vec.rs

// ignore-tidy-filelength
//! 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).
//!
//! Vectors ensure they never allocate more than `isize::MAX` bytes.
//!
//! # Examples
//!
//! You can explicitly create a [`Vec`] with [`Vec::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;
//! ```
//!
//! [`push`]: Vec::push

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

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

use crate::borrow::{Cow, ToOwned};
use crate::boxed::Box;
use crate::collections::TryReserveError;
use crate::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().copied());
///
/// 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 vec = Vec::with_capacity(5);
/// vec.resize(5, 0);
/// assert_eq!(vec, [0, 0, 0, 0, 0]);
/// ```
///
/// For more information, see
/// [Capacity and Reallocation](#capacity-and-reallocation).
///
/// 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!
/// ```
///
/// Use [`get`] and [`get_mut`] if you want to check whether the index is in
/// the `Vec`.
///
/// # 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 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`].
///
/// [`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.
///
/// [`get`]: ../../std/vec/struct.Vec.html#method.get
/// [`get_mut`]: ../../std/vec/struct.Vec.html#method.get_mut
/// [`String`]: crate::string::String
/// [`&str`]: type@str
/// [`shrink_to_fit`]: Vec::shrink_to_fit
/// [`capacity`]: Vec::capacity
/// [`mem::size_of::<T>`]: core::mem::size_of
/// [`len`]: Vec::len
/// [`push`]: Vec::push
/// [`insert`]: Vec::insert
/// [`reserve`]: Vec::reserve
/// [owned slice]: Box
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "vec_type")]
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]
    #[rustc_const_stable(feature = "const_vec_new", since = "1.39.0")]
    #[stable(feature = "rust1", since = "1.0.0")]
    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);
    /// assert_eq!(vec.capacity(), 10);
    ///
    /// // These are all done without reallocating...
    /// for i in 0..10 {
    ///     vec.push(i);
    /// }
    /// assert_eq!(vec.len(), 10);
    /// assert_eq!(vec.capacity(), 10);
    ///
    /// // ...but this may make the vector reallocate
    /// vec.push(11);
    /// assert_eq!(vec.len(), 11);
    /// assert!(vec.capacity() >= 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 }
    }

    /// Decomposes a `Vec<T>` into its raw components.
    ///
    /// Returns the raw pointer to the underlying data, the length of
    /// the vector (in elements), and the allocated capacity of the
    /// data (in elements). These are the same arguments in the same
    /// order as the arguments to [`from_raw_parts`].
    ///
    /// After calling this function, the caller is responsible for the
    /// memory previously managed by the `Vec`. The only way to do
    /// this is to convert the raw pointer, length, and capacity back
    /// into a `Vec` with the [`from_raw_parts`] function, allowing
    /// the destructor to perform the cleanup.
    ///
    /// [`from_raw_parts`]: Vec::from_raw_parts
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(vec_into_raw_parts)]
    /// let v: Vec<i32> = vec![-1, 0, 1];
    ///
    /// let (ptr, len, cap) = v.into_raw_parts();
    ///
    /// let rebuilt = unsafe {
    ///     // We can now make changes to the components, such as
    ///     // transmuting the raw pointer to a compatible type.
    ///     let ptr = ptr as *mut u32;
    ///
    ///     Vec::from_raw_parts(ptr, len, cap)
    /// };
    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
    /// ```
    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
        let mut me = ManuallyDrop::new(self);
        (me.as_mut_ptr(), me.len(), me.capacity())
    }

    /// 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).
    /// * `T` needs to have the same size and alignment as what `ptr` was allocated with.
    ///   (`T` having a less strict alignment is not sufficient, the alignment really
    ///   needs to be equal to satsify the [`dealloc`] requirement that memory must be
    ///   allocated and deallocated with the same layout.)
    /// * `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 with length `size_t`.
    /// It's also not safe to build one from a `Vec<u16>` and its length, because
    /// the allocator cares about the alignment, and these two types have different
    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
    ///
    /// 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`]: crate::string::String
    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    /// use std::mem;
    ///
    /// let v = vec![1, 2, 3];
    ///
    // FIXME Update this when vec_into_raw_parts is stabilized
    /// // Prevent running `v`'s destructor so we are in complete control
    /// // of the allocation.
    /// let mut v = mem::ManuallyDrop::new(v);
    ///
    /// // 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 {
    ///     // 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> {
        unsafe { 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.capacity()
    }

