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    //! A contiguous growable array type with heap-allocated contents, written
    //! `Vec<T>`.
    //!
    //! Vectors have `O(1)` indexing, amortized `O(1)` push (to the end) and
    //! `O(1)` pop (from the end).
    //!
    //! # Examples
    //!
    //! You can explicitly create a [`Vec<T>`] with [`new`]:
    //!
    //! ```
    //! let v: Vec<i32> = Vec::new();
    //! ```
    //!
    //! ...or by using the [`vec!`] macro:
    //!
    //! ```
    //! let v: Vec<i32> = vec![];
    //!
    //! let v = vec![1, 2, 3, 4, 5];
    //!
    //! let v = vec![0; 10]; // ten zeroes
    //! ```
    //!
    //! You can [`push`] values onto the end of a vector (which will grow the vector
    //! as needed):
    //!
    //! ```
    //! let mut v = vec![1, 2];
    //!
    //! v.push(3);
    //! ```
    //!
    //! Popping values works in much the same way:
    //!
    //! ```
    //! let mut v = vec![1, 2];
    //!
    //! let two = v.pop();
    //! ```
    //!
    //! Vectors also support indexing (through the [`Index`] and [`IndexMut`] traits):
    //!
    //! ```
    //! let mut v = vec![1, 2, 3];
    //! let three = v[2];
    //! v[1] = v[1] + 5;
    //! ```
    //!
    //! [`Vec<T>`]: ../../std/vec/struct.Vec.html
    //! [`new`]: ../../std/vec/struct.Vec.html#method.new
    //! [`push`]: ../../std/vec/struct.Vec.html#method.push
    //! [`Index`]: ../../std/ops/trait.Index.html
    //! [`IndexMut`]: ../../std/ops/trait.IndexMut.html
    //! [`vec!`]: ../../std/macro.vec.html

    #![stable(feature = "rust1", since = "1.0.0")]

    use core::cmp::{self, Ordering};
    use core::fmt;
    use core::hash::{self, Hash};
    use core::intrinsics::{arith_offset, assume};
    use core::iter::{FromIterator, FusedIterator, TrustedLen};
    use core::marker::PhantomData;
    use core::mem;
    use core::ops::Bound::{Excluded, Included, Unbounded};
    use core::ops::{Index, IndexMut, RangeBounds};
    use core::ops;
    use core::ptr;
    use core::ptr::NonNull;
    use core::slice;

    use collections::CollectionAllocErr;
    use borrow::ToOwned;
    use borrow::Cow;
    use boxed::Box;
    use raw_vec::RawVec;

    /// A contiguous growable array type, written `Vec<T>` but pronounced 'vector'.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = Vec::new();
    /// vec.push(1);
    /// vec.push(2);
    ///
    /// assert_eq!(vec.len(), 2);
    /// assert_eq!(vec[0], 1);
    ///
    /// assert_eq!(vec.pop(), Some(2));
    /// assert_eq!(vec.len(), 1);
    ///
    /// vec[0] = 7;
    /// assert_eq!(vec[0], 7);
    ///
    /// vec.extend([1, 2, 3].iter().cloned());
    ///
    /// for x in &vec {
    ///     println!("{}", x);
    /// }
    /// assert_eq!(vec, [7, 1, 2, 3]);
    /// ```
    ///
    /// The [`vec!`] macro is provided to make initialization more convenient:
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3];
    /// vec.push(4);
    /// assert_eq!(vec, [1, 2, 3, 4]);
    /// ```
    ///
    /// It can also initialize each element of a `Vec<T>` with a given value.
    /// This may be more efficient than performing allocation and initialization
    /// in separate steps, especially when initializing a vector of zeros:
    ///
    /// ```
    /// let vec = vec![0; 5];
    /// assert_eq!(vec, [0, 0, 0, 0, 0]);
    ///
    /// // The following is equivalent, but potentially slower:
    /// let mut vec1 = Vec::with_capacity(5);
    /// vec1.resize(5, 0);
    /// ```
    ///
    /// Use a `Vec<T>` as an efficient stack:
    ///
    /// ```
    /// let mut stack = Vec::new();
    ///
    /// stack.push(1);
    /// stack.push(2);
    /// stack.push(3);
    ///
    /// while let Some(top) = stack.pop() {
    ///     // Prints 3, 2, 1
    ///     println!("{}", top);
    /// }
    /// ```
    ///
    /// # Indexing
    ///
    /// The `Vec` type allows to access values by index, because it implements the
    /// [`Index`] trait. An example will be more explicit:
    ///
    /// ```
    /// let v = vec![0, 2, 4, 6];
    /// println!("{}", v[1]); // it will display '2'
    /// ```
    ///
    /// However be careful: if you try to access an index which isn't in the `Vec`,
    /// your software will panic! You cannot do this:
    ///
    /// ```should_panic
    /// let v = vec![0, 2, 4, 6];
    /// println!("{}", v[6]); // it will panic!
    /// ```
    ///
    /// In conclusion: always check if the index you want to get really exists
    /// before doing it.
    ///
    /// # Slicing
    ///
    /// A `Vec` can be mutable. Slices, on the other hand, are read-only objects.
    /// To get a slice, use `&`. Example:
    ///
    /// ```
    /// fn read_slice(slice: &[usize]) {
    ///     // ...
    /// }
    ///
    /// let v = vec![0, 1];
    /// read_slice(&v);
    ///
    /// // ... and that's all!
    /// // you can also do it like this:
    /// let x : &[usize] = &v;
    /// ```
    ///
    /// In Rust, it's more common to pass slices as arguments rather than vectors
    /// when you just want to provide a read access. The same goes for [`String`] and
    /// [`&str`].
    ///
    /// # Capacity and reallocation
    ///
    /// The capacity of a vector is the amount of space allocated for any future
    /// elements that will be added onto the vector. This is not to be confused with
    /// the *length* of a vector, which specifies the number of actual elements
    /// within the vector. If a vector's length exceeds its capacity, its capacity
    /// will automatically be increased, but its elements will have to be
    /// reallocated.
    ///
    /// For example, a vector with capacity 10 and length 0 would be an empty vector
    /// with space for 10 more elements. Pushing 10 or fewer elements onto the
    /// vector will not change its capacity or cause reallocation to occur. However,
    /// if the vector's length is increased to 11, it will have to reallocate, which
    /// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
    /// whenever possible to specify how big the vector is expected to get.
    ///
    /// # Guarantees
    ///
    /// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
    /// about its design. This ensures that it's as low-overhead as possible in
    /// the general case, and can be correctly manipulated in primitive ways
    /// by unsafe code. Note that these guarantees refer to an unqualified `Vec<T>`.
    /// If additional type parameters are added (e.g., to support custom allocators),
    /// overriding their defaults may change the behavior.
    ///
    /// Most fundamentally, `Vec` is and always will be a (pointer, capacity, length)
    /// triplet. No more, no less. The order of these fields is completely
    /// unspecified, and you should use the appropriate methods to modify these.
    /// The pointer will never be null, so this type is null-pointer-optimized.
    ///
    /// However, the pointer may not actually point to allocated memory. In particular,
    /// if you construct a `Vec` with capacity 0 via [`Vec::new`], [`vec![]`][`vec!`],
    /// [`Vec::with_capacity(0)`][`Vec::with_capacity`], or by calling [`shrink_to_fit`]
    /// on an empty Vec, it will not allocate memory. Similarly, if you store zero-sized
    /// types inside a `Vec`, it will not allocate space for them. *Note that in this case
    /// the `Vec` may not report a [`capacity`] of 0*. `Vec` will allocate if and only
    /// if [`mem::size_of::<T>`]`() * capacity() > 0`. In general, `Vec`'s allocation
    /// details are very subtle &mdash; if you intend to allocate memory using a `Vec`
    /// and use it for something else (either to pass to unsafe code, or to build your
    /// own memory-backed collection), be sure to deallocate this memory by using
    /// `from_raw_parts` to recover the `Vec` and then dropping it.
    ///
    /// If a `Vec` *has* allocated memory, then the memory it points to is on the heap
    /// (as defined by the allocator Rust is configured to use by default), and its
    /// pointer points to [`len`] initialized, contiguous elements in order (what
    /// you would see if you coerced it to a slice), followed by [`capacity`]` -
    /// `[`len`] logically uninitialized, contiguous elements.
    ///
    /// `Vec` will never perform a "small optimization" where elements are actually
    /// stored on the stack for two reasons:
    ///
    /// * It would make it more difficult for unsafe code to correctly manipulate
    ///   a `Vec`. The contents of a `Vec` wouldn't have a stable address if it were
    ///   only moved, and it would be more difficult to determine if a `Vec` had
    ///   actually allocated memory.
    ///
    /// * It would penalize the general case, incurring an additional branch
    ///   on every access.
    ///
    /// `Vec` will never automatically shrink itself, even if completely empty. This
    /// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
    /// and then filling it back up to the same [`len`] should incur no calls to
    /// the allocator. If you wish to free up unused memory, use
    /// [`shrink_to_fit`][`shrink_to_fit`].
    ///
    /// [`push`] and [`insert`] will never (re)allocate if the reported capacity is
    /// sufficient. [`push`] and [`insert`] *will* (re)allocate if
    /// [`len`]` == `[`capacity`]. That is, the reported capacity is completely
    /// accurate, and can be relied on. It can even be used to manually free the memory
    /// allocated by a `Vec` if desired. Bulk insertion methods *may* reallocate, even
    /// when not necessary.
    ///
    /// `Vec` does not guarantee any particular growth strategy when reallocating
    /// when full, nor when [`reserve`] is called. The current strategy is basic
    /// and it may prove desirable to use a non-constant growth factor. Whatever
    /// strategy is used will of course guarantee `O(1)` amortized [`push`].
    ///
    /// `vec![x; n]`, `vec![a, b, c, d]`, and
    /// [`Vec::with_capacity(n)`][`Vec::with_capacity`], will all produce a `Vec`
    /// with exactly the requested capacity. If [`len`]` == `[`capacity`],
    /// (as is the case for the [`vec!`] macro), then a `Vec<T>` can be converted to
    /// and from a [`Box<[T]>`][owned slice] without reallocating or moving the elements.
    ///
    /// `Vec` will not specifically overwrite any data that is removed from it,
    /// but also won't specifically preserve it. Its uninitialized memory is
    /// scratch space that it may use however it wants. It will generally just do
    /// whatever is most efficient or otherwise easy to implement. Do not rely on
    /// removed data to be erased for security purposes. Even if you drop a `Vec`, its
    /// buffer may simply be reused by another `Vec`. Even if you zero a `Vec`'s memory
    /// first, that may not actually happen because the optimizer does not consider
    /// this a side-effect that must be preserved. There is one case which we will
    /// not break, however: using `unsafe` code to write to the excess capacity,
    /// and then increasing the length to match, is always valid.
    ///
    /// `Vec` does not currently guarantee the order in which elements are dropped.
    /// The order has changed in the past and may change again.
    ///
    /// [`vec!`]: ../../std/macro.vec.html
    /// [`Index`]: ../../std/ops/trait.Index.html
    /// [`String`]: ../../std/string/struct.String.html
    /// [`&str`]: ../../std/primitive.str.html
    /// [`Vec::with_capacity`]: ../../std/vec/struct.Vec.html#method.with_capacity
    /// [`Vec::new`]: ../../std/vec/struct.Vec.html#method.new
    /// [`shrink_to_fit`]: ../../std/vec/struct.Vec.html#method.shrink_to_fit
    /// [`capacity`]: ../../std/vec/struct.Vec.html#method.capacity
    /// [`mem::size_of::<T>`]: ../../std/mem/fn.size_of.html
    /// [`len`]: ../../std/vec/struct.Vec.html#method.len
    /// [`push`]: ../../std/vec/struct.Vec.html#method.push
    /// [`insert`]: ../../std/vec/struct.Vec.html#method.insert
    /// [`reserve`]: ../../std/vec/struct.Vec.html#method.reserve
    /// [owned slice]: ../../std/boxed/struct.Box.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Vec<T> {
        buf: RawVec<T>,
        len: usize,
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Inherent methods
    ////////////////////////////////////////////////////////////////////////////////

