Switch BufMut::bytes_mut to&mut UninitSlice (#433)

The way BufMut uses MaybeUninit can lead to unsoundness. This replaces
MaybeUnit with a type owned by bytes so we can ensure the usage patterns
are sound.

Refs: #328

src/buf/uninit_slice.rs

@@ -0,0 +1,176 @@
+use core::fmt;
+use core::mem::MaybeUninit;
+use core::ops::{
+    Index, IndexMut, Range, RangeFrom, RangeFull, RangeInclusive, RangeTo, RangeToInclusive,
+};
+
+/// Uninitialized byte slice.
+///
+/// Returned by `BufMut::bytes_mut()`, the referenced byte slice may be
+/// uninitialized. The wrapper provides safe access without introducing
+/// undefined behavior.
+///
+/// The safety invariants of this wrapper are:
+///
+///  1. Reading from an `UninitSlice` is undefined behavior.
+///  2. Writing uninitialized bytes to an `UninitSlice` is undefined behavior.
+///
+/// The difference between `&mut UninitSlice` and `&mut [MaybeUninit<u8>]` is
+/// that it is possible in safe code to write uninitialized bytes to an
+/// `&mut [MaybeUninit<u8>]`, which this type prohibits.
+#[repr(transparent)]
+pub struct UninitSlice([MaybeUninit<u8>]);
+
+impl UninitSlice {
+    /// Create a `&mut UninitSlice` from a pointer and a length.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `ptr` references a valid memory region owned
+    /// by the caller representing a byte slice for the duration of `'a`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::buf::UninitSlice;
+    ///
+    /// let bytes = b"hello world".to_vec();
+    /// let ptr = bytes.as_ptr() as *mut _;
+    /// let len = bytes.len();
+    ///
+    /// let slice = unsafe { UninitSlice::from_raw_parts_mut(ptr, len) };
+    /// ```
+    pub unsafe fn from_raw_parts_mut<'a>(ptr: *mut u8, len: usize) -> &'a mut UninitSlice {
+        let maybe_init: &mut [MaybeUninit<u8>] =
+            core::slice::from_raw_parts_mut(ptr as *mut _, len);
+        &mut *(maybe_init as *mut [MaybeUninit<u8>] as *mut UninitSlice)
+    }
+
+    /// Write a single byte at the specified offset.
+    ///
+    /// # Panics
+    ///
+    /// The function panics if `index` is out of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::buf::UninitSlice;
+    ///
+    /// let mut data = [b'f', b'o', b'o'];
+    /// let slice = unsafe { UninitSlice::from_raw_parts_mut(data.as_mut_ptr(), 3) };
+    ///
+    /// slice.write_byte(0, b'b');
+    ///
+    /// assert_eq!(b"boo", &data[..]);
+    /// ```
+    pub fn write_byte(&mut self, index: usize, byte: u8) {
+        assert!(index < self.len());
+
+        unsafe { self[index..].as_mut_ptr().write(byte) }
+    }
+
+    /// Copies bytes  from `src` into `self`.
+    ///
+    /// The length of `src` must be the same as `self`.
+    ///
+    /// # Panics
+    ///
+    /// The function panics if `src` has a different length than `self`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::buf::UninitSlice;
+    ///
+    /// let mut data = [b'f', b'o', b'o'];
+    /// let slice = unsafe { UninitSlice::from_raw_parts_mut(data.as_mut_ptr(), 3) };
+    ///
+    /// slice.copy_from_slice(b"bar");
+    ///
+    /// assert_eq!(b"bar", &data[..]);
+    /// ```
+    pub fn copy_from_slice(&mut self, src: &[u8]) {
+        use core::ptr;
+
+        assert_eq!(self.len(), src.len());
+
+        unsafe {
+            ptr::copy_nonoverlapping(src.as_ptr(), self.as_mut_ptr(), self.len());
+        }
+    }
+
+    /// Return a raw pointer to the slice's buffer.
+    ///
+    /// # Safety
+    ///
+    /// The caller **must not** read from the referenced memory and **must not**
+    /// write **uninitialized** bytes to the slice either.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BufMut;
+    ///
+    /// let mut data = [0, 1, 2];
+    /// let mut slice = &mut data[..];
+    /// let ptr = BufMut::bytes_mut(&mut slice).as_mut_ptr();
+    /// ```
+    pub fn as_mut_ptr(&mut self) -> *mut u8 {
+        self.0.as_mut_ptr() as *mut _
+    }
+
+    /// Returns the number of bytes in the slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BufMut;
+    ///
+    /// let mut data = [0, 1, 2];
+    /// let mut slice = &mut data[..];
+    /// let len = BufMut::bytes_mut(&mut slice).len();
+    ///
+    /// assert_eq!(len, 3);
+    /// ```
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+}
+
+impl fmt::Debug for UninitSlice {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("UninitSlice[...]").finish()
+    }
+}
+
+macro_rules! impl_index {
+    ($($t:ty),*) => {
+        $(
+            impl Index<$t> for UninitSlice {
+                type Output = UninitSlice;
+
+                fn index(&self, index: $t) -> &UninitSlice {
+                    let maybe_uninit: &[MaybeUninit<u8>] = &self.0[index];
+                    unsafe { &*(maybe_uninit as *const [MaybeUninit<u8>] as *const UninitSlice) }
+                }
+            }
+
+            impl IndexMut<$t> for UninitSlice {
+                fn index_mut(&mut self, index: $t) -> &mut UninitSlice {
+                    let maybe_uninit: &mut [MaybeUninit<u8>] = &mut self.0[index];
+                    unsafe { &mut *(maybe_uninit as *mut [MaybeUninit<u8>] as *mut UninitSlice) }
+                }
+            }
+        )*
+    };
+}
+
+impl_index!(
+    Range<usize>,
+    RangeFrom<usize>,
+    RangeFull,
+    RangeInclusive<usize>,
+    RangeTo<usize>,
+    RangeToInclusive<usize>
+);