    /// 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 exceeds `isize::MAX` bytes.
    ///
    /// # 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 `try_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::TryReserveError;
    ///
    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
    ///     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<(), TryReserveError> {
        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 `try_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::TryReserveError;
    ///
    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
    ///     let mut output = Vec::new();
    ///
    ///     // Pre-reserve the memory, exiting if we can't
    ///     output.try_reserve_exact(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<(), TryReserveError> {
        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) {
        // The capacity is never less than the length, and there's nothing to do when
        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
        // by only calling it with a greater capacity.
        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
    ///
    /// 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]: Box
    ///
    /// # 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 me = ManuallyDrop::new(self);
            let buf = ptr::read(&me.buf);
            let len = me.len();
            buf.into_box(len).assume_init()
        }
    }

    /// 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`]: Vec::clear
    /// [`drain`]: Vec::drain
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn truncate(&mut self, len: usize) {
        // This is safe because:
        //
        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
        //   case avoids creating an invalid slice, and
        // * the `len` of the vector is shrunk before calling `drop_in_place`,
        //   such that no value will be dropped twice in case `drop_in_place`
        //   were to panic once (if it panics twice, the program aborts).
        unsafe {
            if len > self.len {
                return;
            }
            let remaining_len = self.len - len;
            let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
            self.len = len;
            ptr::drop_in_place(s);
        }
    }

    /// 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
    }

    /// Returns a raw pointer to the vector's buffer.
    ///
    /// The caller must ensure that the vector outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    /// Modifying the vector may cause its buffer to be reallocated,
    /// which would also make any pointers to it invalid.
    ///
    /// The caller must also ensure that the memory the pointer (non-transitively) points to
    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
    ///
    /// # Examples
    ///
    /// ```
    /// let x = vec![1, 2, 4];
    /// let x_ptr = x.as_ptr();
    ///
    /// unsafe {
    ///     for i in 0..x.len() {
    ///         assert_eq!(*x_ptr.add(i), 1 << i);
    ///     }
    /// }
    /// ```
    ///
    /// [`as_mut_ptr`]: Vec::as_mut_ptr
    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
    #[inline]
    pub fn as_ptr(&self) -> *const T {
        // We shadow the slice method of the same name to avoid going through
        // `deref`, which creates an intermediate reference.
        let ptr = self.buf.ptr();
        unsafe {
            assume(!ptr.is_null());
        }
        ptr
    }

    /// Returns an unsafe mutable pointer to the vector's buffer.
    ///
    /// The caller must ensure that the vector outlives the pointer this
    /// function returns, or else it will end up pointing to garbage.
    /// Modifying the vector may cause its buffer to be reallocated,
    /// which would also make any pointers to it invalid.
    ///
    /// # Examples
    ///
    /// ```
    /// // Allocate vector big enough for 4 elements.
    /// let size = 4;
    /// let mut x: Vec<i32> = Vec::with_capacity(size);
    /// let x_ptr = x.as_mut_ptr();
    ///
    /// // Initialize elements via raw pointer writes, then set length.
    /// unsafe {
    ///     for i in 0..size {
    ///         *x_ptr.add(i) = i as i32;
    ///     }
    ///     x.set_len(size);
    /// }
    /// assert_eq!(&*x, &[0,1,2,3]);
    /// ```
    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
    #[inline]
    pub fn as_mut_ptr(&mut self) -> *mut T {
        // We shadow the slice method of the same name to avoid going through
        // `deref_mut`, which creates an intermediate reference.
        let ptr = self.buf.ptr();
        unsafe {
            assume(!ptr.is_null());
        }
        ptr
    }

