Import bytes-0.5.4

* Add OWNERS and Android.bp

Bug: 143953733
Test: make
Change-Id: Ica69d7252abb1c2967f2c0e11a62d0bd83192fb3

1 files changed, 1552 insertions, 0 deletions

diff --git a/src/bytes_mut.rs b/src/bytes_mut.rs
new file mode 100644
index 0000000..dc4e4b1
--- /dev/null
+++ b/src/bytes_mut.rs
@@ -0,0 +1,1552 @@
+use core::{cmp, fmt, hash, isize, slice, usize};
+use core::mem::{self, ManuallyDrop};
+use core::ops::{Deref, DerefMut};
+use core::ptr::{self, NonNull};
+use core::iter::{FromIterator, Iterator};
+
+use alloc::{vec::Vec, string::String, boxed::Box, borrow::{Borrow, BorrowMut}};
+
+use crate::{Bytes, Buf, BufMut};
+use crate::bytes::Vtable;
+use crate::buf::IntoIter;
+use crate::loom::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+
+/// A unique reference to a contiguous slice of memory.
+///
+/// `BytesMut` represents a unique view into a potentially shared memory region.
+/// Given the uniqueness guarantee, owners of `BytesMut` handles are able to
+/// mutate the memory. It is similar to a `Vec<u8>` but with less copies and
+/// allocations.
+///
+/// # Growth
+///
+/// `BytesMut`'s `BufMut` implementation will implicitly grow its buffer as
+/// necessary. However, explicitly reserving the required space up-front before
+/// a series of inserts will be more efficient.
+///
+/// # Examples
+///
+/// ```
+/// use bytes::{BytesMut, BufMut};
+///
+/// let mut buf = BytesMut::with_capacity(64);
+///
+/// buf.put_u8(b'h');
+/// buf.put_u8(b'e');
+/// buf.put(&b"llo"[..]);
+///
+/// assert_eq!(&buf[..], b"hello");
+///
+/// // Freeze the buffer so that it can be shared
+/// let a = buf.freeze();
+///
+/// // This does not allocate, instead `b` points to the same memory.
+/// let b = a.clone();
+///
+/// assert_eq!(&a[..], b"hello");
+/// assert_eq!(&b[..], b"hello");
+/// ```
+pub struct BytesMut {
+    ptr: NonNull<u8>,
+    len: usize,
+    cap: usize,
+    data: *mut Shared,
+}
+
+// Thread-safe reference-counted container for the shared storage. This mostly
+// the same as `core::sync::Arc` but without the weak counter. The ref counting
+// fns are based on the ones found in `std`.
+//
+// The main reason to use `Shared` instead of `core::sync::Arc` is that it ends
+// up making the overall code simpler and easier to reason about. This is due to
+// some of the logic around setting `Inner::arc` and other ways the `arc` field
+// is used. Using `Arc` ended up requiring a number of funky transmutes and
+// other shenanigans to make it work.
+struct Shared {
+    vec: Vec<u8>,
+    original_capacity_repr: usize,
+    ref_count: AtomicUsize,
+}
+
+// Buffer storage strategy flags.
+const KIND_ARC: usize = 0b0;
+const KIND_VEC: usize = 0b1;
+const KIND_MASK: usize = 0b1;
+
+// The max original capacity value. Any `Bytes` allocated with a greater initial
+// capacity will default to this.
+const MAX_ORIGINAL_CAPACITY_WIDTH: usize = 17;
+// The original capacity algorithm will not take effect unless the originally
+// allocated capacity was at least 1kb in size.
+const MIN_ORIGINAL_CAPACITY_WIDTH: usize = 10;
+// The original capacity is stored in powers of 2 starting at 1kb to a max of
+// 64kb. Representing it as such requires only 3 bits of storage.
+const ORIGINAL_CAPACITY_MASK: usize = 0b11100;
+const ORIGINAL_CAPACITY_OFFSET: usize = 2;
+
+// When the storage is in the `Vec` representation, the pointer can be advanced
+// at most this value. This is due to the amount of storage available to track
+// the offset is usize - number of KIND bits and number of ORIGINAL_CAPACITY
+// bits.
+const VEC_POS_OFFSET: usize = 5;
+const MAX_VEC_POS: usize = usize::MAX >> VEC_POS_OFFSET;
+const NOT_VEC_POS_MASK: usize = 0b11111;
+
+#[cfg(target_pointer_width = "64")]
+const PTR_WIDTH: usize = 64;
+#[cfg(target_pointer_width = "32")]
+const PTR_WIDTH: usize = 32;
+
+/*
+ *
+ * ===== BytesMut =====
+ *
+ */
+
+impl BytesMut {
+    /// Creates a new `BytesMut` with the specified capacity.
+    ///
+    /// The returned `BytesMut` will be able to hold at least `capacity` bytes
+    /// without reallocating. If `capacity` is under `4 * size_of::<usize>() - 1`,
+    /// then `BytesMut` will not allocate.
+    ///
+    /// It is important to note that this function does not specify the length
+    /// of the returned `BytesMut`, but only the capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{BytesMut, BufMut};
+    ///
+    /// let mut bytes = BytesMut::with_capacity(64);
+    ///
+    /// // `bytes` contains no data, even though there is capacity
+    /// assert_eq!(bytes.len(), 0);
+    ///
+    /// bytes.put(&b"hello world"[..]);
+    ///
+    /// assert_eq!(&bytes[..], b"hello world");
+    /// ```
+    #[inline]
+    pub fn with_capacity(capacity: usize) -> BytesMut {
+        BytesMut::from_vec(Vec::with_capacity(capacity))
+    }
+
+    /// Creates a new `BytesMut` with default capacity.
+    ///
+    /// Resulting object has length 0 and unspecified capacity.
