(())
    }

    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        if self.capacity() - self.len() >= len {
            return Ok(());
        }

        bytes::BytesMut::reserve(self, len);

        if self.capacity() - self.len() != len {
            Err(ReserveExactError::ExactSizeMismatch {
                reserved: self.capacity() - self.len(),
                expected: len,
            })
        } else {
            Ok(())
        }
    }
}

#[cfg(feature = "read_buf")]
impl IoBufMut for std::io::BorrowedBuf<'static> {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let total_cap = self.capacity();

        // SAFETY: We reconstruct the full buffer from the filled portion pointer.
        // BorrowedBuf guarantees that the underlying buffer has capacity bytes.
        unsafe {
            let filled_ptr = self.filled().as_ptr() as *mut MaybeUninit<u8>;
            std::slice::from_raw_parts_mut(filled_ptr, total_cap)
        }
    }
}

#[cfg(feature = "arrayvec")]
impl<const N: usize> IoBufMut for arrayvec::ArrayVec<u8, N> {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        // SAFETY: ArrayVec guarantees that the pointer is valid for N bytes
        unsafe { std::slice::from_raw_parts_mut(ptr, N) }
    }
}

#[cfg(feature = "smallvec")]
impl<const N: usize> IoBufMut for smallvec::SmallVec<[u8; N]>
where
    [u8; N]: smallvec::Array<Item = u8>,
{
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        let ptr = self.as_mut_ptr() as *mut MaybeUninit<u8>;
        let cap = self.capacity();
        // SAFETY: SmallVec guarantees that the pointer is valid for `capacity` bytes
        unsafe { std::slice::from_raw_parts_mut(ptr, cap) }
    }

    fn reserve(&mut self, len: usize) -> Result<(), ReserveError> {
        if let Err(e) = smallvec::SmallVec::try_reserve(self, len) {
            return Err(ReserveError::ReserveFailed(Box::new(
                smallvec_err::SmallVecErr(e),
            )));
        }
        Ok(())
    }

    fn reserve_exact(&mut self, len: usize) -> Result<(), ReserveExactError> {
        if self.capacity() - self.len() >= len {
            return Ok(());
        }

        if let Err(e) = smallvec::SmallVec::try_reserve_exact(self, len) {
            return Err(ReserveExactError::ReserveFailed(Box::new(
                smallvec_err::SmallVecErr(e),
            )));
        }

        if self.capacity() - self.len() != len {
            return Err(ReserveExactError::ExactSizeMismatch {
                reserved: self.capacity() - self.len(),
                expected: len,
            });
        }
        Ok(())
    }
}

#[cfg(feature = "memmap2")]
impl IoBufMut for memmap2::MmapMut {
    fn as_uninit(&mut self) -> &mut [MaybeUninit<u8>] {
        // Safety: &mut [u8] is valid &mut [MaybeUninit<u8>]
        unsafe { std::mem::transmute(self.as_mut_slice()) }
    }
}

/// A helper trait for `set_len` like methods.
pub trait SetLen {
    /// Set the buffer length.
    ///
    /// # Safety
    ///
    /// * `len` must be less or equal than `as_uninit().len()`.
    /// * The bytes in the range `[buf_len(), len)` must be initialized.
    unsafe fn set_len(&mut self, len: usize);

    /// Advance the buffer length by `len`.
    ///
    /// # Safety
    ///
    /// * The bytes in the range `[buf_len(), buf_len() + len)` must be
    ///   initialized.
    unsafe fn advance(&mut self, len: usize)
    where
        Self: IoBuf,
    {
        let current_len = (*self).buf_len();
        let new_len = current_len.checked_add(len).expect("length overflow");
        unsafe { self.set_len(new_len) };
    }

    /// Set the buffer length to `len`. If `len` is less than the current
    /// length, this operation is a no-op.
    ///
    /// # Safety
    ///
    /// * `len` must be less or equal than `as_uninit().len()`.
    /// * The bytes in the range `[buf_len(), len)` must be initialized.
    unsafe fn advance_to(&mut self, len: usize)
    where
        Self: IoBuf,
    {
        let current_len = (*self).buf_len();
        if len > current_len {
            unsafe { self.set_len(len) };
        }
    }

    /// Set the vector buffer's total length to `len`. If `len` is less than the
    /// current total length, this operation is a no-op.
    ///
    /// # Safety
    ///
    /// * `len` must be less or equal than `total_len()`.
    /// * The bytes in the range `[total_len(), len)` must be initialized.
    unsafe fn advance_vec_to(&mut self, len: usize)
    where
        Self: IoVectoredBuf,
    {
        let current_len = (*self).total_len();
        if len > current_len {
            unsafe { self.set_len(len) };
        }
    }

    /// Clear the buffer, setting its length to 0 without touching its content
    /// or capacity.
    fn clear(&mut self)
    where
        Self: IoBuf,
    {
        // SAFETY: setting length to 0 is always valid
        unsafe { self.set_len(0) };
    }
}

impl<B: SetLen + ?Sized> SetLen for &'static mut B {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { (**self).set_len(len) }
    }
}

impl<B: SetLen + ?Sized, #[cfg(feature = "allocator_api")] A: Allocator + 'static> SetLen
    for t_alloc!(Box, B, A)
{
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { (**self).set_len(len) }
    }
}

impl<#[cfg(feature = "allocator_api")] A: Allocator + 'static> SetLen for t_alloc!(Vec, u8, A) {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { self.set_len(len) };
    }
}

impl SetLen for [u8] {
    unsafe fn set_len(&mut self, len: usize) {
        debug_assert!(len <= self.len());
    }
}

impl<const N: usize> SetLen for [u8; N] {
    unsafe fn set_len(&mut self, len: usize) {
        debug_assert!(len <= N);
    }
}

