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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A contiguous growable array type with heap-allocated contents, written
    //! `Vec<T>` but pronounced 'vector.'
    //!
    //! 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 alloc::boxed::Box;
    use alloc::heap::EMPTY;
    use alloc::raw_vec::RawVec;
    use borrow::ToOwned;
    use borrow::Cow;
    use core::cmp::Ordering;
    use core::fmt;
    use core::hash::{self, Hash};
    use core::intrinsics::{arith_offset, assume};
    use core::iter::{FromIterator, FusedIterator, TrustedLen};
    use core::mem;
    use core::ops::{InPlace, Index, IndexMut, Place, Placer};
    use core::ops;
    use core::ptr;
    use core::ptr::Shared;
    use core::slice;

    use super::range::RangeArgument;
    use Bound::{Excluded, Included, Unbounded};

    /// 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:
    ///
    /// ```
    /// let vec = vec![0; 5];
    /// assert_eq!(vec, [0, 0, 0, 0, 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:
    ///
    /// ```ignore
    /// 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 subtle enough that it is strongly recommended that you only
    /// free memory allocated by a `Vec` by creating a new `Vec` and 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 elements in order (what you would see
    /// if you coerced it to a slice), followed by [`capacity`]` - `[`len`]
    /// logically uninitialized 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.
    ///
    /// `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")]
        pub 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 this function does not specify the *length*
        /// of the returned vector, but only the *capacity*. 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).
        /// * `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 datastructures. 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);
        }

        /// 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) {
            self.buf.shrink_to_fit(self.len);
        }

        /// Converts the vector into [`Box<[T]>`][owned slice].
        ///
        /// Note that this will drop any excess capacity. Calling this and
        /// converting back to a vector with [`into_vec`] is equivalent to calling
        /// [`shrink_to_fit`].
        ///
        /// [owned slice]: ../../std/boxed/struct.Box.html
        /// [`into_vec`]: ../../std/primitive.slice.html#method.into_vec
        /// [`shrink_to_fit`]: #method.shrink_to_fit
        ///
        /// # 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) {
            unsafe {
                // drop any extra elements
                while len < self.len {
                    // decrement len before the drop_in_place(), so a panic on Drop
                    // doesn't re-drop the just-failed value.
                    self.len -= 1;
                    let len = self.len;
                    ptr::drop_in_place(self.get_unchecked_mut(len));
                }
            }
        }

        /// 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
        }

        /// Sets the length of a vector.
        ///
        /// This will explicitly set the size of the vector, without actually
        /// modifying its buffers, so it is up to the caller to ensure that the
        /// vector is actually the specified size.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::ptr;
        ///
        /// let mut vec = vec!['r', 'u', 's', 't'];
        ///
        /// unsafe {
        ///     ptr::drop_in_place(&mut vec[3]);
        ///     vec.set_len(3);
        /// }
        /// assert_eq!(vec, ['r', 'u', 's']);
        /// ```
        ///
        /// In this example, there is a memory leak since the memory locations
        /// owned by 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]];
        /// unsafe {
        ///     vec.set_len(0);
        /// }
        /// ```
        ///
        /// In this example, the vector gets expanded from zero to four items
        /// without any memory allocations occurring, resulting in vector
        /// values of unallocated memory:
        ///
        /// ```
        /// let mut vec: Vec<char> = Vec::new();
        ///
        /// unsafe {
        ///     vec.set_len(4);
        /// }
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub unsafe fn set_len(&mut self, len: usize) {
            self.len = 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 {
            let length = self.len();
            self.swap(index, length - 1);
            self.pop().unwrap()
        }

        /// Inserts an element at position `index` within the vector, shifting all
        /// elements after it to the right.
        ///
        /// # Panics
        ///
        /// Panics if `index` is out of bounds.
        ///
        /// # 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.buf.double();
            }

            unsafe {
                // infallible
                // The spot to put the new value
                {
                    let p = self.as_mut_ptr().offset(index as isize);
                    // 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().offset(index as isize);
                    // 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
        {
            let len = self.len();
            let mut del = 0;
            {
                let v = &mut **self;

                for i in 0..len {
                    if !f(&v[i]) {
                        del += 1;
                    } else if del > 0 {
                        v.swap(i - del, i);
                    }
                }
            }
            if del > 0 {
                self.truncate(len - del);
            }
        }