+    /// This function does not allocate.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{BytesMut, BufMut};
+    ///
+    /// let mut bytes = BytesMut::new();
+    ///
+    /// assert_eq!(0, bytes.len());
+    ///
+    /// bytes.reserve(2);
+    /// bytes.put_slice(b"xy");
+    ///
+    /// assert_eq!(&b"xy"[..], &bytes[..]);
+    /// ```
+    #[inline]
+    pub fn new() -> BytesMut {
+        BytesMut::with_capacity(0)
+    }
+
+    /// Returns the number of bytes contained in this `BytesMut`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let b = BytesMut::from(&b"hello"[..]);
+    /// assert_eq!(b.len(), 5);
+    /// ```
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    /// Returns true if the `BytesMut` has a length of 0.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let b = BytesMut::with_capacity(64);
+    /// assert!(b.is_empty());
+    /// ```
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len == 0
+    }
+
+    /// Returns the number of bytes the `BytesMut` can hold without reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let b = BytesMut::with_capacity(64);
+    /// assert_eq!(b.capacity(), 64);
+    /// ```
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.cap
+    }
+
+    /// Converts `self` into an immutable `Bytes`.
+    ///
+    /// The conversion is zero cost and is used to indicate that the slice
+    /// referenced by the handle will no longer be mutated. Once the conversion
+    /// is done, the handle can be cloned and shared across threads.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{BytesMut, BufMut};
+    /// use std::thread;
+    ///
+    /// let mut b = BytesMut::with_capacity(64);
+    /// b.put(&b"hello world"[..]);
+    /// let b1 = b.freeze();
+    /// let b2 = b1.clone();
+    ///
+    /// let th = thread::spawn(move || {
+    ///     assert_eq!(&b1[..], b"hello world");
+    /// });
+    ///
+    /// assert_eq!(&b2[..], b"hello world");
+    /// th.join().unwrap();
+    /// ```
+    #[inline]
+    pub fn freeze(mut self) -> Bytes {
+        if self.kind() == KIND_VEC {
+            // Just re-use `Bytes` internal Vec vtable
+            unsafe {
+                let (off, _) = self.get_vec_pos();
+                let vec = rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off);
+                mem::forget(self);
+                vec.into()
+            }
+        } else {
+            debug_assert_eq!(self.kind(), KIND_ARC);
+
+            let ptr = self.ptr.as_ptr();
+            let len = self.len;
+            let data = AtomicPtr::new(self.data as _);
+            mem::forget(self);
+            unsafe {
+                Bytes::with_vtable(ptr, len, data, &SHARED_VTABLE)
+            }
+        }
+    }
+
+    /// Splits the bytes into two at the given index.
+    ///
+    /// Afterwards `self` contains elements `[0, at)`, and the returned
+    /// `BytesMut` contains elements `[at, capacity)`.
+    ///
+    /// This is an `O(1)` operation that just increases the reference count
+    /// and sets a few indices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut a = BytesMut::from(&b"hello world"[..]);
+    /// let mut b = a.split_off(5);
+    ///
+    /// a[0] = b'j';
+    /// b[0] = b'!';
+    ///
+    /// assert_eq!(&a[..], b"jello");
+    /// assert_eq!(&b[..], b"!world");
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at > capacity`.
+    #[must_use = "consider BytesMut::truncate if you don't need the other half"]
+    pub fn split_off(&mut self, at: usize) -> BytesMut {
+        assert!(
+            at <= self.capacity(),
+            "split_off out of bounds: {:?} <= {:?}",
+            at,
+            self.capacity(),
+        );
+        unsafe {
+            let mut other = self.shallow_clone();
+            other.set_start(at);
+            self.set_end(at);
+            other
+        }
+    }
+
+    /// Removes the bytes from the current view, returning them in a new
+    /// `BytesMut` handle.
+    ///
+    /// Afterwards, `self` will be empty, but will retain any additional
+    /// capacity that it had before the operation. This is identical to
+    /// `self.split_to(self.len())`.
+    ///
+    /// This is an `O(1)` operation that just increases the reference count and
+    /// sets a few indices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::{BytesMut, BufMut};
+    ///
+    /// let mut buf = BytesMut::with_capacity(1024);
+    /// buf.put(&b"hello world"[..]);
+    ///
+    /// let other = buf.split();
+    ///
+    /// assert!(buf.is_empty());
+    /// assert_eq!(1013, buf.capacity());
+    ///
+    /// assert_eq!(other, b"hello world"[..]);
+    /// ```
+    #[must_use = "consider BytesMut::advance(len()) if you don't need the other half"]
+    pub fn split(&mut self) -> BytesMut {
+        let len = self.len();
+        self.split_to(len)
+    }
+
+    /// Splits the buffer into two at the given index.
+    ///
+    /// Afterwards `self` contains elements `[at, len)`, and the returned `BytesMut`
+    /// contains elements `[0, at)`.
+    ///
+    /// This is an `O(1)` operation that just increases the reference count and
+    /// sets a few indices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut a = BytesMut::from(&b"hello world"[..]);
+    /// let mut b = a.split_to(5);
+    ///
+    /// a[0] = b'!';
+    /// b[0] = b'j';
+    ///
+    /// assert_eq!(&a[..], b"!world");
+    /// assert_eq!(&b[..], b"jello");
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at > len`.
+    #[must_use = "consider BytesMut::advance if you don't need the other half"]
+    pub fn split_to(&mut self, at: usize) -> BytesMut {
+        assert!(
+            at <= self.len(),
+            "split_to out of bounds: {:?} <= {:?}",
+            at,
+            self.len(),
+        );
+
+        unsafe {
+            let mut other = self.shallow_clone();
+            other.set_end(at);
+            self.set_start(at);
+            other
+        }
+    }
+
+    /// Shortens the buffer, keeping the first `len` bytes and dropping the
+    /// rest.
+    ///
+    /// If `len` is greater than the buffer's current length, this has no
+    /// effect.