    impl<T> Vec<T> {
        /// Constructs a new, empty `Vec<T>`.
        ///
        /// The vector will not allocate until elements are pushed onto it.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![allow(unused_mut)]
        /// let mut vec: Vec<i32> = Vec::new();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_const_unstable(feature = "const_vec_new")]
        pub const fn new() -> Vec<T> {
            Vec {
                buf: RawVec::new(),
                len: 0,
            }
        }

        /// Constructs a new, empty `Vec<T>` with the specified capacity.
        ///
        /// The vector will be able to hold exactly `capacity` elements without
        /// reallocating. If `capacity` is 0, the vector will not allocate.
        ///
        /// It is important to note that although the returned vector has the
        /// *capacity* specified, the vector will have a zero *length*. For an
        /// explanation of the difference between length and capacity, see
        /// *[Capacity and reallocation]*.
        ///
        /// [Capacity and reallocation]: #capacity-and-reallocation
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = Vec::with_capacity(10);
        ///
        /// // The vector contains no items, even though it has capacity for more
        /// assert_eq!(vec.len(), 0);
        ///
        /// // These are all done without reallocating...
        /// for i in 0..10 {
        ///     vec.push(i);
        /// }
        ///
        /// // ...but this may make the vector reallocate
        /// vec.push(11);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(capacity: usize) -> Vec<T> {
            Vec {
                buf: RawVec::with_capacity(capacity),
                len: 0,
            }
        }

        /// Creates a `Vec<T>` directly from the raw components of another vector.
        ///
        /// # Safety
        ///
        /// This is highly unsafe, due to the number of invariants that aren't
        /// checked:
        ///
        /// * `ptr` needs to have been previously allocated via [`String`]/`Vec<T>`
        ///   (at least, it's highly likely to be incorrect if it wasn't).
        /// * `ptr`'s `T` needs to have the same size and alignment as it was allocated with.
        /// * `length` needs to be less than or equal to `capacity`.
        /// * `capacity` needs to be the capacity that the pointer was allocated with.
        ///
        /// Violating these may cause problems like corrupting the allocator's
        /// internal data structures. For example it is **not** safe
        /// to build a `Vec<u8>` from a pointer to a C `char` array and a `size_t`.
        ///
        /// The ownership of `ptr` is effectively transferred to the
        /// `Vec<T>` which may then deallocate, reallocate or change the
        /// contents of memory pointed to by the pointer at will. Ensure
        /// that nothing else uses the pointer after calling this
        /// function.
        ///
        /// [`String`]: ../../std/string/struct.String.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::ptr;
        /// use std::mem;
        ///
        /// fn main() {
        ///     let mut v = vec![1, 2, 3];
        ///
        ///     // Pull out the various important pieces of information about `v`
        ///     let p = v.as_mut_ptr();
        ///     let len = v.len();
        ///     let cap = v.capacity();
        ///
        ///     unsafe {
        ///         // Cast `v` into the void: no destructor run, so we are in
        ///         // complete control of the allocation to which `p` points.
        ///         mem::forget(v);
        ///
        ///         // Overwrite memory with 4, 5, 6
        ///         for i in 0..len as isize {
        ///             ptr::write(p.offset(i), 4 + i);
        ///         }
        ///
        ///         // Put everything back together into a Vec
        ///         let rebuilt = Vec::from_raw_parts(p, len, cap);
        ///         assert_eq!(rebuilt, [4, 5, 6]);
        ///     }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Vec<T> {
            Vec {
                buf: RawVec::from_raw_parts(ptr, capacity),
                len: length,
            }
        }

        /// Returns the number of elements the vector can hold without
        /// reallocating.
        ///
        /// # Examples
        ///
        /// ```
        /// let vec: Vec<i32> = Vec::with_capacity(10);
        /// assert_eq!(vec.capacity(), 10);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn capacity(&self) -> usize {
            self.buf.cap()
        }

        /// Reserves capacity for at least `additional` more elements to be inserted
        /// in the given `Vec<T>`. The collection may reserve more space to avoid
        /// frequent reallocations. After calling `reserve`, capacity will be
        /// greater than or equal to `self.len() + additional`. Does nothing if
        /// capacity is already sufficient.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1];
        /// vec.reserve(10);
        /// assert!(vec.capacity() >= 11);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve(&mut self, additional: usize) {
            self.buf.reserve(self.len, additional);
        }

        /// Reserves the minimum capacity for exactly `additional` more elements to
        /// be inserted in the given `Vec<T>`. After calling `reserve_exact`,
        /// capacity will be greater than or equal to `self.len() + additional`.
        /// Does nothing if the capacity is already sufficient.
        ///
        /// Note that the allocator may give the collection more space than it
        /// requests. Therefore capacity can not be relied upon to be precisely
        /// minimal. Prefer `reserve` if future insertions are expected.
        ///
        /// # Panics
        ///
        /// Panics if the new capacity overflows `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1];
        /// vec.reserve_exact(10);
        /// assert!(vec.capacity() >= 11);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn reserve_exact(&mut self, additional: usize) {
            self.buf.reserve_exact(self.len, additional);
        }

        /// Tries to reserve capacity for at least `additional` more elements to be inserted
        /// in the given `Vec<T>`. The collection may reserve more space to avoid
        /// frequent reallocations. After calling `reserve`, capacity will be
        /// greater than or equal to `self.len() + additional`. Does nothing if
        /// capacity is already sufficient.
        ///
        /// # Errors
        ///
        /// If the capacity overflows, or the allocator reports a failure, then an error
        /// is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(try_reserve)]
        /// use std::collections::CollectionAllocErr;
        ///
        /// fn process_data(data: &[u32]) -> Result<Vec<u32>, CollectionAllocErr> {
        ///     let mut output = Vec::new();
        ///
        ///     // Pre-reserve the memory, exiting if we can't
        ///     output.try_reserve(data.len())?;
        ///
        ///     // Now we know this can't OOM in the middle of our complex work
        ///     output.extend(data.iter().map(|&val| {
        ///         val * 2 + 5 // very complicated
        ///     }));
        ///
        ///     Ok(output)
        /// }
        /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
        /// ```
        #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
        pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
            self.buf.try_reserve(self.len, additional)
        }

        /// Tries to reserves the minimum capacity for exactly `additional` more elements to
        /// be inserted in the given `Vec<T>`. After calling `reserve_exact`,
        /// capacity will be greater than or equal to `self.len() + additional`.
        /// Does nothing if the capacity is already sufficient.
        ///
        /// Note that the allocator may give the collection more space than it
        /// requests. Therefore capacity can not be relied upon to be precisely
        /// minimal. Prefer `reserve` if future insertions are expected.
        ///
        /// # Errors
        ///
        /// If the capacity overflows, or the allocator reports a failure, then an error
        /// is returned.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(try_reserve)]
        /// use std::collections::CollectionAllocErr;
        ///
        /// fn process_data(data: &[u32]) -> Result<Vec<u32>, CollectionAllocErr> {
        ///     let mut output = Vec::new();
        ///
        ///     // Pre-reserve the memory, exiting if we can't
        ///     output.try_reserve(data.len())?;
        ///
        ///     // Now we know this can't OOM in the middle of our complex work
        ///     output.extend(data.iter().map(|&val| {
        ///         val * 2 + 5 // very complicated
        ///     }));
        ///
        ///     Ok(output)
        /// }
        /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
        /// ```
        #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
        pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
            self.buf.try_reserve_exact(self.len, additional)
        }