        /// Removes 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 consecutive elements in the vector according to a predicate.
        ///
        /// The `same_bucket` function is passed references to two elements from the vector, and
        /// returns `true` if the elements compare equal, or `false` if they do not. Only the first
        /// of adjacent equal items is kept.
        ///
        /// If the vector is sorted, this removes all duplicates.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::ascii::AsciiExt;
        ///
        /// 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, mut same_bucket: F) where F: FnMut(&mut T, &mut T) -> bool {
            unsafe {
                // Although we have a mutable reference to `self`, we cannot make
                // *arbitrary* changes. The `same_bucket` calls could panic, so we
                // must ensure that the vector is in a valid state at all time.
                //
                // The way that we handle this is by using swaps; we iterate
                // over all the elements, swapping as we go so that at the end
                // the elements we wish to keep are in the front, and those we
                // wish to reject are at the back. We can then truncate the
                // vector. This operation is still O(n).
                //
                // Example: We start in this state, where `r` represents "next
                // read" and `w` represents "next_write`.
                //
                //           r
                //     +---+---+---+---+---+---+
                //     | 0 | 1 | 1 | 2 | 3 | 3 |
                //     +---+---+---+---+---+---+
                //           w
                //
                // Comparing self[r] against self[w-1], this is not a duplicate, so
                // we swap self[r] and self[w] (no effect as r==w) and then increment both
                // r and w, leaving us with:
                //
                //               r
                //     +---+---+---+---+---+---+
                //     | 0 | 1 | 1 | 2 | 3 | 3 |
                //     +---+---+---+---+---+---+
                //               w
                //
                // Comparing self[r] against self[w-1], this value is a duplicate,
                // so we increment `r` but leave everything else unchanged:
                //
                //                   r
                //     +---+---+---+---+---+---+
                //     | 0 | 1 | 1 | 2 | 3 | 3 |
                //     +---+---+---+---+---+---+
                //               w
                //
                // Comparing self[r] against self[w-1], this is not a duplicate,
                // so swap self[r] and self[w] and advance r and w:
                //
                //                       r
                //     +---+---+---+---+---+---+
                //     | 0 | 1 | 2 | 1 | 3 | 3 |
                //     +---+---+---+---+---+---+
                //                   w
                //
                // Not a duplicate, repeat:
                //
                //                           r
                //     +---+---+---+---+---+---+
                //     | 0 | 1 | 2 | 3 | 1 | 3 |
                //     +---+---+---+---+---+---+
                //                       w
                //
                // Duplicate, advance r. End of vec. Truncate to w.

                let ln = self.len();
                if ln <= 1 {
                    return;
                }

                // Avoid bounds checks by using raw pointers.
                let p = self.as_mut_ptr();
                let mut r: usize = 1;
                let mut w: usize = 1;

                while r < ln {
                    let p_r = p.offset(r as isize);
                    let p_wm1 = p.offset((w - 1) as isize);
                    if !same_bucket(&mut *p_r, &mut *p_wm1) {
                        if r != w {
                            let p_w = p_wm1.offset(1);
                            mem::swap(&mut *p_r, &mut *p_w);
                        }
                        w += 1;
                    }
                    r += 1;
                }

                self.truncate(w);
            }
        }

        /// 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.buf.double();
            }
            unsafe {
                let end = self.as_mut_ptr().offset(self.len as isize);
                ptr::write(end, value);
                self.len += 1;
            }
        }

        /// Returns a place for insertion at the back of the `Vec`.
        ///
        /// Using this method with placement syntax is equivalent to [`push`](#method.push),
        /// but may be more efficient.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(collection_placement)]
        /// #![feature(placement_in_syntax)]
        ///
        /// let mut vec = vec![1, 2];
        /// vec.place_back() <- 3;
        /// vec.place_back() <- 4;
        /// assert_eq!(&vec, &[1, 2, 3, 4]);
        /// ```
        #[unstable(feature = "collection_placement",
                   reason = "placement protocol is subject to change",
                   issue = "30172")]
        pub fn place_back(&mut self) -> PlaceBack<T> {
            PlaceBack { vec: self }
        }