+    ///
+    /// The [`split_off`] method can emulate `truncate`, but this causes the
+    /// excess bytes to be returned instead of dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::from(&b"hello world"[..]);
+    /// buf.truncate(5);
+    /// assert_eq!(buf, b"hello"[..]);
+    /// ```
+    ///
+    /// [`split_off`]: #method.split_off
+    pub fn truncate(&mut self, len: usize) {
+        if len <= self.len() {
+            unsafe { self.set_len(len); }
+        }
+    }
+
+    /// Clears the buffer, removing all data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::from(&b"hello world"[..]);
+    /// buf.clear();
+    /// assert!(buf.is_empty());
+    /// ```
+    pub fn clear(&mut self) {
+        self.truncate(0);
+    }
+
+    /// Resizes the buffer so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is greater than `len`, the buffer is extended by the
+    /// difference with each additional byte set to `value`. If `new_len` is
+    /// less than `len`, the buffer is simply truncated.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::new();
+    ///
+    /// buf.resize(3, 0x1);
+    /// assert_eq!(&buf[..], &[0x1, 0x1, 0x1]);
+    ///
+    /// buf.resize(2, 0x2);
+    /// assert_eq!(&buf[..], &[0x1, 0x1]);
+    ///
+    /// buf.resize(4, 0x3);
+    /// assert_eq!(&buf[..], &[0x1, 0x1, 0x3, 0x3]);
+    /// ```
+    pub fn resize(&mut self, new_len: usize, value: u8) {
+        let len = self.len();
+        if new_len > len {
+            let additional = new_len - len;
+            self.reserve(additional);
+            unsafe {
+                let dst = self.bytes_mut().as_mut_ptr();
+                ptr::write_bytes(dst, value, additional);
+                self.set_len(new_len);
+            }
+        } else {
+            self.truncate(new_len);
+        }
+    }
+
+    /// Sets the length of the buffer.
+    ///
+    /// This will explicitly set the size of the buffer without actually
+    /// modifying the data, so it is up to the caller to ensure that the data
+    /// has been initialized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut b = BytesMut::from(&b"hello world"[..]);
+    ///
+    /// unsafe {
+    ///     b.set_len(5);
+    /// }
+    ///
+    /// assert_eq!(&b[..], b"hello");
+    ///
+    /// unsafe {
+    ///     b.set_len(11);
+    /// }
+    ///
+    /// assert_eq!(&b[..], b"hello world");
+    /// ```
+    pub unsafe fn set_len(&mut self, len: usize) {
+        debug_assert!(len <= self.cap, "set_len out of bounds");
+        self.len = len;
+    }
+
+    /// Reserves capacity for at least `additional` more bytes to be inserted
+    /// into the given `BytesMut`.
+    ///
+    /// More than `additional` bytes may be reserved in order to avoid frequent
+    /// reallocations. A call to `reserve` may result in an allocation.
+    ///
+    /// Before allocating new buffer space, the function will attempt to reclaim
+    /// space in the existing buffer. If the current handle references a small
+    /// view in the original buffer and all other handles have been dropped,
+    /// and the requested capacity is less than or equal to the existing
+    /// buffer's capacity, then the current view will be copied to the front of
+    /// the buffer and the handle will take ownership of the full buffer.
+    ///
+    /// # Examples
+    ///
+    /// In the following example, a new buffer is allocated.
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::from(&b"hello"[..]);
+    /// buf.reserve(64);
+    /// assert!(buf.capacity() >= 69);
+    /// ```
+    ///
+    /// In the following example, the existing buffer is reclaimed.
+    ///
+    /// ```
+    /// use bytes::{BytesMut, BufMut};
+    ///
+    /// let mut buf = BytesMut::with_capacity(128);
+    /// buf.put(&[0; 64][..]);
+    ///
+    /// let ptr = buf.as_ptr();
+    /// let other = buf.split();
+    ///
+    /// assert!(buf.is_empty());
+    /// assert_eq!(buf.capacity(), 64);
+    ///
+    /// drop(other);
+    /// buf.reserve(128);
+    ///
+    /// assert_eq!(buf.capacity(), 128);
+    /// assert_eq!(buf.as_ptr(), ptr);
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity overflows `usize`.
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        let len = self.len();
+        let rem = self.capacity() - len;
+
+        if additional <= rem {
+            // The handle can already store at least `additional` more bytes, so
+            // there is no further work needed to be done.
+            return;
+        }
+
+        self.reserve_inner(additional);
+    }
+
+    // In separate function to allow the short-circuits in `reserve` to
+    // be inline-able. Significant helps performance.
+    fn reserve_inner(&mut self, additional: usize) {
+        let len = self.len();
+        let kind = self.kind();
+
+        if kind == KIND_VEC {
+            // If there's enough free space before the start of the buffer, then
+            // just copy the data backwards and reuse the already-allocated
+            // space.
+            //
+            // Otherwise, since backed by a vector, use `Vec::reserve`
+            unsafe {
+                let (off, prev) = self.get_vec_pos();
+
+                // Only reuse space if we stand to gain at least capacity/2
+                // bytes of space back
+                if off >= additional && off >= (self.cap / 2) {
+                    // There's space - reuse it
+                    //
+                    // Just move the pointer back to the start after copying
+                    // data back.
+                    let base_ptr = self.ptr.as_ptr().offset(-(off as isize));
+                    ptr::copy(self.ptr.as_ptr(), base_ptr, self.len);
+                    self.ptr = vptr(base_ptr);
+                    self.set_vec_pos(0, prev);
+
+                    // Length stays constant, but since we moved backwards we
+                    // can gain capacity back.
+                    self.cap += off;
+                } else {
+                    // No space - allocate more
+                    let mut v = ManuallyDrop::new(rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off));
+                    v.reserve(additional);
+
+                    // Update the info
+                    self.ptr = vptr(v.as_mut_ptr().offset(off as isize));
+                    self.len = v.len() - off;
+                    self.cap = v.capacity() - off;
+                }
+
+                return;
+            }
+        }
+
+        debug_assert_eq!(kind, KIND_ARC);
+        let shared: *mut Shared = self.data as _;
+
+
+        // Reserving involves abandoning the currently shared buffer and
+        // allocating a new vector with the requested capacity.