        /// Shrinks the capacity of the vector as much as possible.
        ///
        /// It will drop down as close as possible to the length but the allocator
        /// may still inform the vector that there is space for a few more elements.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = Vec::with_capacity(10);
        /// vec.extend([1, 2, 3].iter().cloned());
        /// assert_eq!(vec.capacity(), 10);
        /// vec.shrink_to_fit();
        /// assert!(vec.capacity() >= 3);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn shrink_to_fit(&mut self) {
            if self.capacity() != self.len {
                self.buf.shrink_to_fit(self.len);
            }
        }

        /// Shrinks the capacity of the vector with a lower bound.
        ///
        /// The capacity will remain at least as large as both the length
        /// and the supplied value.
        ///
        /// Panics if the current capacity is smaller than the supplied
        /// minimum capacity.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(shrink_to)]
        /// let mut vec = Vec::with_capacity(10);
        /// vec.extend([1, 2, 3].iter().cloned());
        /// assert_eq!(vec.capacity(), 10);
        /// vec.shrink_to(4);
        /// assert!(vec.capacity() >= 4);
        /// vec.shrink_to(0);
        /// assert!(vec.capacity() >= 3);
        /// ```
        #[unstable(feature = "shrink_to", reason = "new API", issue="56431")]
        pub fn shrink_to(&mut self, min_capacity: usize) {
            self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
        }

        /// Converts the vector into [`Box<[T]>`][owned slice].
        ///
        /// Note that this will drop any excess capacity.
        ///
        /// [owned slice]: ../../std/boxed/struct.Box.html
        ///
        /// # Examples
        ///
        /// ```
        /// let v = vec![1, 2, 3];
        ///
        /// let slice = v.into_boxed_slice();
        /// ```
        ///
        /// Any excess capacity is removed:
        ///
        /// ```
        /// let mut vec = Vec::with_capacity(10);
        /// vec.extend([1, 2, 3].iter().cloned());
        ///
        /// assert_eq!(vec.capacity(), 10);
        /// let slice = vec.into_boxed_slice();
        /// assert_eq!(slice.into_vec().capacity(), 3);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_boxed_slice(mut self) -> Box<[T]> {
            unsafe {
                self.shrink_to_fit();
                let buf = ptr::read(&self.buf);
                mem::forget(self);
                buf.into_box()
            }
        }

        /// Shortens the vector, keeping the first `len` elements and dropping
        /// the rest.
        ///
        /// If `len` is greater than the vector's current length, this has no
        /// effect.
        ///
        /// The [`drain`] method can emulate `truncate`, but causes the excess
        /// elements to be returned instead of dropped.
        ///
        /// Note that this method has no effect on the allocated capacity
        /// of the vector.
        ///
        /// # Examples
        ///
        /// Truncating a five element vector to two elements:
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3, 4, 5];
        /// vec.truncate(2);
        /// assert_eq!(vec, [1, 2]);
        /// ```
        ///
        /// No truncation occurs when `len` is greater than the vector's current
        /// length:
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// vec.truncate(8);
        /// assert_eq!(vec, [1, 2, 3]);
        /// ```
        ///
        /// Truncating when `len == 0` is equivalent to calling the [`clear`]
        /// method.
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// vec.truncate(0);
        /// assert_eq!(vec, []);
        /// ```
        ///
        /// [`clear`]: #method.clear
        /// [`drain`]: #method.drain
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn truncate(&mut self, len: usize) {
            let current_len = self.len;
            unsafe {
                let mut ptr = self.as_mut_ptr().add(self.len);
                // Set the final length at the end, keeping in mind that
                // dropping an element might panic. Works around a missed
                // optimization, as seen in the following issue:
                // https://github.com/rust-lang/rust/issues/51802
                let mut local_len = SetLenOnDrop::new(&mut self.len);

                // drop any extra elements
                for _ in len..current_len {
                    local_len.decrement_len(1);
                    ptr = ptr.offset(-1);
                    ptr::drop_in_place(ptr);
                }
            }
        }

        /// Extracts a slice containing the entire vector.
        ///
        /// Equivalent to `&s[..]`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::{self, Write};
        /// let buffer = vec![1, 2, 3, 5, 8];
        /// io::sink().write(buffer.as_slice()).unwrap();
        /// ```
        #[inline]
        #[stable(feature = "vec_as_slice", since = "1.7.0")]
        pub fn as_slice(&self) -> &[T] {
            self
        }

        /// Extracts a mutable slice of the entire vector.
        ///
        /// Equivalent to `&mut s[..]`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::{self, Read};
        /// let mut buffer = vec![0; 3];
        /// io::repeat(0b101).read_exact(buffer.as_mut_slice()).unwrap();
        /// ```
        #[inline]
        #[stable(feature = "vec_as_slice", since = "1.7.0")]
        pub fn as_mut_slice(&mut self) -> &mut [T] {
            self
        }

        /// Forces the length of the vector to `new_len`.
        ///
        /// This is a low-level operation that maintains none of the normal
        /// invariants of the type.  Normally changing the length of a vector
        /// is done using one of the safe operations instead, such as
        /// [`truncate`], [`resize`], [`extend`], or [`clear`].
        ///
        /// [`truncate`]: #method.truncate
        /// [`resize`]: #method.resize
        /// [`extend`]: #method.extend-1
        /// [`clear`]: #method.clear
        ///
        /// # Safety
        ///
        /// - `new_len` must be less than or equal to [`capacity()`].
        /// - The elements at `old_len..new_len` must be initialized.
        ///
        /// [`capacity()`]: #method.capacity
        ///
        /// # Examples
        ///
        /// This method can be useful for situations in which the vector
        /// is serving as a buffer for other code, particularly over FFI:
        ///
        /// ```no_run
        /// # #![allow(dead_code)]
        /// # // This is just a minimal skeleton for the doc example;
        /// # // don't use this as a starting point for a real library.
        /// # pub struct StreamWrapper { strm: *mut std::ffi::c_void }
        /// # const Z_OK: i32 = 0;
        /// # extern "C" {
        /// #     fn deflateGetDictionary(
        /// #         strm: *mut std::ffi::c_void,
        /// #         dictionary: *mut u8,
        /// #         dictLength: *mut usize,
        /// #     ) -> i32;
        /// # }
        /// # impl StreamWrapper {
        /// pub fn get_dictionary(&self) -> Option<Vec<u8>> {
        ///     // Per the FFI method's docs, "32768 bytes is always enough".
        ///     let mut dict = Vec::with_capacity(32_768);
        ///     let mut dict_length = 0;
        ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
        ///     // 1. `dict_length` elements were initialized.
        ///     // 2. `dict_length` <= the capacity (32_768)
        ///     // which makes `set_len` safe to call.
        ///     unsafe {
        ///         // Make the FFI call...
        ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
        ///         if r == Z_OK {
        ///             // ...and update the length to what was initialized.
        ///             dict.set_len(dict_length);
        ///             Some(dict)
        ///         } else {
        ///             None
        ///         }
        ///     }
        /// }
        /// # }
        /// ```
        ///
        /// While the following example is sound, there is a memory leak since
        /// the inner vectors were not freed prior to the `set_len` call:
        ///
        /// ```
        /// let mut vec = vec![vec![1, 0, 0],
        ///                    vec![0, 1, 0],
        ///                    vec![0, 0, 1]];
        /// // SAFETY:
        /// // 1. `old_len..0` is empty so no elements need to be initialized.
        /// // 2. `0 <= capacity` always holds whatever `capacity` is.
        /// unsafe {
        ///     vec.set_len(0);
        /// }
        /// ```
        ///
        /// Normally, here, one would use [`clear`] instead to correctly drop
        /// the contents and thus not leak memory.
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn set_len(&mut self, new_len: usize) {
            debug_assert!(new_len <= self.capacity());

            self.len = new_len;
        }

        /// Removes an element from the vector and returns it.
        ///
        /// The removed element is replaced by the last element of the vector.
        ///
        /// This does not preserve ordering, but is O(1).
        ///
        /// # Panics
        ///
        /// Panics if `index` is out of bounds.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = vec!["foo", "bar", "baz", "qux"];
        ///
        /// assert_eq!(v.swap_remove(1), "bar");
        /// assert_eq!(v, ["foo", "qux", "baz"]);
        ///
        /// assert_eq!(v.swap_remove(0), "foo");
        /// assert_eq!(v, ["baz", "qux"]);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn swap_remove(&mut self, index: usize) -> T {
            unsafe {
                // We replace self[index] with the last element. Note that if the
                // bounds check on hole succeeds there must be a last element (which
                // can be self[index] itself).
                let hole: *mut T = &mut self[index];
                let last = ptr::read(self.get_unchecked(self.len - 1));
                self.len -= 1;
                ptr::replace(hole, last)
            }
        }

        /// Inserts an element at position `index` within the vector, shifting all
        /// elements after it to the right.
        ///
        /// # Panics
        ///
        /// Panics if `index > len`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// vec.insert(1, 4);
        /// assert_eq!(vec, [1, 4, 2, 3]);
        /// vec.insert(4, 5);
        /// assert_eq!(vec, [1, 4, 2, 3, 5]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn insert(&mut self, index: usize, element: T) {
            let len = self.len();
            assert!(index <= len);

            // space for the new element
            if len == self.buf.cap() {
                self.reserve(1);
            }

            unsafe {
                // infallible
                // The spot to put the new value
                {
                    let p = self.as_mut_ptr().add(index);
                    // Shift everything over to make space. (Duplicating the
                    // `index`th element into two consecutive places.)
                    ptr::copy(p, p.offset(1), len - index);
                    // Write it in, overwriting the first copy of the `index`th
                    // element.
                    ptr::write(p, element);
                }
                self.set_len(len + 1);
            }
        }

        /// Removes and returns the element at position `index` within the vector,
        /// shifting all elements after it to the left.
        ///
        /// # Panics
        ///
        /// Panics if `index` is out of bounds.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = vec![1, 2, 3];
        /// assert_eq!(v.remove(1), 2);
        /// assert_eq!(v, [1, 3]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn remove(&mut self, index: usize) -> T {
            let len = self.len();
            assert!(index < len);
            unsafe {
                // infallible
                let ret;
                {
                    // the place we are taking from.
                    let ptr = self.as_mut_ptr().add(index);
                    // copy it out, unsafely having a copy of the value on
                    // the stack and in the vector at the same time.
                    ret = ptr::read(ptr);

