core/io/

borrowed_buf.rs

#![unstable(feature = "core_io_borrowed_buf", issue = "117693")]

use crate::fmt::{self, Debug, Formatter};
use crate::mem::{self, MaybeUninit};

/// A borrowed buffer of initially uninitialized bytes, which is incrementally filled.
///
/// This type makes it safer to work with `MaybeUninit` buffers, such as to read into a buffer
/// without having to initialize it first. It tracks the region of bytes that have been filled and
/// the region that remains uninitialized.
///
/// The contents of the buffer can be visualized as:
/// ```not_rust
/// [                         capacity                            ]
/// [ len: filled and initialized | capacity - len: uninitialized ]
/// ```
///
/// Note that `BorrowedBuf` does not distinguish between uninitialized data and data that was
/// previously initialized but no longer contains valid data.
///
/// A `BorrowedBuf` is created around some existing data (or capacity for data) via a unique
/// reference (`&mut`). The `BorrowedBuf` can be configured (e.g., using `clear` or `set_len`), but
/// cannot be directly written. To write into the buffer, use `unfilled` to create a
/// `BorrowedCursor`. The cursor has write-only access to the unfilled portion of the buffer.
///
/// The lifetime `'data` is a bound on the lifetime of the underlying data.
pub struct BorrowedBuf<'data> {
    /// The buffer's underlying data.
    buf: &'data mut [MaybeUninit<u8>],
    /// The length of `self.buf` which is known to be filled.
    filled: usize,
}

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

/// Creates a new `BorrowedBuf` from a fully initialized slice.
impl<'data> From<&'data mut [u8]> for BorrowedBuf<'data> {
    #[inline]
    fn from(slice: &'data mut [u8]) -> BorrowedBuf<'data> {
        BorrowedBuf {
            // SAFETY: Always in bounds. We treat the buffer as uninitialized, even though it's
            // already initialized.
            buf: unsafe { (slice as *mut [u8]).as_uninit_slice_mut().unwrap() },
            filled: 0,
        }
    }
}

/// Creates a new `BorrowedBuf` from an uninitialized buffer.
///
/// Use `set_filled` if part of the buffer is known to be already filled.
impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data> {
    #[inline]
    fn from(buf: &'data mut [MaybeUninit<u8>]) -> BorrowedBuf<'data> {
        BorrowedBuf { buf, filled: 0 }
    }
}

/// Creates a new `BorrowedBuf` from a cursor.
///
/// Use `BorrowedCursor::with_unfilled_buf` instead for a safer alternative.
impl<'data> From<BorrowedCursor<'data>> for BorrowedBuf<'data> {
    #[inline]
    fn from(buf: BorrowedCursor<'data>) -> BorrowedBuf<'data> {
        BorrowedBuf {
            // SAFETY: Always in bounds. We treat the buffer as uninitialized.
            buf: unsafe { buf.buf.buf.get_unchecked_mut(buf.buf.filled..) },
            filled: 0,
        }
    }
}

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

    /// Returns the length of the filled part of the buffer.
    #[inline]
    pub fn len(&self) -> usize {
        self.filled
    }

    /// Returns a shared reference to the filled portion of the buffer.
    #[inline]
    pub fn filled(&self) -> &[u8] {
        // SAFETY: We only slice the filled part of the buffer, which is always valid
        unsafe {
            let buf = self.buf.get_unchecked(..self.filled);
            buf.assume_init_ref()
        }
    }

    /// Returns a mutable reference to the filled portion of the buffer.
    #[inline]
    pub fn filled_mut(&mut self) -> &mut [u8] {
        // SAFETY: We only slice the filled part of the buffer, which is always valid
        unsafe {
            let buf = self.buf.get_unchecked_mut(..self.filled);
            buf.assume_init_mut()
        }
    }

    /// Returns a shared reference to the filled portion of the buffer with its original lifetime.
    #[inline]
    pub fn into_filled(self) -> &'data [u8] {
        // SAFETY: We only slice the filled part of the buffer, which is always valid
        unsafe {
            let buf = self.buf.get_unchecked(..self.filled);
            buf.assume_init_ref()
        }
    }

    /// Returns a mutable reference to the filled portion of the buffer with its original lifetime.
    #[inline]
    pub fn into_filled_mut(self) -> &'data mut [u8] {
        // SAFETY: We only slice the filled part of the buffer, which is always valid
        unsafe {
            let buf = self.buf.get_unchecked_mut(..self.filled);
            buf.assume_init_mut()
        }
    }

    /// Returns a cursor over the unfilled part of the buffer.
    #[inline]
    pub fn unfilled<'this>(&'this mut self) -> BorrowedCursor<'this> {
        BorrowedCursor {
            // SAFETY: we never assign into `BorrowedCursor::buf`, so treating its
            // lifetime covariantly is safe.
            buf: unsafe {
                mem::transmute::<&'this mut BorrowedBuf<'data>, &'this mut BorrowedBuf<'this>>(self)
            },
        }
    }