+        //
+        // Compute the new capacity
+        let mut new_cap = len.checked_add(additional).expect("overflow");
+
+        let original_capacity;
+        let original_capacity_repr;
+
+        unsafe {
+            original_capacity_repr = (*shared).original_capacity_repr;
+            original_capacity = original_capacity_from_repr(original_capacity_repr);
+
+            // First, try to reclaim the buffer. This is possible if the current
+            // handle is the only outstanding handle pointing to the buffer.
+            if (*shared).is_unique() {
+                // This is the only handle to the buffer. It can be reclaimed.
+                // However, before doing the work of copying data, check to make
+                // sure that the vector has enough capacity.
+                let v = &mut (*shared).vec;
+
+                if v.capacity() >= new_cap {
+                    // The capacity is sufficient, reclaim the buffer
+                    let ptr = v.as_mut_ptr();
+
+                    ptr::copy(self.ptr.as_ptr(), ptr, len);
+
+                    self.ptr = vptr(ptr);
+                    self.cap = v.capacity();
+
+                    return;
+                }
+
+                // The vector capacity is not sufficient. The reserve request is
+                // asking for more than the initial buffer capacity. Allocate more
+                // than requested if `new_cap` is not much bigger than the current
+                // capacity.
+                //
+                // There are some situations, using `reserve_exact` that the
+                // buffer capacity could be below `original_capacity`, so do a
+                // check.
+                let double = v.capacity().checked_shl(1).unwrap_or(new_cap);
+
+                new_cap = cmp::max(
+                    cmp::max(double, new_cap),
+                    original_capacity);
+            } else {
+                new_cap = cmp::max(new_cap, original_capacity);
+            }
+        }
+
+        // Create a new vector to store the data
+        let mut v = ManuallyDrop::new(Vec::with_capacity(new_cap));
+
+        // Copy the bytes
+        v.extend_from_slice(self.as_ref());
+
+        // Release the shared handle. This must be done *after* the bytes are
+        // copied.
+        unsafe { release_shared(shared) };
+
+        // Update self
+        let data = (original_capacity_repr << ORIGINAL_CAPACITY_OFFSET) | KIND_VEC;
+        self.data = data as _;
+        self.ptr = vptr(v.as_mut_ptr());
+        self.len = v.len();
+        self.cap = v.capacity();
+    }
+
+    /// Appends given bytes to this `BytesMut`.
+    ///
+    /// If this `BytesMut` object does not have enough capacity, it is resized
+    /// first.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::with_capacity(0);
+    /// buf.extend_from_slice(b"aaabbb");
+    /// buf.extend_from_slice(b"cccddd");
+    ///
+    /// assert_eq!(b"aaabbbcccddd", &buf[..]);
+    /// ```
+    pub fn extend_from_slice(&mut self, extend: &[u8]) {
+        let cnt = extend.len();
+        self.reserve(cnt);
+
+        unsafe {
+            let dst = self.maybe_uninit_bytes();
+            // Reserved above
+            debug_assert!(dst.len() >= cnt);
+
+            ptr::copy_nonoverlapping(
+                extend.as_ptr(),
+                dst.as_mut_ptr() as *mut u8,
+                cnt);
+
+        }
+
+        unsafe { self.advance_mut(cnt); }
+    }
+
+    /// Absorbs a `BytesMut` that was previously split off.
+    ///
+    /// If the two `BytesMut` objects were previously contiguous, i.e., if
+    /// `other` was created by calling `split_off` on this `BytesMut`, then
+    /// this is an `O(1)` operation that just decreases a reference
+    /// count and sets a few indices. Otherwise this method degenerates to
+    /// `self.extend_from_slice(other.as_ref())`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use bytes::BytesMut;
+    ///
+    /// let mut buf = BytesMut::with_capacity(64);
+    /// buf.extend_from_slice(b"aaabbbcccddd");
+    ///
+    /// let split = buf.split_off(6);
+    /// assert_eq!(b"aaabbb", &buf[..]);
+    /// assert_eq!(b"cccddd", &split[..]);
+    ///
+    /// buf.unsplit(split);
+    /// assert_eq!(b"aaabbbcccddd", &buf[..]);
+    /// ```
+    pub fn unsplit(&mut self, other: BytesMut) {
+        if self.is_empty() {
+            *self = other;
+            return;
+        }
+
+        if let Err(other) = self.try_unsplit(other) {
+            self.extend_from_slice(other.as_ref());
+        }
+    }
+
+    // private
+
+    // For now, use a `Vec` to manage the memory for us, but we may want to
+    // change that in the future to some alternate allocator strategy.
+    //
+    // Thus, we don't expose an easy way to construct from a `Vec` since an
+    // internal change could make a simple pattern (`BytesMut::from(vec)`)
+    // suddenly a lot more expensive.
+    #[inline]
+    pub(crate) fn from_vec(mut vec: Vec<u8>) -> BytesMut {
+        let ptr = vptr(vec.as_mut_ptr());
+        let len = vec.len();
+        let cap = vec.capacity();
+        mem::forget(vec);
+
+        let original_capacity_repr = original_capacity_to_repr(cap);
+        let data = (original_capacity_repr << ORIGINAL_CAPACITY_OFFSET) | KIND_VEC;
+
+        BytesMut {
+            ptr,
+            len,
+            cap,
+            data: data as *mut _,
+        }
+    }
+
+    #[inline]
+    fn as_slice(&self) -> &[u8] {
+        unsafe {
+            slice::from_raw_parts(self.ptr.as_ptr(), self.len)
+        }
+    }
+
+    #[inline]
+    fn as_slice_mut(&mut self) -> &mut [u8] {
+        unsafe {
+            slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len)
+        }
+    }
+
+    unsafe fn set_start(&mut self, start: usize) {
+        // Setting the start to 0 is a no-op, so return early if this is the
+        // case.