                    // Shift everything down to fill in that spot.
                    ptr::copy(ptr.offset(1), ptr, len - index - 1);
                }
                self.set_len(len - 1);
                ret
            }
        }

        /// Retains only the elements specified by the predicate.
        ///
        /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
        /// This method operates in place and preserves the order of the retained
        /// elements.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3, 4];
        /// vec.retain(|&x| x%2 == 0);
        /// assert_eq!(vec, [2, 4]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn retain<F>(&mut self, mut f: F)
            where F: FnMut(&T) -> bool
        {
            self.drain_filter(|x| !f(x));
        }

        /// Removes all but the first of consecutive elements in the vector that resolve to the same
        /// key.
        ///
        /// If the vector is sorted, this removes all duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![10, 20, 21, 30, 20];
        ///
        /// vec.dedup_by_key(|i| *i / 10);
        ///
        /// assert_eq!(vec, [10, 20, 30, 20]);
        /// ```
        #[stable(feature = "dedup_by", since = "1.16.0")]
        #[inline]
        pub fn dedup_by_key<F, K>(&mut self, mut key: F) where F: FnMut(&mut T) -> K, K: PartialEq {
            self.dedup_by(|a, b| key(a) == key(b))
        }

        /// Removes all but the first of consecutive elements in the vector satisfying a given equality
        /// relation.
        ///
        /// The `same_bucket` function is passed references to two elements from the vector and
        /// must determine if the elements compare equal. The elements are passed in opposite order
        /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
        ///
        /// If the vector is sorted, this removes all duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec!["foo", "bar", "Bar", "baz", "bar"];
        ///
        /// vec.dedup_by(|a, b| a.eq_ignore_ascii_case(b));
        ///
        /// assert_eq!(vec, ["foo", "bar", "baz", "bar"]);
        /// ```
        #[stable(feature = "dedup_by", since = "1.16.0")]
        pub fn dedup_by<F>(&mut self, same_bucket: F) where F: FnMut(&mut T, &mut T) -> bool {
            let len = {
                let (dedup, _) = self.as_mut_slice().partition_dedup_by(same_bucket);
                dedup.len()
            };
            self.truncate(len);
        }

        /// Appends an element to the back of a collection.
        ///
        /// # Panics
        ///
        /// Panics if the number of elements in the vector overflows a `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2];
        /// vec.push(3);
        /// assert_eq!(vec, [1, 2, 3]);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn push(&mut self, value: T) {
            // This will panic or abort if we would allocate > isize::MAX bytes
            // or if the length increment would overflow for zero-sized types.
            if self.len == self.buf.cap() {
                self.reserve(1);
            }
            unsafe {
                let end = self.as_mut_ptr().add(self.len);
                ptr::write(end, value);
                self.len += 1;
            }
        }

        /// Removes the last element from a vector and returns it, or [`None`] if it
        /// is empty.
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// assert_eq!(vec.pop(), Some(3));
        /// assert_eq!(vec, [1, 2]);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn pop(&mut self) -> Option<T> {
            if self.len == 0 {
                None
            } else {
                unsafe {
                    self.len -= 1;
                    Some(ptr::read(self.get_unchecked(self.len())))
                }
            }
        }

        /// Moves all the elements of `other` into `Self`, leaving `other` empty.
        ///
        /// # Panics
        ///
        /// Panics if the number of elements in the vector overflows a `usize`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// let mut vec2 = vec![4, 5, 6];
        /// vec.append(&mut vec2);
        /// assert_eq!(vec, [1, 2, 3, 4, 5, 6]);
        /// assert_eq!(vec2, []);
        /// ```
        #[inline]
        #[stable(feature = "append", since = "1.4.0")]
        pub fn append(&mut self, other: &mut Self) {
            unsafe {
                self.append_elements(other.as_slice() as _);
                other.set_len(0);
            }
        }

        /// Appends elements to `Self` from other buffer.
        #[inline]
        unsafe fn append_elements(&mut self, other: *const [T]) {
            let count = (*other).len();
            self.reserve(count);
            let len = self.len();
            ptr::copy_nonoverlapping(other as *const T, self.get_unchecked_mut(len), count);
            self.len += count;
        }

        /// Creates a draining iterator that removes the specified range in the vector
        /// and yields the removed items.
        ///
        /// Note 1: The element range is removed even if the iterator is only
        /// partially consumed or not consumed at all.
        ///
        /// Note 2: It is unspecified how many elements are removed from the vector
        /// if the `Drain` value is leaked.
        ///
        /// # Panics
        ///
        /// Panics if the starting point is greater than the end point or if
        /// the end point is greater than the length of the vector.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = vec![1, 2, 3];
        /// let u: Vec<_> = v.drain(1..).collect();
        /// assert_eq!(v, &[1]);
        /// assert_eq!(u, &[2, 3]);
        ///
        /// // A full range clears the vector
        /// v.drain(..);
        /// assert_eq!(v, &[]);
        /// ```
        #[stable(feature = "drain", since = "1.6.0")]
        pub fn drain<R>(&mut self, range: R) -> Drain<T>
            where R: RangeBounds<usize>
        {
            // Memory safety
            //
            // When the Drain is first created, it shortens the length of
            // the source vector to make sure no uninitialized or moved-from elements
            // are accessible at all if the Drain's destructor never gets to run.
            //
            // Drain will ptr::read out the values to remove.
            // When finished, remaining tail of the vec is copied back to cover
            // the hole, and the vector length is restored to the new length.
            //
            let len = self.len();
            let start = match range.start_bound() {
                Included(&n) => n,
                Excluded(&n) => n + 1,
                Unbounded    => 0,
            };
            let end = match range.end_bound() {
                Included(&n) => n + 1,
                Excluded(&n) => n,
                Unbounded    => len,
            };
            assert!(start <= end);
            assert!(end <= len);

            unsafe {
                // set self.vec length's to start, to be safe in case Drain is leaked
                self.set_len(start);
                // Use the borrow in the IterMut to indicate borrowing behavior of the
                // whole Drain iterator (like &mut T).
                let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start),
                                                            end - start);
                Drain {
                    tail_start: end,
                    tail_len: len - end,
                    iter: range_slice.iter(),
                    vec: NonNull::from(self),
                }
            }
        }

        /// Clears the vector, removing all values.
        ///
        /// Note that this method has no effect on the allocated capacity
        /// of the vector.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = vec![1, 2, 3];
        ///
        /// v.clear();
        ///
        /// assert!(v.is_empty());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn clear(&mut self) {
            self.truncate(0)
        }

        /// Returns the number of elements in the vector, also referred to
        /// as its 'length'.
        ///
        /// # Examples
        ///
        /// ```
        /// let a = vec![1, 2, 3];
        /// assert_eq!(a.len(), 3);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn len(&self) -> usize {
            self.len
        }

        /// Returns `true` if the vector contains no elements.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = Vec::new();
        /// assert!(v.is_empty());
        ///
        /// v.push(1);
        /// assert!(!v.is_empty());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// Splits the collection into two at the given index.
        ///
        /// Returns a newly allocated `Self`. `self` contains elements `[0, at)`,
        /// and the returned `Self` contains elements `[at, len)`.
        ///
        /// Note that the capacity of `self` does not change.
        ///
        /// # Panics
        ///
        /// Panics if `at > len`.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1,2,3];
        /// let vec2 = vec.split_off(1);
        /// assert_eq!(vec, [1]);
        /// assert_eq!(vec2, [2, 3]);
        /// ```
        #[inline]
        #[stable(feature = "split_off", since = "1.4.0")]
        pub fn split_off(&mut self, at: usize) -> Self {
            assert!(at <= self.len(), "`at` out of bounds");

            let other_len = self.len - at;
            let mut other = Vec::with_capacity(other_len);

            // Unsafely `set_len` and copy items to `other`.
            unsafe {
                self.set_len(at);
                other.set_len(other_len);

                ptr::copy_nonoverlapping(self.as_ptr().add(at),
                                         other.as_mut_ptr(),
                                         other.len());
            }
            other
        }

        /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
        ///
        /// If `new_len` is greater than `len`, the `Vec` is extended by the
        /// difference, with each additional slot filled with the result of
        /// calling the closure `f`. The return values from `f` will end up
        /// in the `Vec` in the order they have been generated.
        ///
        /// If `new_len` is less than `len`, the `Vec` is simply truncated.
        ///
        /// This method uses a closure to create new values on every push. If
        /// you'd rather [`Clone`] a given value, use [`resize`]. If you want
        /// to use the [`Default`] trait to generate values, you can pass
        /// [`Default::default()`] as the second argument..
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 3];
        /// vec.resize_with(5, Default::default);
        /// assert_eq!(vec, [1, 2, 3, 0, 0]);
        ///
        /// let mut vec = vec![];
        /// let mut p = 1;
        /// vec.resize_with(4, || { p *= 2; p });
        /// assert_eq!(vec, [2, 4, 8, 16]);
        /// ```
        ///
        /// [`resize`]: #method.resize
        /// [`Clone`]: ../../std/clone/trait.Clone.html
        #[stable(feature = "vec_resize_with", since = "1.33.0")]
        pub fn resize_with<F>(&mut self, new_len: usize, f: F)
            where F: FnMut() -> T
        {
            let len = self.len();
            if new_len > len {
                self.extend_with(new_len - len, ExtendFunc(f));
            } else {
                self.truncate(new_len);
            }
        }
    }

    impl<T: Clone> Vec<T> {
        /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
        ///
        /// If `new_len` is greater than `len`, the `Vec` is extended by the
        /// difference, with each additional slot filled with `value`.
        /// If `new_len` is less than `len`, the `Vec` is simply truncated.
        ///
        /// This method requires [`Clone`] to be able clone the passed value. If
        /// you need more flexibility (or want to rely on [`Default`] instead of
        /// [`Clone`]), use [`resize_with`].
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec!["hello"];
        /// vec.resize(3, "world");
        /// assert_eq!(vec, ["hello", "world", "world"]);
        ///
        /// let mut vec = vec![1, 2, 3, 4];
        /// vec.resize(2, 0);
        /// assert_eq!(vec, [1, 2]);
        /// ```
        ///
        /// [`Clone`]: ../../std/clone/trait.Clone.html
        /// [`Default`]: ../../std/default/trait.Default.html
        /// [`resize_with`]: #method.resize_with
        #[stable(feature = "vec_resize", since = "1.5.0")]
        pub fn resize(&mut self, new_len: usize, value: T) {
            let len = self.len();

            if new_len > len {
                self.extend_with(new_len - len, ExtendElement(value))
            } else {
                self.truncate(new_len);
            }
        }