    /// 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`]: Vec::truncate
    /// [`resize`]: Vec::resize
    /// [`extend`]: Extend::extend
    /// [`clear`]: Vec::clear
    ///
    /// # Safety
    ///
    /// - `new_len` must be less than or equal to [`capacity()`].
    /// - The elements at `old_len..new_len` must be initialized.
    ///
    /// [`capacity()`]: Vec::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 {
        #[cold]
        #[inline(never)]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("swap_remove index (is {}) should be < len (is {})", index, len);
        }

        let len = self.len();
        if index >= len {
            assert_failed(index, len);
        }
        unsafe {
            // We replace self[index] with the last element. Note that if the
            // bounds check above succeeds there must be a last element (which
            // can be self[index] itself).
            let last = ptr::read(self.as_ptr().add(len - 1));
            let hole = self.as_mut_ptr().add(index);
            self.set_len(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) {
        #[cold]
        #[inline(never)]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("insertion index (is {}) should be <= len (is {})", index, len);
        }

        let len = self.len();
        if index > len {
            assert_failed(index, len);
        }

        // space for the new element
        if len == self.buf.capacity() {
            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 {
        #[cold]
        #[inline(never)]
        fn assert_failed(index: usize, len: usize) -> ! {
            panic!("removal index (is {}) should be < len (is {})", index, len);
        }

        let len = self.len();
        if index >= len {
            assert_failed(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, visiting each element exactly once in the
    /// original order, 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]);
    /// ```
    ///
    /// The exact order may be useful for tracking external state, like an index.
    ///
    /// ```
    /// let mut vec = vec![1, 2, 3, 4, 5];
    /// let keep = [false, true, true, false, true];
    /// let mut i = 0;
    /// vec.retain(|_| (keep[i], i += 1).0);
    /// assert_eq!(vec, [2, 3, 5]);
    /// ```
    #[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 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 new capacity exceeds `isize::MAX` bytes.
    ///
    /// # 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.capacity() {
            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.
    ///
    /// # 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.as_ptr().add(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 = unsafe { (*other).len() };
        self.reserve(count);
        let len = self.len();
        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
        self.len += count;
    }

    /// Creates a draining iterator that removes the specified range in the vector
    /// and yields the removed items.
    ///
    /// When the iterator **is** dropped, all elements in the range are removed
    /// from the vector, even if the iterator was not fully consumed. If the
    /// iterator **is not** dropped (with [`mem::forget`] for example), it is
    /// unspecified how many elements are removed.
    ///
    /// # 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,
        };

        #[cold]
        #[inline(never)]
        fn start_assert_failed(start: usize, end: usize) -> ! {
            panic!("start drain index (is {}) should be <= end drain index (is {})", start, end);
        }

        #[cold]
        #[inline(never)]
        fn end_assert_failed(end: usize, len: usize) -> ! {
            panic!("end drain index (is {}) should be <= len (is {})", end, len);
        }

        if start > end {
            start_assert_failed(start, end);
        }
        if end > len {
            end_assert_failed(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 vector containing the elements in the range
    /// `[at, len)`. After the call, the original vector will be left containing
    /// the elements `[0, at)` with its previous capacity unchanged.
    ///
    /// # 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]
    #[must_use = "use `.truncate()` if you don't need the other half"]
    #[stable(feature = "split_off", since = "1.4.0")]
    pub fn split_off(&mut self, at: usize) -> Self {
        #[cold]
        #[inline(never)]
        fn assert_failed(at: usize, len: usize) -> ! {
            panic!("`at` split index (is {}) should be <= len (is {})", at, len);
        }

        if at > self.len() {
            assert_failed(at, self.len());
        }

        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 [`Vec::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]);
    /// ```
    #[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);
        }
    }