+        if start == 0 {
+            return;
+        }
+
+        debug_assert!(start <= self.cap, "internal: set_start out of bounds");
+
+        let kind = self.kind();
+
+        if kind == KIND_VEC {
+            // Setting the start when in vec representation is a little more
+            // complicated. First, we have to track how far ahead the
+            // "start" of the byte buffer from the beginning of the vec. We
+            // also have to ensure that we don't exceed the maximum shift.
+            let (mut pos, prev) = self.get_vec_pos();
+            pos += start;
+
+            if pos <= MAX_VEC_POS {
+                self.set_vec_pos(pos, prev);
+            } else {
+                // The repr must be upgraded to ARC. This will never happen
+                // on 64 bit systems and will only happen on 32 bit systems
+                // when shifting past 134,217,727 bytes. As such, we don't
+                // worry too much about performance here.
+                self.promote_to_shared(/*ref_count = */1);
+            }
+        }
+
+        // Updating the start of the view is setting `ptr` to point to the
+        // new start and updating the `len` field to reflect the new length
+        // of the view.
+        self.ptr = vptr(self.ptr.as_ptr().offset(start as isize));
+
+        if self.len >= start {
+            self.len -= start;
+        } else {
+            self.len = 0;
+        }
+
+        self.cap -= start;
+    }
+
+    unsafe fn set_end(&mut self, end: usize) {
+        debug_assert_eq!(self.kind(), KIND_ARC);
+        assert!(end <= self.cap, "set_end out of bounds");
+
+        self.cap = end;
+        self.len = cmp::min(self.len, end);
+    }
+
+    fn try_unsplit(&mut self, other: BytesMut) -> Result<(), BytesMut> {
+        if other.is_empty() {
+            return Ok(());
+        }
+
+        let ptr = unsafe { self.ptr.as_ptr().offset(self.len as isize) };
+        if ptr == other.ptr.as_ptr() &&
+           self.kind() == KIND_ARC &&
+           other.kind() == KIND_ARC &&
+           self.data == other.data
+        {
+            // Contiguous blocks, just combine directly
+            self.len += other.len;
+            self.cap += other.cap;
+            Ok(())
+        } else {
+            Err(other)
+        }
+    }
+
+    #[inline]
+    fn kind(&self) -> usize {
+        self.data as usize & KIND_MASK
+    }
+
+    unsafe fn promote_to_shared(&mut self, ref_cnt: usize) {
+        debug_assert_eq!(self.kind(), KIND_VEC);
+        debug_assert!(ref_cnt == 1 || ref_cnt == 2);
+
+        let original_capacity_repr =
+            (self.data as usize & ORIGINAL_CAPACITY_MASK) >> ORIGINAL_CAPACITY_OFFSET;
+
+        // The vec offset cannot be concurrently mutated, so there
+        // should be no danger reading it.
+        let off = (self.data as usize) >> VEC_POS_OFFSET;
+
+        // First, allocate a new `Shared` instance containing the
+        // `Vec` fields. It's important to note that `ptr`, `len`,
+        // and `cap` cannot be mutated without having `&mut self`.
+        // This means that these fields will not be concurrently
+        // updated and since the buffer hasn't been promoted to an
+        // `Arc`, those three fields still are the components of the
+        // vector.
+        let shared = Box::new(Shared {
+            vec: rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off),
+            original_capacity_repr,
+            ref_count: AtomicUsize::new(ref_cnt),
+        });
+
+        let shared = Box::into_raw(shared);
+
+        // The pointer should be aligned, so this assert should
+        // always succeed.
+        debug_assert_eq!(shared as usize & KIND_MASK, KIND_ARC);
+
+        self.data = shared as _;
+    }
+
+    /// Makes an exact shallow clone of `self`.
+    ///
+    /// The kind of `self` doesn't matter, but this is unsafe
+    /// because the clone will have the same offsets. You must
+    /// be sure the returned value to the user doesn't allow
+    /// two views into the same range.
+    #[inline]
+    unsafe fn shallow_clone(&mut self) -> BytesMut {
+        if self.kind() == KIND_ARC {
+            increment_shared(self.data);
+            ptr::read(self)
+        } else {
+            self.promote_to_shared(/*ref_count = */2);
+            ptr::read(self)
+        }
+    }
+
+    #[inline]
+    unsafe fn get_vec_pos(&mut self) -> (usize, usize) {
+        debug_assert_eq!(self.kind(), KIND_VEC);
+
+        let prev = self.data as usize;
+        (prev >> VEC_POS_OFFSET, prev)
+    }
+
+    #[inline]
+    unsafe fn set_vec_pos(&mut self, pos: usize, prev: usize) {
+        debug_assert_eq!(self.kind(), KIND_VEC);
+        debug_assert!(pos <= MAX_VEC_POS);
+
+        self.data = ((pos << VEC_POS_OFFSET) | (prev & NOT_VEC_POS_MASK)) as *mut _;
+    }
+
+    #[inline]
+    fn maybe_uninit_bytes(&mut self) -> &mut [mem::MaybeUninit<u8>] {
+        unsafe {
+            let ptr = self.ptr.as_ptr().offset(self.len as isize);
+            let len = self.cap - self.len;
+
+            slice::from_raw_parts_mut(ptr as *mut mem::MaybeUninit<u8>, len)
+        }
+    }
+}
+
+impl Drop for BytesMut {
+    fn drop(&mut self) {
+        let kind = self.kind();
+
+        if kind == KIND_VEC {
+            unsafe {
+                let (off, _) = self.get_vec_pos();
+
+                // Vector storage, free the vector
+                let _ = rebuild_vec(self.ptr.as_ptr(), self.len, self.cap, off);
+            }
+        } else if kind == KIND_ARC {
+            unsafe { release_shared(self.data as _) };
+        }
+    }
+}
+
+impl Buf for BytesMut {
+    #[inline]
+    fn remaining(&self) -> usize {
+        self.len()
+    }
+
+    #[inline]
+    fn bytes(&self) -> &[u8] {
+        self.as_slice()
+    }
+
+    #[inline]
+    fn advance(&mut self, cnt: usize) {
+        assert!(
+            cnt <= self.remaining(),
+            "cannot advance past `remaining`: {:?} <= {:?}",
+            cnt,
+            self.remaining(),
+        );
+        unsafe { self.set_start(cnt); }
+    }
+
+    fn to_bytes(&mut self) -> crate::Bytes {
+        self.split().freeze()
+    }
+}
+
+impl BufMut for BytesMut {
+    #[inline]
+    fn remaining_mut(&self) -> usize {
+        usize::MAX - self.len()
+    }
+
+    #[inline]
+    unsafe fn advance_mut(&mut self, cnt: usize) {
+        let new_len = self.len() + cnt;
+        assert!(new_len <= self.cap, "new_len = {}; capacity = {}", new_len, self.cap);
+        self.len = new_len;
+    }
+
+    #[inline]
+    fn bytes_mut(&mut self) -> &mut [mem::MaybeUninit<u8>] {
+        if self.capacity() == self.len() {
+            self.reserve(64);
+        }
+        self.maybe_uninit_bytes()
+    }
+
+    // Specialize these methods so they can skip checking `remaining_mut`
+    // and `advance_mut`.