        /// 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) {
            self.reserve(other.len());
            let len = self.len();
            unsafe {
                ptr::copy_nonoverlapping(other.as_ptr(), self.get_unchecked_mut(len), other.len());
            }

            self.len += other.len();
            unsafe {
                other.set_len(0);
            }
        }

        /// Create 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: RangeArgument<usize>
        {
            // Memory safety
            //
            // When the Drain is first created, it shortens the length of
            // the source vector to make sure no uninitalized 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() {
                Included(&n) => n,
                Excluded(&n) => n + 1,
                Unbounded    => 0,
            };
            let end = match range.end() {
                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().offset(start as isize),
                                                            end - start);
                Drain {
                    tail_start: end,
                    tail_len: len - end,
                    iter: range_slice.iter(),
                    vec: Shared::new(self as *mut _),
                }
            }
        }

        /// 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.
        ///
        /// # 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().offset(at as isize),
                                         other.as_mut_ptr(),
                                         other.len());
            }
            other
        }
    }

    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.
        ///
        /// # 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]);
        /// ```
        #[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_element(new_len - len, value);
            } else {
                self.truncate(new_len);
            }
        }

        /// Extend the vector by `n` additional clones of `value`.
        fn extend_with_element(&mut self, n: usize, value: T) {
            self.reserve(n);

            unsafe {
                let mut ptr = self.as_mut_ptr().offset(self.len() as isize);
                // 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.clone());
                    ptr = ptr.offset(1);
                    // Increment the length in every step in case clone() panics
                    local_len.increment_len(1);
                }

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

                // len set by scope guard
            }
        }

        /// 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]);
        /// ```
        #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
        pub fn extend_from_slice(&mut self, other: &[T]) {
            self.spec_extend(other.iter())
        }
    }

    // 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;
        }
    }

    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.
        ///
        /// 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 = match self.iter().position(|x| *x == *item) {
                Some(x) => x,
                None => return None,
            };
            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> {
        let mut v = Vec::with_capacity(n);
        v.extend_with_element(n, elem);
        v
    }

    ////////////////////////////////////////////////////////////////////////////////
    // 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>) {
            // drop anything in self that will not be overwritten
            self.truncate(other.len());
            let len = self.len();

            // reuse the contained values' allocations/resources.
            self.clone_from_slice(&other[..len]);

            // self.len <= other.len due to the truncate above, so the
            // slice here is always in-bounds.
            self.extend_from_slice(&other[len..]);
        }
    }

    #[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")]
    impl<T> Index<usize> for Vec<T> {
        type Output = T;

        #[inline]
        fn index(&self, index: usize) -> &T {
            // NB built-in indexing via `&[T]`
            &(**self)[index]
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> IndexMut<usize> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: usize) -> &mut T {
            // NB built-in indexing via `&mut [T]`
            &mut (**self)[index]
        }
    }

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

        #[inline]
        fn index(&self, index: ops::Range<usize>) -> &[T] {
            Index::index(&**self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::Index<ops::RangeTo<usize>> for Vec<T> {
        type Output = [T];

        #[inline]
        fn index(&self, index: ops::RangeTo<usize>) -> &[T] {
            Index::index(&**self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::Index<ops::RangeFrom<usize>> for Vec<T> {
        type Output = [T];

        #[inline]
        fn index(&self, index: ops::RangeFrom<usize>) -> &[T] {
            Index::index(&**self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::Index<ops::RangeFull> for Vec<T> {
        type Output = [T];

        #[inline]
        fn index(&self, _index: ops::RangeFull) -> &[T] {
            self
        }
    }
    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<T> ops::Index<ops::RangeInclusive<usize>> for Vec<T> {
        type Output = [T];

        #[inline]
        fn index(&self, index: ops::RangeInclusive<usize>) -> &[T] {
            Index::index(&**self, index)
        }
    }
    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<T> ops::Index<ops::RangeToInclusive<usize>> for Vec<T> {
        type Output = [T];