    /// Consumes and leaks the `Vec`, returning a mutable reference to the contents,
    /// `&'a mut [T]`. Note that the type `T` must outlive the chosen lifetime
    /// `'a`. If the type has only static references, or none at all, then this
    /// may be chosen to be `'static`.
    ///
    /// This function is similar to the `leak` function on `Box`.
    ///
    /// This function is mainly useful for data that lives for the remainder of
    /// the program's life. Dropping the returned reference will cause a memory
    /// leak.
    ///
    /// # Examples
    ///
    /// Simple usage:
    ///
    /// ```
    /// let x = vec![1, 2, 3];
    /// let static_ref: &'static mut [usize] = x.leak();
    /// static_ref[0] += 1;
    /// assert_eq!(static_ref, &[2, 2, 3]);
    /// ```
    #[stable(feature = "vec_leak", since = "1.47.0")]
    #[inline]
    pub fn leak<'a>(self) -> &'a mut [T]
    where
        T: 'a, // Technically not needed, but kept to be explicit.
    {
        Box::leak(self.into_boxed_slice())
    }

    /// Returns the remaining spare capacity of the vector as a slice of
    /// `MaybeUninit<T>`.
    ///
    /// The returned slice can be used to fill the vector with data (e.g. by
    /// reading from a file) before marking the data as initialized using the
    /// [`set_len`] method.
    ///
    /// [`set_len`]: Vec::set_len
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(vec_spare_capacity, maybe_uninit_extra)]
    ///
    /// // Allocate vector big enough for 10 elements.
    /// let mut v = Vec::with_capacity(10);
    ///
    /// // Fill in the first 3 elements.
    /// let uninit = v.spare_capacity_mut();
    /// uninit[0].write(0);
    /// uninit[1].write(1);
    /// uninit[2].write(2);
    ///
    /// // Mark the first 3 elements of the vector as being initialized.
    /// unsafe {
    ///     v.set_len(3);
    /// }
    ///
    /// assert_eq!(&v, &[0, 1, 2]);
    /// ```
    #[unstable(feature = "vec_spare_capacity", issue = "75017")]
    #[inline]
    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
        unsafe {
            slice::from_raw_parts_mut(
                self.as_mut_ptr().add(self.len) as *mut MaybeUninit<T>,
                self.buf.capacity() - self.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 `T` to implement [`Clone`],
    /// in order to be able to clone the passed value.
    /// If you need more flexibility (or want to rely on [`Default`] instead of
    /// [`Clone`]), use [`Vec::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]);
    /// ```
    #[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`]: Vec::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
    ///
    /// ```
    /// # #![allow(deprecated)]
    /// #![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`]: Vec::resize
    #[unstable(feature = "vec_resize_default", issue = "41758")]
    #[rustc_deprecated(
        reason = "This is moving towards being removed in favor \
                  of `.resize_with(Default::default)`.  If you disagree, please comment \
                  in the tracking issue.",
        since = "1.33.0"
    )]
    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 generalizes `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 }
    }

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

impl Drop for SetLenOnDrop<'_> {
    #[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)
    }
}

impl<T> Vec<T> {
    /// Removes the first instance of `item` from the vector if the item exists.
    ///
    /// This method will be removed soon.
    #[unstable(feature = "vec_remove_item", reason = "recently added", issue = "40062")]
    #[rustc_deprecated(
        reason = "Removing the first item equal to a needle is already easily possible \
            with iterators and the current Vec methods. Furthermore, having a method for \
            one particular case of removal (linear search, only the first item, no swap remove) \
            but not for others is inconsistent. This method will be removed soon.",
        since = "1.46.0"
    )]
    pub fn remove_item<V>(&mut self, item: &V) -> Option<T>
    where
        T: PartialEq<V>,
    {
        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 i8 {
    #[inline]
    fn from_elem(elem: i8, n: usize) -> Vec<i8> {
        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 as u8, n);
            v.set_len(n);
            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
    }
}

#[rustc_specialization_trait]
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!(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!(bool, |x| x == false);
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> IsZero for *const T {
    #[inline]
    fn is_zero(&self) -> bool {
        (*self).is_null()
    }
}

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

// `Option<&T>` and `Option<Box<T>>` are guaranteed to represent `None` as null.
// For fat pointers, the bytes that would be the pointer metadata in the `Some`
// variant are padding in the `None` variant, so ignoring them and
// zero-initializing instead is ok.
// `Option<&mut T>` never implements `Clone`, so there's no need for an impl of
// `SpecFromElem`.