+
+    fn put<T: crate::Buf>(&mut self, mut src: T) where Self: Sized {
+        while src.has_remaining() {
+            let s = src.bytes();
+            let l = s.len();
+            self.extend_from_slice(s);
+            src.advance(l);
+        }
+    }
+
+    fn put_slice(&mut self, src: &[u8]) {
+        self.extend_from_slice(src);
+    }
+}
+
+impl AsRef<[u8]> for BytesMut {
+    #[inline]
+    fn as_ref(&self) -> &[u8] {
+        self.as_slice()
+    }
+}
+
+impl Deref for BytesMut {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        self.as_ref()
+    }
+}
+
+impl AsMut<[u8]> for BytesMut {
+    fn as_mut(&mut self) -> &mut [u8] {
+        self.as_slice_mut()
+    }
+}
+
+impl DerefMut for BytesMut {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut [u8] {
+        self.as_mut()
+    }
+}
+
+impl<'a> From<&'a [u8]> for BytesMut {
+    fn from(src: &'a [u8]) -> BytesMut {
+        BytesMut::from_vec(src.to_vec())
+    }
+}
+
+impl<'a> From<&'a str> for BytesMut {
+    fn from(src: &'a str) -> BytesMut {
+        BytesMut::from(src.as_bytes())
+    }
+}
+
+impl From<BytesMut> for Bytes {
+    fn from(src: BytesMut) -> Bytes {
+        src.freeze()
+    }
+}
+
+impl PartialEq for BytesMut {
+    fn eq(&self, other: &BytesMut) -> bool {
+        self.as_slice() == other.as_slice()
+    }
+}
+
+impl PartialOrd for BytesMut {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        self.as_slice().partial_cmp(other.as_slice())
+    }
+}
+
+impl Ord for BytesMut {
+    fn cmp(&self, other: &BytesMut) -> cmp::Ordering {
+        self.as_slice().cmp(other.as_slice())
+    }
+}
+
+impl Eq for BytesMut {
+}
+
+impl Default for BytesMut {
+    #[inline]
+    fn default() -> BytesMut {
+        BytesMut::new()
+    }
+}
+
+impl hash::Hash for BytesMut {
+    fn hash<H>(&self, state: &mut H) where H: hash::Hasher {
+        let s: &[u8] = self.as_ref();
+        s.hash(state);
+    }
+}
+
+impl Borrow<[u8]> for BytesMut {
+    fn borrow(&self) -> &[u8] {
+        self.as_ref()
+    }
+}
+
+impl BorrowMut<[u8]> for BytesMut {
+    fn borrow_mut(&mut self) -> &mut [u8] {
+        self.as_mut()
+    }
+}
+
+impl fmt::Write for BytesMut {
+    #[inline]
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        if self.remaining_mut() >= s.len() {
+            self.put_slice(s.as_bytes());
+            Ok(())
+        } else {
+            Err(fmt::Error)
+        }
+    }
+
+    #[inline]
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> fmt::Result {
+        fmt::write(self, args)
+    }
+}
+
+impl Clone for BytesMut {
+    fn clone(&self) -> BytesMut {
+        BytesMut::from(&self[..])
+    }
+}
+
+impl IntoIterator for BytesMut {
+    type Item = u8;
+    type IntoIter = IntoIter<BytesMut>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        IntoIter::new(self)
+    }
+}
+
+impl<'a> IntoIterator for &'a BytesMut {
+    type Item = &'a u8;
+    type IntoIter = core::slice::Iter<'a, u8>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.as_ref().into_iter()
+    }
+}
+
+impl Extend<u8> for BytesMut {
+    fn extend<T>(&mut self, iter: T) where T: IntoIterator<Item = u8> {
+        let iter = iter.into_iter();
+
+        let (lower, _) = iter.size_hint();
+        self.reserve(lower);
+
+        // TODO: optimize
+        // 1. If self.kind() == KIND_VEC, use Vec::extend
+        // 2. Make `reserve` inline-able
+        for b in iter {
+            self.reserve(1);
+            self.put_u8(b);
+        }
+    }
+}
+
+impl<'a> Extend<&'a u8> for BytesMut {
+    fn extend<T>(&mut self, iter: T) where T: IntoIterator<Item = &'a u8> {
+        self.extend(iter.into_iter().map(|b| *b))
+    }
+}
+
+impl FromIterator<u8> for BytesMut {
+    fn from_iter<T: IntoIterator<Item = u8>>(into_iter: T) -> Self {
+        BytesMut::from_vec(Vec::from_iter(into_iter))
+    }
+}
+
+impl<'a> FromIterator<&'a u8> for BytesMut {
+    fn from_iter<T: IntoIterator<Item = &'a u8>>(into_iter: T) -> Self {
+        BytesMut::from_iter(into_iter.into_iter().map(|b| *b))
+    }
+}
+
+/*
+ *
+ * ===== Inner =====
+ *
+ */
+
+unsafe fn increment_shared(ptr: *mut Shared) {
+    let old_size = (*ptr).ref_count.fetch_add(1, Ordering::Relaxed);
+
+    if old_size > isize::MAX as usize {
+        crate::abort();
+    }
+}
+
+unsafe fn release_shared(ptr: *mut Shared) {
+    // `Shared` storage... follow the drop steps from Arc.