    /// Clears the buffer, resetting the filled region to empty.
    ///
    /// The contents of the buffer are not modified.
    #[inline]
    pub fn clear(&mut self) -> &mut Self {
        self.filled = 0;
        self
    }
}

/// A writeable view of the unfilled portion of a [`BorrowedBuf`].
///
/// Data can be written directly to the cursor by using [`append`](BorrowedCursor::append) or
/// indirectly by getting a slice of part or all of the cursor and writing into the slice. In the
/// indirect case, the caller must call [`advance`](BorrowedCursor::advance) after writing to inform
/// the cursor how many bytes have been written.
///
/// Once data is written to the cursor, it becomes part of the filled portion of the underlying
/// `BorrowedBuf` and can no longer be accessed or re-written by the cursor. I.e., the cursor tracks
/// the unfilled part of the underlying `BorrowedBuf`.
///
/// The lifetime `'a` is a bound on the lifetime of the underlying buffer (which means it is a bound
/// on the data in that buffer by transitivity).
#[derive(Debug)]
pub struct BorrowedCursor<'a> {
    /// The underlying buffer.
    // Safety invariant: we treat the type of buf as covariant in the lifetime of `BorrowedBuf` when
    // we create a `BorrowedCursor`. This is only safe if we never replace `buf` by assigning into
    // it, so don't do that!
    buf: &'a mut BorrowedBuf<'a>,
}

impl<'a> BorrowedCursor<'a> {
    /// Reborrows this cursor by cloning it with a smaller lifetime.
    ///
    /// Since a cursor maintains unique access to its underlying buffer, the borrowed cursor is
    /// not accessible while the new cursor exists.
    #[inline]
    pub fn reborrow<'this>(&'this mut self) -> BorrowedCursor<'this> {
        BorrowedCursor {
            // SAFETY: we never assign into `BorrowedCursor::buf`, so treating its
            // lifetime covariantly is safe.
            buf: unsafe {
                mem::transmute::<&'this mut BorrowedBuf<'a>, &'this mut BorrowedBuf<'this>>(
                    self.buf,
                )
            },
        }
    }

    /// Returns the available space in the cursor.
    #[inline]
    pub fn capacity(&self) -> usize {
        self.buf.capacity() - self.buf.filled
    }

    /// Returns the number of bytes written to the `BorrowedBuf` this cursor was created from.
    ///
    /// In particular, the count returned is shared by all reborrows of the cursor.
    #[inline]
    pub fn written(&self) -> usize {
        self.buf.filled
    }

    /// Returns a mutable reference to the whole cursor.
    ///
    /// # Safety
    ///
    /// The caller must not uninitialize any previously initialized bytes.
    #[inline]
    pub unsafe fn as_mut(&mut self) -> &mut [MaybeUninit<u8>] {
        // SAFETY: always in bounds
        unsafe { self.buf.buf.get_unchecked_mut(self.buf.filled..) }
    }

    /// Advances the cursor by asserting that `n` bytes have been filled.
    ///
    /// After advancing, the `n` bytes are no longer accessible via the cursor and can only be
    /// accessed via the underlying buffer. I.e., the buffer's filled portion grows by `n` elements
    /// and its unfilled portion (and the capacity of this cursor) shrinks by `n` elements.
    ///
    /// # Safety
    ///
    /// The caller must ensure that the first `n` bytes of the cursor have been initialized. `n`
    /// must not exceed the remaining capacity of this cursor.
    #[inline]
    pub unsafe fn advance(&mut self, n: usize) -> &mut Self {
        self.buf.filled += n;
        self
    }

    /// Appends data to the cursor, advancing position within its buffer.
    ///
    /// # Panics
    ///
    /// Panics if `self.capacity()` is less than `buf.len()`.
    #[inline]
    pub fn append(&mut self, buf: &[u8]) {
        assert!(self.capacity() >= buf.len());

        // SAFETY: we do not de-initialize any of the elements of the slice
        unsafe {
            self.as_mut()[..buf.len()].write_copy_of_slice(buf);
        }

        self.buf.filled += buf.len();
    }

    /// Runs the given closure with a `BorrowedBuf` containing the unfilled part
    /// of the cursor.
    ///
    /// This enables inspecting what was written to the cursor.
    ///
    /// # Panics
    ///
    /// Panics if the `BorrowedBuf` given to the closure is replaced by another
    /// one.
    pub fn with_unfilled_buf<T>(&mut self, f: impl FnOnce(&mut BorrowedBuf<'_>) -> T) -> T {
        let mut buf = BorrowedBuf::from(self.reborrow());
        let prev_ptr = buf.buf as *const _;
        let res = f(&mut buf);

        // Check that the caller didn't replace the `BorrowedBuf`.
        // This is necessary for the safety of the code below: if the check wasn't
        // there, one could mark some bytes as initialized even though there aren't.
        assert!(core::ptr::addr_eq(prev_ptr, buf.buf));

        // SAFETY: These bytes were filled in the `BorrowedBuf`, so they're filled in the cursor
        // too, because the buffer wasn't replaced.
        self.buf.filled += buf.filled;

        res
    }
}