#[cfg(feature = "bytes")]
impl SetLen for bytes::BytesMut {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { self.set_len(len) };
    }
}

#[cfg(feature = "read_buf")]
impl SetLen for std::io::BorrowedBuf<'static> {
    unsafe fn set_len(&mut self, len: usize) {
        debug_assert!(self.capacity() >= len);

        // SAFETY: `len` range is initialized guaranteed by invariant of `set_len`
        #[allow(unused_unsafe)]
        unsafe {
            self.clear().unfilled().advance(len)
        };
    }
}

#[cfg(feature = "arrayvec")]
impl<const N: usize> SetLen for arrayvec::ArrayVec<u8, N> {
    unsafe fn set_len(&mut self, len: usize) {
        if (**self).buf_len() < len {
            unsafe { self.set_len(len) };
        }
    }
}

#[cfg(feature = "smallvec")]
impl<const N: usize> SetLen for smallvec::SmallVec<[u8; N]>
where
    [u8; N]: smallvec::Array<Item = u8>,
{
    unsafe fn set_len(&mut self, len: usize) {
        if (**self).buf_len() < len {
            unsafe { self.set_len(len) };
        }
    }
}

#[cfg(feature = "memmap2")]
impl SetLen for memmap2::MmapMut {
    unsafe fn set_len(&mut self, len: usize) {
        debug_assert!(len <= self.len())
    }
}

impl<T: IoBufMut> SetLen for [T] {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { default_set_len(self.iter_mut(), len) }
    }
}

impl<T: IoBufMut, const N: usize> SetLen for [T; N] {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { default_set_len(self.iter_mut(), len) }
    }
}

impl<T: IoBufMut, #[cfg(feature = "allocator_api")] A: Allocator + 'static> SetLen
    for t_alloc!(Vec, T, A)
{
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { default_set_len(self.iter_mut(), len) }
    }
}

#[cfg(feature = "arrayvec")]
impl<T: IoBufMut, const N: usize> SetLen for arrayvec::ArrayVec<T, N> {
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { default_set_len(self.iter_mut(), len) }
    }
}

#[cfg(feature = "smallvec")]
impl<T: IoBufMut, const N: usize> SetLen for smallvec::SmallVec<[T; N]>
where
    [T; N]: smallvec::Array<Item = T>,
{
    unsafe fn set_len(&mut self, len: usize) {
        unsafe { default_set_len(self.iter_mut(), len) }
    }
}

/// # Safety
/// * `len` should be less or equal than the sum of `buf_capacity()` of all
///   buffers.
/// * The bytes in the range `[buf_len(), new_len)` of each buffer must be
///   initialized
unsafe fn default_set_len<'a, B: IoBufMut>(
    iter: impl IntoIterator<Item = &'a mut B>,
    mut len: usize,
) {
    let mut iter = iter.into_iter();
    while len > 0 {
        let Some(curr) = iter.next() else { return };
        let sub = (*curr).buf_capacity().min(len);
        unsafe { curr.set_len(sub) };
        len -= sub;
    }
}

#[cfg(test)]
mod test {
    use crate::IoBufMut;

    #[test]
    fn test_vec_reserve() {
        let mut buf = Vec::new();
        IoBufMut::reserve(&mut buf, 10).unwrap();
        assert!(buf.capacity() >= 10);

        let mut buf = Vec::new();
        IoBufMut::reserve_exact(&mut buf, 10).unwrap();
        assert!(buf.capacity() == 10);

        let mut buf = Box::new(Vec::new());
        IoBufMut::reserve_exact(&mut buf, 10).unwrap();
        assert!(buf.capacity() == 10);
    }

    #[test]
    #[cfg(feature = "bytes")]
    fn test_bytes_reserve() {
        let mut buf = bytes::BytesMut::new();
        IoBufMut::reserve(&mut buf, 10).unwrap();
        assert!(buf.capacity() >= 10);
    }

    #[test]
    #[cfg(feature = "smallvec")]
    fn test_smallvec_reserve() {
        let mut buf = smallvec::SmallVec::<[u8; 8]>::new();
        IoBufMut::reserve(&mut buf, 10).unwrap();
        assert!(buf.capacity() >= 10);
    }

    #[test]
    #[cfg(feature = "memmap2")]
    fn tests_memmap2() {
        use std::{
            fs::{OpenOptions, remove_file},
            io::{Seek, SeekFrom, Write},
        };

        use memmap2::MmapOptions;

        use super::*;

        let path = std::env::temp_dir().join("compio_buf_mmap_mut_test");

        let mut file = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .truncate(true)
            .open(&path)
            .unwrap();
        let data = b"hello memmap2";
        file.write_all(data).unwrap();
        file.flush().unwrap();
        file.seek(SeekFrom::Start(0)).unwrap();
        let mmap = unsafe { MmapOptions::new().map(&file).unwrap() };

        let uninit_slice = mmap.as_init();
        assert_eq!(uninit_slice, data);

        remove_file(path).unwrap();
    }

    #[test]
    fn test_other_reserve() {
        let mut buf = [1, 1, 4, 5, 1, 4];
        let res = IoBufMut::reserve(&mut buf, 10);
        assert!(res.is_err_and(|x| x.is_not_supported()));
        assert!(buf.buf_capacity() == 6);
    }

    #[test]
    fn test_extend() {
        let mut buf = Vec::from(b"hello");
        IoBufMut::extend_from_slice(&mut buf, b" world").unwrap();
        assert_eq!(buf.as_slice(), b"hello world");

        let mut buf = [];
        let res = IoBufMut::extend_from_slice(&mut buf, b" ");
        assert!(res.is_err_and(|x| x.is_not_supported()));
    }
}