+    if (*ptr).ref_count.fetch_sub(1, Ordering::Release) != 1 {
+        return;
+    }
+
+    // This fence is needed to prevent reordering of use of the data and
+    // deletion of the data.  Because it is marked `Release`, the decreasing
+    // of the reference count synchronizes with this `Acquire` fence. This
+    // means that use of the data happens before decreasing the reference
+    // count, which happens before this fence, which happens before the
+    // deletion of the data.
+    //
+    // As explained in the [Boost documentation][1],
+    //
+    // > It is important to enforce any possible access to the object in one
+    // > thread (through an existing reference) to *happen before* deleting
+    // > the object in a different thread. This is achieved by a "release"
+    // > operation after dropping a reference (any access to the object
+    // > through this reference must obviously happened before), and an
+    // > "acquire" operation before deleting the object.
+    //
+    // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
+    atomic::fence(Ordering::Acquire);
+
+    // Drop the data
+    Box::from_raw(ptr);
+}
+
+impl Shared {
+    fn is_unique(&self) -> bool {
+        // The goal is to check if the current handle is the only handle
+        // that currently has access to the buffer. This is done by
+        // checking if the `ref_count` is currently 1.
+        //
+        // The `Acquire` ordering synchronizes with the `Release` as
+        // part of the `fetch_sub` in `release_shared`. The `fetch_sub`
+        // operation guarantees that any mutations done in other threads
+        // are ordered before the `ref_count` is decremented. As such,
+        // this `Acquire` will guarantee that those mutations are
+        // visible to the current thread.
+        self.ref_count.load(Ordering::Acquire) == 1
+    }
+}
+
+fn original_capacity_to_repr(cap: usize) -> usize {
+    let width = PTR_WIDTH - ((cap >> MIN_ORIGINAL_CAPACITY_WIDTH).leading_zeros() as usize);
+    cmp::min(width, MAX_ORIGINAL_CAPACITY_WIDTH - MIN_ORIGINAL_CAPACITY_WIDTH)
+}
+
+fn original_capacity_from_repr(repr: usize) -> usize {
+    if repr == 0 {
+        return 0;
+    }
+
+    1 << (repr + (MIN_ORIGINAL_CAPACITY_WIDTH - 1))
+}
+
+/*
+#[test]
+fn test_original_capacity_to_repr() {
+    assert_eq!(original_capacity_to_repr(0), 0);
+
+    let max_width = 32;
+
+    for width in 1..(max_width + 1) {
+        let cap = 1 << width - 1;
+
+        let expected = if width < MIN_ORIGINAL_CAPACITY_WIDTH {
+            0
+        } else if width < MAX_ORIGINAL_CAPACITY_WIDTH {
+            width - MIN_ORIGINAL_CAPACITY_WIDTH
+        } else {
+            MAX_ORIGINAL_CAPACITY_WIDTH - MIN_ORIGINAL_CAPACITY_WIDTH
+        };
+
+        assert_eq!(original_capacity_to_repr(cap), expected);
+
+        if width > 1 {
+            assert_eq!(original_capacity_to_repr(cap + 1), expected);
+        }
+
+        //  MIN_ORIGINAL_CAPACITY_WIDTH must be bigger than 7 to pass tests below
+        if width == MIN_ORIGINAL_CAPACITY_WIDTH + 1 {
+            assert_eq!(original_capacity_to_repr(cap - 24), expected - 1);
+            assert_eq!(original_capacity_to_repr(cap + 76), expected);
+        } else if width == MIN_ORIGINAL_CAPACITY_WIDTH + 2 {
+            assert_eq!(original_capacity_to_repr(cap - 1), expected - 1);
+            assert_eq!(original_capacity_to_repr(cap - 48), expected - 1);
+        }
+    }
+}
+
+#[test]
+fn test_original_capacity_from_repr() {
+    assert_eq!(0, original_capacity_from_repr(0));
+
+    let min_cap = 1 << MIN_ORIGINAL_CAPACITY_WIDTH;
+
+    assert_eq!(min_cap, original_capacity_from_repr(1));
+    assert_eq!(min_cap * 2, original_capacity_from_repr(2));
+    assert_eq!(min_cap * 4, original_capacity_from_repr(3));
+    assert_eq!(min_cap * 8, original_capacity_from_repr(4));
+    assert_eq!(min_cap * 16, original_capacity_from_repr(5));
+    assert_eq!(min_cap * 32, original_capacity_from_repr(6));
+    assert_eq!(min_cap * 64, original_capacity_from_repr(7));
+}
+*/
+
+unsafe impl Send for BytesMut {}
+unsafe impl Sync for BytesMut {}
+
+/*
+ *
+ * ===== PartialEq / PartialOrd =====
+ *
+ */
+
+impl PartialEq<[u8]> for BytesMut {
+    fn eq(&self, other: &[u8]) -> bool {
+        &**self == other
+    }
+}
+
+impl PartialOrd<[u8]> for BytesMut {
+    fn partial_cmp(&self, other: &[u8]) -> Option<cmp::Ordering> {
+        (**self).partial_cmp(other)
+    }
+}
+
+impl PartialEq<BytesMut> for [u8] {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for [u8] {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        <[u8] as PartialOrd<[u8]>>::partial_cmp(self, other)
+    }
+}
+
+impl PartialEq<str> for BytesMut {
+    fn eq(&self, other: &str) -> bool {
+        &**self == other.as_bytes()
+    }
+}
+
+impl PartialOrd<str> for BytesMut {
+    fn partial_cmp(&self, other: &str) -> Option<cmp::Ordering> {
+        (**self).partial_cmp(other.as_bytes())
+    }
+}
+
+impl PartialEq<BytesMut> for str {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for str {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        <[u8] as PartialOrd<[u8]>>::partial_cmp(self.as_bytes(), other)
+    }
+}
+
+impl PartialEq<Vec<u8>> for BytesMut {
+    fn eq(&self, other: &Vec<u8>) -> bool {
+        *self == &other[..]