        /// Clones and appends all elements in a slice to the `Vec`.
        ///
        /// Iterates over the slice `other`, clones each element, and then appends
        /// it to this `Vec`. The `other` vector is traversed in-order.
        ///
        /// Note that this function is same as [`extend`] except that it is
        /// specialized to work with slices instead. If and when Rust gets
        /// specialization this function will likely be deprecated (but still
        /// available).
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1];
        /// vec.extend_from_slice(&[2, 3, 4]);
        /// assert_eq!(vec, [1, 2, 3, 4]);
        /// ```
        ///
        /// [`extend`]: #method.extend
        #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
        pub fn extend_from_slice(&mut self, other: &[T]) {
            self.spec_extend(other.iter())
        }
    }

    impl<T: Default> Vec<T> {
        /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
        ///
        /// If `new_len` is greater than `len`, the `Vec` is extended by the
        /// difference, with each additional slot filled with [`Default::default()`].
        /// If `new_len` is less than `len`, the `Vec` is simply truncated.
        ///
        /// This method uses [`Default`] to create new values on every push. If
        /// you'd rather [`Clone`] a given value, use [`resize`].
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(vec_resize_default)]
        ///
        /// let mut vec = vec![1, 2, 3];
        /// vec.resize_default(5);
        /// assert_eq!(vec, [1, 2, 3, 0, 0]);
        ///
        /// let mut vec = vec![1, 2, 3, 4];
        /// vec.resize_default(2);
        /// assert_eq!(vec, [1, 2]);
        /// ```
        ///
        /// [`resize`]: #method.resize
        /// [`Default::default()`]: ../../std/default/trait.Default.html#tymethod.default
        /// [`Default`]: ../../std/default/trait.Default.html
        /// [`Clone`]: ../../std/clone/trait.Clone.html
        #[unstable(feature = "vec_resize_default", issue = "41758")]
        pub fn resize_default(&mut self, new_len: usize) {
            let len = self.len();

            if new_len > len {
                self.extend_with(new_len - len, ExtendDefault);
            } else {
                self.truncate(new_len);
            }
        }
    }

    // This code generalises `extend_with_{element,default}`.
    trait ExtendWith<T> {
        fn next(&mut self) -> T;
        fn last(self) -> T;
    }

    struct ExtendElement<T>(T);
    impl<T: Clone> ExtendWith<T> for ExtendElement<T> {
        fn next(&mut self) -> T { self.0.clone() }
        fn last(self) -> T { self.0 }
    }

    struct ExtendDefault;
    impl<T: Default> ExtendWith<T> for ExtendDefault {
        fn next(&mut self) -> T { Default::default() }
        fn last(self) -> T { Default::default() }
    }

    struct ExtendFunc<F>(F);
    impl<T, F: FnMut() -> T> ExtendWith<T> for ExtendFunc<F> {
        fn next(&mut self) -> T { (self.0)() }
        fn last(mut self) -> T { (self.0)() }
    }

    impl<T> Vec<T> {
        /// Extend the vector by `n` values, using the given generator.
        fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, mut value: E) {
            self.reserve(n);

            unsafe {
                let mut ptr = self.as_mut_ptr().add(self.len());
                // Use SetLenOnDrop to work around bug where compiler
                // may not realize the store through `ptr` through self.set_len()
                // don't alias.
                let mut local_len = SetLenOnDrop::new(&mut self.len);

                // Write all elements except the last one
                for _ in 1..n {
                    ptr::write(ptr, value.next());
                    ptr = ptr.offset(1);
                    // Increment the length in every step in case next() panics
                    local_len.increment_len(1);
                }

                if n > 0 {
                    // We can write the last element directly without cloning needlessly
                    ptr::write(ptr, value.last());
                    local_len.increment_len(1);
                }

                // len set by scope guard
            }
        }
    }

    // Set the length of the vec when the `SetLenOnDrop` value goes out of scope.
    //
    // The idea is: The length field in SetLenOnDrop is a local variable
    // that the optimizer will see does not alias with any stores through the Vec's data
    // pointer. This is a workaround for alias analysis issue #32155
    struct SetLenOnDrop<'a> {
        len: &'a mut usize,
        local_len: usize,
    }

    impl<'a> SetLenOnDrop<'a> {
        #[inline]
        fn new(len: &'a mut usize) -> Self {
            SetLenOnDrop { local_len: *len, len: len }
        }

        #[inline]
        fn increment_len(&mut self, increment: usize) {
            self.local_len += increment;
        }

        #[inline]
        fn decrement_len(&mut self, decrement: usize) {
            self.local_len -= decrement;
        }
    }

    impl<'a> Drop for SetLenOnDrop<'a> {
        #[inline]
        fn drop(&mut self) {
            *self.len = self.local_len;
        }
    }

    impl<T: PartialEq> Vec<T> {
        /// Removes consecutive repeated elements in the vector according to the
        /// [`PartialEq`] trait implementation.
        ///
        /// If the vector is sorted, this removes all duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut vec = vec![1, 2, 2, 3, 2];
        ///
        /// vec.dedup();
        ///
        /// assert_eq!(vec, [1, 2, 3, 2]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn dedup(&mut self) {
            self.dedup_by(|a, b| a == b)
        }

        /// Removes the first instance of `item` from the vector if the item exists.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(vec_remove_item)]
        /// let mut vec = vec![1, 2, 3, 1];
        ///
        /// vec.remove_item(&1);
        ///
        /// assert_eq!(vec, vec![2, 3, 1]);
        /// ```
        #[unstable(feature = "vec_remove_item", reason = "recently added", issue = "40062")]
        pub fn remove_item(&mut self, item: &T) -> Option<T> {
            let pos = self.iter().position(|x| *x == *item)?;
            Some(self.remove(pos))
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Internal methods and functions
    ////////////////////////////////////////////////////////////////////////////////

    #[doc(hidden)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> {
        <T as SpecFromElem>::from_elem(elem, n)
    }

    // Specialization trait used for Vec::from_elem
    trait SpecFromElem: Sized {
        fn from_elem(elem: Self, n: usize) -> Vec<Self>;
    }

    impl<T: Clone> SpecFromElem for T {
        default fn from_elem(elem: Self, n: usize) -> Vec<Self> {
            let mut v = Vec::with_capacity(n);
            v.extend_with(n, ExtendElement(elem));
            v
        }
    }

    impl SpecFromElem for u8 {
        #[inline]
        fn from_elem(elem: u8, n: usize) -> Vec<u8> {
            if elem == 0 {
                return Vec {
                    buf: RawVec::with_capacity_zeroed(n),
                    len: n,
                }
            }
            unsafe {
                let mut v = Vec::with_capacity(n);
                ptr::write_bytes(v.as_mut_ptr(), elem, n);
                v.set_len(n);
                v
            }
        }
    }

    impl<T: Clone + IsZero> SpecFromElem for T {
        #[inline]
        fn from_elem(elem: T, n: usize) -> Vec<T> {
            if elem.is_zero() {
                return Vec {
                    buf: RawVec::with_capacity_zeroed(n),
                    len: n,
                }
            }
            let mut v = Vec::with_capacity(n);
            v.extend_with(n, ExtendElement(elem));
            v
        }
    }

    unsafe trait IsZero {
        /// Whether this value is zero
        fn is_zero(&self) -> bool;
    }

    macro_rules! impl_is_zero {
        ($t: ty, $is_zero: expr) => {
            unsafe impl IsZero for $t {
                #[inline]
                fn is_zero(&self) -> bool {
                    $is_zero(*self)
                }
            }
        }
    }

    impl_is_zero!(i8, |x| x == 0);
    impl_is_zero!(i16, |x| x == 0);
    impl_is_zero!(i32, |x| x == 0);
    impl_is_zero!(i64, |x| x == 0);
    impl_is_zero!(i128, |x| x == 0);
    impl_is_zero!(isize, |x| x == 0);

    impl_is_zero!(u16, |x| x == 0);
    impl_is_zero!(u32, |x| x == 0);
    impl_is_zero!(u64, |x| x == 0);
    impl_is_zero!(u128, |x| x == 0);
    impl_is_zero!(usize, |x| x == 0);

    impl_is_zero!(char, |x| x == '\0');

    impl_is_zero!(f32, |x: f32| x.to_bits() == 0);
    impl_is_zero!(f64, |x: f64| x.to_bits() == 0);

    unsafe impl<T: ?Sized> IsZero for *const T {
        #[inline]
        fn is_zero(&self) -> bool {
            (*self).is_null()
        }
    }

    unsafe impl<T: ?Sized> IsZero for *mut T {
        #[inline]
        fn is_zero(&self) -> bool {
            (*self).is_null()
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Common trait implementations for Vec
    ////////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for Vec<T> {
        #[cfg(not(test))]
        fn clone(&self) -> Vec<T> {
            <[T]>::to_vec(&**self)
        }

        // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
        // required for this method definition, is not available. Instead use the
        // `slice::to_vec`  function which is only available with cfg(test)
        // NB see the slice::hack module in slice.rs for more information
        #[cfg(test)]
        fn clone(&self) -> Vec<T> {
            ::slice::to_vec(&**self)
        }

        fn clone_from(&mut self, other: &Vec<T>) {
            other.as_slice().clone_into(self);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Hash> Hash for Vec<T> {
        #[inline]
        fn hash<H: hash::Hasher>(&self, state: &mut H) {
            Hash::hash(&**self, state)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented(
        message="vector indices are of type `usize` or ranges of `usize`",
        label="vector indices are of type `usize` or ranges of `usize`",
    )]
    impl<T, I> Index<I> for Vec<T>
    where
        I: ::core::slice::SliceIndex<[T]>,
    {
        type Output = I::Output;

        #[inline]
        fn index(&self, index: I) -> &Self::Output {
            Index::index(&**self, index)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_on_unimplemented(
        message="vector indices are of type `usize` or ranges of `usize`",
        label="vector indices are of type `usize` or ranges of `usize`",
    )]
    impl<T, I> IndexMut<I> for Vec<T>
    where
        I: ::core::slice::SliceIndex<[T]>,
    {
        #[inline]
        fn index_mut(&mut self, index: I) -> &mut Self::Output {
            IndexMut::index_mut(&mut **self, index)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::Deref for Vec<T> {
        type Target = [T];

        fn deref(&self) -> &[T] {
            unsafe {
                let p = self.buf.ptr();
                assume(!p.is_null());
                slice::from_raw_parts(p, self.len)
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::DerefMut for Vec<T> {
        fn deref_mut(&mut self) -> &mut [T] {
            unsafe {
                let ptr = self.buf.ptr();
                assume(!ptr.is_null());
                slice::from_raw_parts_mut(ptr, self.len)
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> FromIterator<T> for Vec<T> {
        #[inline]
        fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
            <Self as SpecExtend<T, I::IntoIter>>::from_iter(iter.into_iter())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> IntoIterator for Vec<T> {
        type Item = T;
        type IntoIter = IntoIter<T>;

        /// Creates a consuming iterator, that is, one that moves each value out of
        /// the vector (from start to end). The vector cannot be used after calling
        /// this.
        ///
        /// # Examples
        ///
        /// ```
        /// let v = vec!["a".to_string(), "b".to_string()];
        /// for s in v.into_iter() {
        ///     // s has type String, not &String
        ///     println!("{}", s);
        /// }
        /// ```
        #[inline]
        fn into_iter(mut self) -> IntoIter<T> {
            unsafe {
                let begin = self.as_mut_ptr();
                assume(!begin.is_null());
                let end = if mem::size_of::<T>() == 0 {
                    arith_offset(begin as *const i8, self.len() as isize) as *const T
                } else {
                    begin.add(self.len()) as *const T
                };
                let cap = self.buf.cap();
                mem::forget(self);
                IntoIter {
                    buf: NonNull::new_unchecked(begin),
                    phantom: PhantomData,
                    cap,
                    ptr: begin,
                    end,
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> IntoIterator for &'a Vec<T> {
        type Item = &'a T;
        type IntoIter = slice::Iter<'a, T>;

        fn into_iter(self) -> slice::Iter<'a, T> {
            self.iter()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> IntoIterator for &'a mut Vec<T> {
        type Item = &'a mut T;
        type IntoIter = slice::IterMut<'a, T>;

        fn into_iter(self) -> slice::IterMut<'a, T> {
            self.iter_mut()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Extend<T> for Vec<T> {
        #[inline]
        fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
            <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
        }
    }

    // Specialization trait used for Vec::from_iter and Vec::extend
    trait SpecExtend<T, I> {
        fn from_iter(iter: I) -> Self;
        fn spec_extend(&mut self, iter: I);
    }

    impl<T, I> SpecExtend<T, I> for Vec<T>
        where I: Iterator<Item=T>,
    {
        default fn from_iter(mut iterator: I) -> Self {
            // Unroll the first iteration, as the vector is going to be
            // expanded on this iteration in every case when the iterable is not
            // empty, but the loop in extend_desugared() is not going to see the
            // vector being full in the few subsequent loop iterations.
            // So we get better branch prediction.
            let mut vector = match iterator.next() {
                None => return Vec::new(),
                Some(element) => {
                    let (lower, _) = iterator.size_hint();
                    let mut vector = Vec::with_capacity(lower.saturating_add(1));
                    unsafe {
                        ptr::write(vector.get_unchecked_mut(0), element);
                        vector.set_len(1);
                    }
                    vector
                }
            };
            <Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
            vector
        }

        default fn spec_extend(&mut self, iter: I) {
            self.extend_desugared(iter)
        }
    }

    impl<T, I> SpecExtend<T, I> for Vec<T>
        where I: TrustedLen<Item=T>,
    {
        default fn from_iter(iterator: I) -> Self {
            let mut vector = Vec::new();
            vector.spec_extend(iterator);
            vector
        }

        default fn spec_extend(&mut self, iterator: I) {
            // This is the case for a TrustedLen iterator.
            let (low, high) = iterator.size_hint();
            if let Some(high_value) = high {
                debug_assert_eq!(low, high_value,
                                 "TrustedLen iterator's size hint is not exact: {:?}",
                                 (low, high));
            }
            if let Some(additional) = high {
                self.reserve(additional);
                unsafe {
                    let mut ptr = self.as_mut_ptr().add(self.len());
                    let mut local_len = SetLenOnDrop::new(&mut self.len);
                    iterator.for_each(move |element| {
                        ptr::write(ptr, element);
                        ptr = ptr.offset(1);
                        // NB can't overflow since we would have had to alloc the address space
                        local_len.increment_len(1);
                    });
                }
            } else {
                self.extend_desugared(iterator)
            }
        }
    }

    impl<T> SpecExtend<T, IntoIter<T>> for Vec<T> {
        fn from_iter(iterator: IntoIter<T>) -> Self {
            // A common case is passing a vector into a function which immediately
            // re-collects into a vector. We can short circuit this if the IntoIter
            // has not been advanced at all.
            if iterator.buf.as_ptr() as *const _ == iterator.ptr {
                unsafe {
                    let vec = Vec::from_raw_parts(iterator.buf.as_ptr(),
                                                  iterator.len(),
                                                  iterator.cap);
                    mem::forget(iterator);
                    vec
                }
            } else {
                let mut vector = Vec::new();
                vector.spec_extend(iterator);
                vector
            }
        }

        fn spec_extend(&mut self, mut iterator: IntoIter<T>) {
            unsafe {
                self.append_elements(iterator.as_slice() as _);
            }
            iterator.ptr = iterator.end;
        }
    }

    impl<'a, T: 'a, I> SpecExtend<&'a T, I> for Vec<T>
        where I: Iterator<Item=&'a T>,
              T: Clone,
    {
        default fn from_iter(iterator: I) -> Self {
            SpecExtend::from_iter(iterator.cloned())
        }

        default fn spec_extend(&mut self, iterator: I) {
            self.spec_extend(iterator.cloned())
        }
    }

    impl<'a, T: 'a> SpecExtend<&'a T, slice::Iter<'a, T>> for Vec<T>
        where T: Copy,
    {
        fn spec_extend(&mut self, iterator: slice::Iter<'a, T>) {
            let slice = iterator.as_slice();
            self.reserve(slice.len());
            unsafe {
                let len = self.len();
                self.set_len(len + slice.len());
                self.get_unchecked_mut(len..).copy_from_slice(slice);
            }
        }
    }

    impl<T> Vec<T> {
        fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
            // This is the case for a general iterator.
            //
            // This function should be the moral equivalent of:
            //
            //      for item in iterator {
            //          self.push(item);
            //      }
            while let Some(element) = iterator.next() {
                let len = self.len();
                if len == self.capacity() {
                    let (lower, _) = iterator.size_hint();
                    self.reserve(lower.saturating_add(1));
                }
                unsafe {
                    ptr::write(self.get_unchecked_mut(len), element);
                    // NB can't overflow since we would have had to alloc the address space
                    self.set_len(len + 1);
                }
            }
        }

        /// Creates a splicing iterator that replaces the specified range in the vector
        /// with the given `replace_with` iterator and yields the removed items.
        /// `replace_with` does not need to be the same length as `range`.
        ///
        /// Note 1: The element range is removed even if the iterator is not
        /// consumed until the end.
        ///
        /// Note 2: It is unspecified how many elements are removed from the vector,
        /// if the `Splice` value is leaked.
        ///
        /// Note 3: The input iterator `replace_with` is only consumed
        /// when the `Splice` value is dropped.
        ///
        /// Note 4: This is optimal if:
        ///
        /// * The tail (elements in the vector after `range`) is empty,
        /// * or `replace_with` yields fewer elements than `range`’s length
        /// * or the lower bound of its `size_hint()` is exact.
        ///
        /// Otherwise, a temporary vector is allocated and the tail is moved twice.
        ///
        /// # Panics
        ///
        /// Panics if the starting point is greater than the end point or if
        /// the end point is greater than the length of the vector.
        ///
        /// # Examples
        ///
        /// ```
        /// let mut v = vec![1, 2, 3];
        /// let new = [7, 8];
        /// let u: Vec<_> = v.splice(..2, new.iter().cloned()).collect();
        /// assert_eq!(v, &[7, 8, 3]);
        /// assert_eq!(u, &[1, 2]);
        /// ```
        #[inline]
        #[stable(feature = "vec_splice", since = "1.21.0")]
        pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<I::IntoIter>
            where R: RangeBounds<usize>, I: IntoIterator<Item=T>
        {
            Splice {
                drain: self.drain(range),
                replace_with: replace_with.into_iter(),
            }
        }