        #[inline]
        fn index(&self, index: ops::RangeToInclusive<usize>) -> &[T] {
            Index::index(&**self, index)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::IndexMut<ops::Range<usize>> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: ops::Range<usize>) -> &mut [T] {
            IndexMut::index_mut(&mut **self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::IndexMut<ops::RangeTo<usize>> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut [T] {
            IndexMut::index_mut(&mut **self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::IndexMut<ops::RangeFrom<usize>> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut [T] {
            IndexMut::index_mut(&mut **self, index)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ops::IndexMut<ops::RangeFull> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, _index: ops::RangeFull) -> &mut [T] {
            self
        }
    }
    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<T> ops::IndexMut<ops::RangeInclusive<usize>> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut [T] {
            IndexMut::index_mut(&mut **self, index)
        }
    }
    #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")]
    impl<T> ops::IndexMut<ops::RangeToInclusive<usize>> for Vec<T> {
        #[inline]
        fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut [T] {
            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.offset(self.len() as isize) as *const T
                };
                let cap = self.buf.cap();
                mem::forget(self);
                IntoIter {
                    buf: Shared::new(begin),
                    cap: cap,
                    ptr: begin,
                    end: 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(mut 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
        }

        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().offset(self.len() as isize);
                    let mut local_len = SetLenOnDrop::new(&mut self.len);
                    for element in iterator {
                        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 == iterator.ptr as *mut T {
                unsafe {
                    let vec = Vec::from_raw_parts(*iterator.buf as *mut T,
                                                  iterator.len(),
                                                  iterator.cap);
                    mem::forget(iterator);
                    vec
                }
            } else {
                let mut vector = Vec::new();
                vector.spec_extend(iterator);
                vector
            }
        }
    }

    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);
                }
            }
        }
    }

    #[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_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 = "1.17.0")]
    impl<T> From<Box<[T]>> for Vec<T> {
        fn from(s: Box<[T]>) -> Vec<T> {
            s.into_vec()
        }
    }

    #[stable(feature = "box_from_vec", since = "1.17.0")]
    impl<T> Into<Box<[T]>> for Vec<T> {
        fn into(self) -> Box<[T]> {
            self.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 = "1.7.0")]
    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 = "1.7.0")]
    impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]> {
        fn from(v: Vec<T>) -> Cow<'a, [T]> {
            Cow::Owned(v)
        }
    }

    #[stable(feature = "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: Shared<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;

                        // Use a non-null pointer value
                        Some(ptr::read(EMPTY as *mut T))
                    } 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 = match self.ptr.offset_to(self.end) {
                Some(x) => x as usize,
                None => (self.end as usize).wrapping_sub(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;

                        // Use a non-null pointer value
                        Some(ptr::read(EMPTY as *mut T))
                    } 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
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    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_mut_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: Shared<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
            while let Some(_) = self.next() {}

            if self.tail_len > 0 {
                unsafe {
                    let source_vec = &mut *self.vec.as_mut_ptr();
                    // memmove back untouched tail, update to new length
                    let start = source_vec.len();
                    let tail = self.tail_start;
                    let src = source_vec.as_ptr().offset(tail as isize);
                    let dst = source_vec.as_mut_ptr().offset(start as isize);
                    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()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<'a, T> FusedIterator for Drain<'a, T> {}

    /// A place for insertion at the back of a `Vec`.
    ///
    /// See [`Vec::place_back`](struct.Vec.html#method.place_back) for details.
    #[must_use = "places do nothing unless written to with `<-` syntax"]
    #[unstable(feature = "collection_placement",
               reason = "struct name and placement protocol are subject to change",
               issue = "30172")]
    #[derive(Debug)]
    pub struct PlaceBack<'a, T: 'a> {
        vec: &'a mut Vec<T>,
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Placer<T> for PlaceBack<'a, T> {
        type Place = PlaceBack<'a, T>;

        fn make_place(self) -> Self {
            // 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.vec.len == self.vec.buf.cap() {
                self.vec.buf.double();
            }
            self
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> Place<T> for PlaceBack<'a, T> {
        fn pointer(&mut self) -> *mut T {
            unsafe { self.vec.as_mut_ptr().offset(self.vec.len as isize) }
        }
    }

    #[unstable(feature = "collection_placement",
               reason = "placement protocol is subject to change",
               issue = "30172")]
    impl<'a, T> InPlace<T> for PlaceBack<'a, T> {
        type Owner = &'a mut T;

        unsafe fn finalize(mut self) -> &'a mut T {
            let ptr = self.pointer();
            self.vec.len += 1;
            &mut *ptr
        }
    }

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