+    }
+}
+
+impl PartialOrd<Vec<u8>> for BytesMut {
+    fn partial_cmp(&self, other: &Vec<u8>) -> Option<cmp::Ordering> {
+        (**self).partial_cmp(&other[..])
+    }
+}
+
+impl PartialEq<BytesMut> for Vec<u8> {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for Vec<u8> {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        other.partial_cmp(self)
+    }
+}
+
+impl PartialEq<String> for BytesMut {
+    fn eq(&self, other: &String) -> bool {
+        *self == &other[..]
+    }
+}
+
+impl PartialOrd<String> for BytesMut {
+    fn partial_cmp(&self, other: &String) -> Option<cmp::Ordering> {
+        (**self).partial_cmp(other.as_bytes())
+    }
+}
+
+impl PartialEq<BytesMut> for String {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for String {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        <[u8] as PartialOrd<[u8]>>::partial_cmp(self.as_bytes(), other)
+    }
+}
+
+impl<'a, T: ?Sized> PartialEq<&'a T> for BytesMut
+    where BytesMut: PartialEq<T>
+{
+    fn eq(&self, other: &&'a T) -> bool {
+        *self == **other
+    }
+}
+
+impl<'a, T: ?Sized> PartialOrd<&'a T> for BytesMut
+    where BytesMut: PartialOrd<T>
+{
+    fn partial_cmp(&self, other: &&'a T) -> Option<cmp::Ordering> {
+        self.partial_cmp(*other)
+    }
+}
+
+impl PartialEq<BytesMut> for &[u8] {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for &[u8] {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        <[u8] as PartialOrd<[u8]>>::partial_cmp(self, other)
+    }
+}
+
+impl PartialEq<BytesMut> for &str {
+    fn eq(&self, other: &BytesMut) -> bool {
+        *other == *self
+    }
+}
+
+impl PartialOrd<BytesMut> for &str {
+    fn partial_cmp(&self, other: &BytesMut) -> Option<cmp::Ordering> {
+        other.partial_cmp(self)
+    }
+}
+
+impl PartialEq<BytesMut> for Bytes {
+    fn eq(&self, other: &BytesMut) -> bool {
+        &other[..] == &self[..]
+    }
+}
+
+impl PartialEq<Bytes> for BytesMut {
+    fn eq(&self, other: &Bytes) -> bool {
+        &other[..] == &self[..]
+    }
+}
+
+fn vptr(ptr: *mut u8) -> NonNull<u8> {
+    if cfg!(debug_assertions) {
+        NonNull::new(ptr).expect("Vec pointer should be non-null")
+    } else {
+        unsafe { NonNull::new_unchecked(ptr) }
+    }
+}
+
+unsafe fn rebuild_vec(ptr: *mut u8, mut len: usize, mut cap: usize, off: usize) -> Vec<u8> {
+    let ptr = ptr.offset(-(off as isize));
+    len += off;
+    cap += off;
+
+    Vec::from_raw_parts(ptr, len, cap)
+}
+
+// ===== impl SharedVtable =====
+
+static SHARED_VTABLE: Vtable = Vtable {
+    clone: shared_v_clone,
+    drop: shared_v_drop,
+};
+
+unsafe fn shared_v_clone(data: &AtomicPtr<()>, ptr: *const u8, len: usize) -> Bytes {
+    let shared = data.load(Ordering::Acquire) as *mut Shared;
+    increment_shared(shared);
+
+    let data = AtomicPtr::new(shared as _);
+    Bytes::with_vtable(ptr, len, data, &SHARED_VTABLE)
+}
+
+unsafe fn shared_v_drop(data: &mut AtomicPtr<()>, _ptr: *const u8, _len: usize) {
+    let shared = (*data.get_mut()) as *mut Shared;
+    release_shared(shared as *mut Shared);
+}
+
+// compile-fails
+
+/// ```compile_fail
+/// use bytes::BytesMut;
+/// #[deny(unused_must_use)]
+/// {
+///     let mut b1 = BytesMut::from("hello world");
+///     b1.split_to(6);
+/// }
+/// ```
+fn _split_to_must_use() {}
+
+/// ```compile_fail
+/// use bytes::BytesMut;
+/// #[deny(unused_must_use)]
+/// {
+///     let mut b1 = BytesMut::from("hello world");
+///     b1.split_off(6);
+/// }
+/// ```
+fn _split_off_must_use() {}
+
+/// ```compile_fail
+/// use bytes::BytesMut;
+/// #[deny(unused_must_use)]
+/// {
+///     let mut b1 = BytesMut::from("hello world");
+///     b1.split();
+/// }
+/// ```
+fn _split_must_use() {}
+
+// fuzz tests
+#[cfg(all(test, loom))]
+mod fuzz {
+    use std::sync::Arc;
+    use loom::thread;
+
+    use crate::Bytes;
+    use super::BytesMut;
+
+    #[test]
+    fn bytes_mut_cloning_frozen() {
+        loom::model(|| {
+            let a = BytesMut::from(&b"abcdefgh"[..]).split().freeze();
+            let addr = a.as_ptr() as usize;
+
+            // test the Bytes::clone is Sync by putting it in an Arc
+            let a1 = Arc::new(a);
+            let a2 = a1.clone();
+
+            let t1 = thread::spawn(move || {
+                let b: Bytes = (*a1).clone();
+                assert_eq!(b.as_ptr() as usize, addr);
+            });
+
+            let t2 = thread::spawn(move || {
+                let b: Bytes = (*a2).clone();
+                assert_eq!(b.as_ptr() as usize, addr);
+            });
+
+            t1.join().unwrap();
+            t2.join().unwrap();
+        });
+    }
+}