        /// Creates an iterator which uses a closure to determine if an element should be removed.
        ///
        /// If the closure returns true, then the element is removed and yielded.
        /// If the closure returns false, the element will remain in the vector and will not be yielded
        /// by the iterator.
        ///
        /// Using this method is equivalent to the following code:
        ///
        /// ```
        /// # let some_predicate = |x: &mut i32| { *x == 2 || *x == 3 || *x == 6 };
        /// # let mut vec = vec![1, 2, 3, 4, 5, 6];
        /// let mut i = 0;
        /// while i != vec.len() {
        ///     if some_predicate(&mut vec[i]) {
        ///         let val = vec.remove(i);
        ///         // your code here
        ///     } else {
        ///         i += 1;
        ///     }
        /// }
        ///
        /// # assert_eq!(vec, vec![1, 4, 5]);
        /// ```
        ///
        /// But `drain_filter` is easier to use. `drain_filter` is also more efficient,
        /// because it can backshift the elements of the array in bulk.
        ///
        /// Note that `drain_filter` also lets you mutate every element in the filter closure,
        /// regardless of whether you choose to keep or remove it.
        ///
        ///
        /// # Examples
        ///
        /// Splitting an array into evens and odds, reusing the original allocation:
        ///
        /// ```
        /// #![feature(drain_filter)]
        /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
        ///
        /// let evens = numbers.drain_filter(|x| *x % 2 == 0).collect::<Vec<_>>();
        /// let odds = numbers;
        ///
        /// assert_eq!(evens, vec![2, 4, 6, 8, 14]);
        /// assert_eq!(odds, vec![1, 3, 5, 9, 11, 13, 15]);
        /// ```
        #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
        pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<T, F>
            where F: FnMut(&mut T) -> bool,
        {
            let old_len = self.len();

            // Guard against us getting leaked (leak amplification)
            unsafe { self.set_len(0); }

            DrainFilter {
                vec: self,
                idx: 0,
                del: 0,
                old_len,
                pred: filter,
            }
        }
    }

    /// Extend implementation that copies elements out of references before pushing them onto the Vec.
    ///
    /// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
    /// append the entire slice at once.
    ///
    /// [`copy_from_slice`]: ../../std/primitive.slice.html#method.copy_from_slice
    #[stable(feature = "extend_ref", since = "1.2.0")]
    impl<'a, T: 'a + Copy> Extend<&'a T> for Vec<T> {
        fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
            self.spec_extend(iter.into_iter())
        }
    }

    macro_rules! __impl_slice_eq1 {
        ($Lhs: ty, $Rhs: ty) => {
            __impl_slice_eq1! { $Lhs, $Rhs, Sized }
        };
        ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
                #[inline]
                fn eq(&self, other: &$Rhs) -> bool { self[..] == other[..] }
                #[inline]
                fn ne(&self, other: &$Rhs) -> bool { self[..] != other[..] }
            }
        }
    }

    __impl_slice_eq1! { Vec<A>, Vec<B> }
    __impl_slice_eq1! { Vec<A>, &'b [B] }
    __impl_slice_eq1! { Vec<A>, &'b mut [B] }
    __impl_slice_eq1! { Cow<'a, [A]>, &'b [B], Clone }
    __impl_slice_eq1! { Cow<'a, [A]>, &'b mut [B], Clone }
    __impl_slice_eq1! { Cow<'a, [A]>, Vec<B>, Clone }

    macro_rules! array_impls {
        ($($N: expr)+) => {
            $(
                // NOTE: some less important impls are omitted to reduce code bloat
                __impl_slice_eq1! { Vec<A>, [B; $N] }
                __impl_slice_eq1! { Vec<A>, &'b [B; $N] }
                // __impl_slice_eq1! { Vec<A>, &'b mut [B; $N] }
                // __impl_slice_eq1! { Cow<'a, [A]>, [B; $N], Clone }
                // __impl_slice_eq1! { Cow<'a, [A]>, &'b [B; $N], Clone }
                // __impl_slice_eq1! { Cow<'a, [A]>, &'b mut [B; $N], Clone }
            )+
        }
    }

    array_impls! {
         0  1  2  3  4  5  6  7  8  9
        10 11 12 13 14 15 16 17 18 19
        20 21 22 23 24 25 26 27 28 29
        30 31 32
    }

    /// Implements comparison of vectors, lexicographically.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: PartialOrd> PartialOrd for Vec<T> {
        #[inline]
        fn partial_cmp(&self, other: &Vec<T>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Eq> Eq for Vec<T> {}

    /// Implements ordering of vectors, lexicographically.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> Ord for Vec<T> {
        #[inline]
        fn cmp(&self, other: &Vec<T>) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T> Drop for Vec<T> {
        fn drop(&mut self) {
            unsafe {
                // use drop for [T]
                ptr::drop_in_place(&mut self[..]);
            }
            // RawVec handles deallocation
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for Vec<T> {
        /// Creates an empty `Vec<T>`.
        fn default() -> Vec<T> {
            Vec::new()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Debug> fmt::Debug for Vec<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> AsRef<Vec<T>> for Vec<T> {
        fn as_ref(&self) -> &Vec<T> {
            self
        }
    }

    #[stable(feature = "vec_as_mut", since = "1.5.0")]
    impl<T> AsMut<Vec<T>> for Vec<T> {
        fn as_mut(&mut self) -> &mut Vec<T> {
            self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> AsRef<[T]> for Vec<T> {
        fn as_ref(&self) -> &[T] {
            self
        }
    }

    #[stable(feature = "vec_as_mut", since = "1.5.0")]
    impl<T> AsMut<[T]> for Vec<T> {
        fn as_mut(&mut self) -> &mut [T] {
            self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T: Clone> From<&'a [T]> for Vec<T> {
        #[cfg(not(test))]
        fn from(s: &'a [T]) -> Vec<T> {
            s.to_vec()
        }
        #[cfg(test)]
        fn from(s: &'a [T]) -> Vec<T> {
            ::slice::to_vec(s)
        }
    }

    #[stable(feature = "vec_from_mut", since = "1.19.0")]
    impl<'a, T: Clone> From<&'a mut [T]> for Vec<T> {
        #[cfg(not(test))]
        fn from(s: &'a mut [T]) -> Vec<T> {
            s.to_vec()
        }
        #[cfg(test)]
        fn from(s: &'a mut [T]) -> Vec<T> {
            ::slice::to_vec(s)
        }
    }

    #[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
    impl<'a, T> From<Cow<'a, [T]>> for Vec<T> where [T]: ToOwned<Owned=Vec<T>> {
        fn from(s: Cow<'a, [T]>) -> Vec<T> {
            s.into_owned()
        }
    }

    // note: test pulls in libstd, which causes errors here
    #[cfg(not(test))]
    #[stable(feature = "vec_from_box", since = "1.18.0")]
    impl<T> From<Box<[T]>> for Vec<T> {
        fn from(s: Box<[T]>) -> Vec<T> {
            s.into_vec()
        }
    }

    // note: test pulls in libstd, which causes errors here
    #[cfg(not(test))]
    #[stable(feature = "box_from_vec", since = "1.20.0")]
    impl<T> From<Vec<T>> for Box<[T]> {
        fn from(v: Vec<T>) -> Box<[T]> {
            v.into_boxed_slice()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> From<&'a str> for Vec<u8> {
        fn from(s: &'a str) -> Vec<u8> {
            From::from(s.as_bytes())
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Clone-on-write
    ////////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "cow_from_vec", since = "1.8.0")]
    impl<'a, T: Clone> From<&'a [T]> for Cow<'a, [T]> {
        fn from(s: &'a [T]) -> Cow<'a, [T]> {
            Cow::Borrowed(s)
        }
    }

    #[stable(feature = "cow_from_vec", since = "1.8.0")]
    impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]> {
        fn from(v: Vec<T>) -> Cow<'a, [T]> {
            Cow::Owned(v)
        }
    }

    #[stable(feature = "cow_from_vec_ref", since = "1.28.0")]
    impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]> {
        fn from(v: &'a Vec<T>) -> Cow<'a, [T]> {
            Cow::Borrowed(v.as_slice())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> FromIterator<T> for Cow<'a, [T]> where T: Clone {
        fn from_iter<I: IntoIterator<Item = T>>(it: I) -> Cow<'a, [T]> {
            Cow::Owned(FromIterator::from_iter(it))
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Iterators
    ////////////////////////////////////////////////////////////////////////////////

    /// An iterator that moves out of a vector.
    ///
    /// This `struct` is created by the `into_iter` method on [`Vec`][`Vec`] (provided
    /// by the [`IntoIterator`] trait).
    ///
    /// [`Vec`]: struct.Vec.html
    /// [`IntoIterator`]: ../../std/iter/trait.IntoIterator.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoIter<T> {
        buf: NonNull<T>,
        phantom: PhantomData<T>,
        cap: usize,
        ptr: *const T,
        end: *const T,
    }

    #[stable(feature = "vec_intoiter_debug", since = "1.13.0")]
    impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("IntoIter")
                .field(&self.as_slice())
                .finish()
        }
    }

    impl<T> IntoIter<T> {
        /// Returns the remaining items of this iterator as a slice.
        ///
        /// # Examples
        ///
        /// ```
        /// let vec = vec!['a', 'b', 'c'];
        /// let mut into_iter = vec.into_iter();
        /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
        /// let _ = into_iter.next().unwrap();
        /// assert_eq!(into_iter.as_slice(), &['b', 'c']);
        /// ```
        #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
        pub fn as_slice(&self) -> &[T] {
            unsafe {
                slice::from_raw_parts(self.ptr, self.len())
            }
        }

        /// Returns the remaining items of this iterator as a mutable slice.
        ///
        /// # Examples
        ///
        /// ```
        /// let vec = vec!['a', 'b', 'c'];
        /// let mut into_iter = vec.into_iter();
        /// assert_eq!(into_iter.as_slice(), &['a', 'b', 'c']);
        /// into_iter.as_mut_slice()[2] = 'z';
        /// assert_eq!(into_iter.next().unwrap(), 'a');
        /// assert_eq!(into_iter.next().unwrap(), 'b');
        /// assert_eq!(into_iter.next().unwrap(), 'z');
        /// ```
        #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
        pub fn as_mut_slice(&mut self) -> &mut [T] {
            unsafe {
                slice::from_raw_parts_mut(self.ptr as *mut T, self.len())
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for IntoIter<T> {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Sync> Sync for IntoIter<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Iterator for IntoIter<T> {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<T> {
            unsafe {
                if self.ptr as *const _ == self.end {
                    None
                } else {
                    if mem::size_of::<T>() == 0 {
                        // purposefully don't use 'ptr.offset' because for
                        // vectors with 0-size elements this would return the
                        // same pointer.
                        self.ptr = arith_offset(self.ptr as *const i8, 1) as *mut T;

                        // Make up a value of this ZST.
                        Some(mem::zeroed())
                    } else {
                        let old = self.ptr;
                        self.ptr = self.ptr.offset(1);

                        Some(ptr::read(old))
                    }
                }
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let exact = if mem::size_of::<T>() == 0 {
                (self.end as usize).wrapping_sub(self.ptr as usize)
            } else {
                unsafe { self.end.offset_from(self.ptr) as usize }
            };
            (exact, Some(exact))
        }

        #[inline]
        fn count(self) -> usize {
            self.len()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> DoubleEndedIterator for IntoIter<T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> {
            unsafe {
                if self.end == self.ptr {
                    None
                } else {
                    if mem::size_of::<T>() == 0 {
                        // See above for why 'ptr.offset' isn't used
                        self.end = arith_offset(self.end as *const i8, -1) as *mut T;

                        // Make up a value of this ZST.
                        Some(mem::zeroed())
                    } else {
                        self.end = self.end.offset(-1);

                        Some(ptr::read(self.end))
                    }
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for IntoIter<T> {
        fn is_empty(&self) -> bool {
            self.ptr == self.end
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for IntoIter<T> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<T> TrustedLen for IntoIter<T> {}

    #[stable(feature = "vec_into_iter_clone", since = "1.8.0")]
    impl<T: Clone> Clone for IntoIter<T> {
        fn clone(&self) -> IntoIter<T> {
            self.as_slice().to_owned().into_iter()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T> Drop for IntoIter<T> {
        fn drop(&mut self) {
            // destroy the remaining elements
            for _x in self.by_ref() {}

            // RawVec handles deallocation
            let _ = unsafe { RawVec::from_raw_parts(self.buf.as_ptr(), self.cap) };
        }
    }

    /// A draining iterator for `Vec<T>`.
    ///
    /// This `struct` is created by the [`drain`] method on [`Vec`].
    ///
    /// [`drain`]: struct.Vec.html#method.drain
    /// [`Vec`]: struct.Vec.html
    #[stable(feature = "drain", since = "1.6.0")]
    pub struct Drain<'a, T: 'a> {
        /// Index of tail to preserve
        tail_start: usize,
        /// Length of tail
        tail_len: usize,
        /// Current remaining range to remove
        iter: slice::Iter<'a, T>,
        vec: NonNull<Vec<T>>,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<'a, T: 'a + fmt::Debug> fmt::Debug for Drain<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_tuple("Drain")
             .field(&self.iter.as_slice())
             .finish()
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl<'a, T: Sync> Sync for Drain<'a, T> {}
    #[stable(feature = "drain", since = "1.6.0")]
    unsafe impl<'a, T: Send> Send for Drain<'a, T> {}

    #[stable(feature = "drain", since = "1.6.0")]
    impl<'a, T> Iterator for Drain<'a, T> {
        type Item = T;

        #[inline]
        fn next(&mut self) -> Option<T> {
            self.iter.next().map(|elt| unsafe { ptr::read(elt as *const _) })
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<T> {
            self.iter.next_back().map(|elt| unsafe { ptr::read(elt as *const _) })
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<'a, T> Drop for Drain<'a, T> {
        fn drop(&mut self) {
            // exhaust self first
            self.for_each(drop);

            if self.tail_len > 0 {
                unsafe {
                    let source_vec = self.vec.as_mut();
                    // memmove back untouched tail, update to new length
                    let start = source_vec.len();
                    let tail = self.tail_start;
                    if tail != start {
                        let src = source_vec.as_ptr().add(tail);
                        let dst = source_vec.as_mut_ptr().add(start);
                        ptr::copy(src, dst, self.tail_len);
                    }
                    source_vec.set_len(start + self.tail_len);
                }
            }
        }
    }

    #[stable(feature = "drain", since = "1.6.0")]
    impl<'a, T> ExactSizeIterator for Drain<'a, T> {
        fn is_empty(&self) -> bool {
            self.iter.is_empty()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<'a, T> FusedIterator for Drain<'a, T> {}

    /// A splicing iterator for `Vec`.
    ///
    /// This struct is created by the [`splice()`] method on [`Vec`]. See its
    /// documentation for more.
    ///
    /// [`splice()`]: struct.Vec.html#method.splice
    /// [`Vec`]: struct.Vec.html
    #[derive(Debug)]
    #[stable(feature = "vec_splice", since = "1.21.0")]
    pub struct Splice<'a, I: Iterator + 'a> {
        drain: Drain<'a, I::Item>,
        replace_with: I,
    }

    #[stable(feature = "vec_splice", since = "1.21.0")]
    impl<'a, I: Iterator> Iterator for Splice<'a, I> {
        type Item = I::Item;

        fn next(&mut self) -> Option<Self::Item> {
            self.drain.next()
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.drain.size_hint()
        }
    }

    #[stable(feature = "vec_splice", since = "1.21.0")]
    impl<'a, I: Iterator> DoubleEndedIterator for Splice<'a, I> {
        fn next_back(&mut self) -> Option<Self::Item> {
            self.drain.next_back()
        }
    }

    #[stable(feature = "vec_splice", since = "1.21.0")]
    impl<'a, I: Iterator> ExactSizeIterator for Splice<'a, I> {}

    #[stable(feature = "vec_splice", since = "1.21.0")]
    impl<'a, I: Iterator> Drop for Splice<'a, I> {
        fn drop(&mut self) {
            self.drain.by_ref().for_each(drop);

            unsafe {
                if self.drain.tail_len == 0 {
                    self.drain.vec.as_mut().extend(self.replace_with.by_ref());
                    return
                }

                // First fill the range left by drain().
                if !self.drain.fill(&mut self.replace_with) {
                    return
                }

                // There may be more elements. Use the lower bound as an estimate.
                // FIXME: Is the upper bound a better guess? Or something else?
                let (lower_bound, _upper_bound) = self.replace_with.size_hint();
                if lower_bound > 0  {
                    self.drain.move_tail(lower_bound);
                    if !self.drain.fill(&mut self.replace_with) {
                        return
                    }
                }

                // Collect any remaining elements.
                // This is a zero-length vector which does not allocate if `lower_bound` was exact.
                let mut collected = self.replace_with.by_ref().collect::<Vec<I::Item>>().into_iter();
                // Now we have an exact count.
                if collected.len() > 0 {
                    self.drain.move_tail(collected.len());
                    let filled = self.drain.fill(&mut collected);
                    debug_assert!(filled);
                    debug_assert_eq!(collected.len(), 0);
                }
            }
            // Let `Drain::drop` move the tail back if necessary and restore `vec.len`.
        }
    }

    /// Private helper methods for `Splice::drop`
    impl<'a, T> Drain<'a, T> {
        /// The range from `self.vec.len` to `self.tail_start` contains elements
        /// that have been moved out.
        /// Fill that range as much as possible with new elements from the `replace_with` iterator.
        /// Return whether we filled the entire range. (`replace_with.next()` didn’t return `None`.)
        unsafe fn fill<I: Iterator<Item=T>>(&mut self, replace_with: &mut I) -> bool {
            let vec = self.vec.as_mut();
            let range_start = vec.len;
            let range_end = self.tail_start;
            let range_slice = slice::from_raw_parts_mut(
                vec.as_mut_ptr().add(range_start),
                range_end - range_start);

            for place in range_slice {
                if let Some(new_item) = replace_with.next() {
                    ptr::write(place, new_item);
                    vec.len += 1;
                } else {
                    return false
                }
            }
            true
        }

        /// Make room for inserting more elements before the tail.
        unsafe fn move_tail(&mut self, extra_capacity: usize) {
            let vec = self.vec.as_mut();
            let used_capacity = self.tail_start + self.tail_len;
            vec.buf.reserve(used_capacity, extra_capacity);

            let new_tail_start = self.tail_start + extra_capacity;
            let src = vec.as_ptr().add(self.tail_start);
            let dst = vec.as_mut_ptr().add(new_tail_start);
            ptr::copy(src, dst, self.tail_len);
            self.tail_start = new_tail_start;
        }
    }

    /// An iterator produced by calling `drain_filter` on Vec.
    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    #[derive(Debug)]
    pub struct DrainFilter<'a, T: 'a, F>
        where F: FnMut(&mut T) -> bool,
    {
        vec: &'a mut Vec<T>,
        idx: usize,
        del: usize,
        old_len: usize,
        pred: F,
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
        where F: FnMut(&mut T) -> bool,
    {
        type Item = T;

        fn next(&mut self) -> Option<T> {
            unsafe {
                while self.idx != self.old_len {
                    let i = self.idx;
                    self.idx += 1;
                    let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
                    if (self.pred)(&mut v[i]) {
                        self.del += 1;
                        return Some(ptr::read(&v[i]));
                    } else if self.del > 0 {
                        let del = self.del;
                        let src: *const T = &v[i];
                        let dst: *mut T = &mut v[i - del];
                        // This is safe because self.vec has length 0
                        // thus its elements will not have Drop::drop
                        // called on them in the event of a panic.
                        ptr::copy_nonoverlapping(src, dst, 1);
                    }
                }
                None
            }
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            (0, Some(self.old_len - self.idx))
        }
    }

    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
    impl<'a, T, F> Drop for DrainFilter<'a, T, F>
        where F: FnMut(&mut T) -> bool,
    {
        fn drop(&mut self) {
            self.for_each(drop);
            unsafe {
                self.vec.set_len(self.old_len - self.del);
            }
        }
    }

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