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

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

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

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

    fn deref(&self) -> &[T] {
        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
    }
}

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

#[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> {
        crate::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: 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: SliceIndex<[T]>> Index<I> for Vec<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: SliceIndex<[T]>> IndexMut<I> for Vec<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> 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(self) -> IntoIter<T> {
        unsafe {
            let mut me = ManuallyDrop::new(self);
            let begin = me.as_mut_ptr();
            let end = if mem::size_of::<T>() == 0 {
                arith_offset(begin as *const i8, me.len() as isize) as *const T
            } else {
                begin.add(me.len()) as *const T
            };
            let cap = me.buf.capacity();
            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())
    }

    #[inline]
    fn extend_one(&mut self, item: T) {
        self.push(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

// 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.as_mut_ptr(), 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 it = ManuallyDrop::new(iterator);
                Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap)
            }
        } 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();
            let dst_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(len), slice.len());
            dst_slice.copy_from_slice(slice);
            self.set_len(len + slice.len());
        }
    }
}

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.as_mut_ptr().add(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`.
    ///
    /// `range` is removed even if the iterator is not consumed until the end.
    ///
    /// It is unspecified how many elements are removed from the vector
    /// if the `Splice` value is leaked.
    ///
    /// The input iterator `replace_with` is only consumed when the `Splice` value is dropped.
    ///
    /// 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, panic_flag: false }
    }
}

/// 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())
    }

    #[inline]
    fn extend_one(&mut self, &item: &'a T) {
        self.push(item);
    }

    #[inline]
    fn extend_reserve(&mut self, additional: usize) {
        self.reserve(additional);
    }
}

macro_rules! __impl_slice_eq1 {
    ([$($vars:tt)*] $lhs:ty, $rhs:ty $(where $ty:ty: $bound:ident)?, #[$stability:meta]) => {
        #[$stability]
        impl<A, B, $($vars)*> PartialEq<$rhs> for $lhs
        where
            A: PartialEq<B>,
            $($ty: $bound)?
        {
            #[inline]
            fn eq(&self, other: &$rhs) -> bool { self[..] == other[..] }
            #[inline]
            fn ne(&self, other: &$rhs) -> bool { self[..] != other[..] }
        }
    }
}

__impl_slice_eq1! { [] Vec<A>, Vec<B>, #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [] Vec<A>, &[B], #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [] Vec<A>, &mut [B], #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [] &[A], Vec<B>, #[stable(feature = "partialeq_vec_for_ref_slice", since = "1.46.0")] }
__impl_slice_eq1! { [] &mut [A], Vec<B>, #[stable(feature = "partialeq_vec_for_ref_slice", since = "1.46.0")] }
__impl_slice_eq1! { [] Cow<'_, [A]>, Vec<B> where A: Clone, #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [] Cow<'_, [A]>, &[B] where A: Clone, #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [] Cow<'_, [A]>, &mut [B] where A: Clone, #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [const N: usize] Vec<A>, [B; N], #[stable(feature = "rust1", since = "1.0.0")] }
__impl_slice_eq1! { [const N: usize] Vec<A>, &[B; N], #[stable(feature = "rust1", since = "1.0.0")] }

// NOTE: some less important impls are omitted to reduce code bloat
// FIXME(Centril): Reconsider this?
//__impl_slice_eq1! { [const N: usize] Vec<A>, &mut [B; N], }
//__impl_slice_eq1! { [const N: usize] [A; N], Vec<B>, }
//__impl_slice_eq1! { [const N: usize] &[A; N], Vec<B>, }
//__impl_slice_eq1! { [const N: usize] &mut [A; N], Vec<B>, }
//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, [B; N], }
//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &[B; N], }
//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &mut [B; N], }

/// 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]
            // use a raw slice to refer to the elements of the vector as weakest necessary type;
            // could avoid questions of validity in certain cases
            ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
        }
        // 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<T: Clone> From<&[T]> for Vec<T> {
    #[cfg(not(test))]
    fn from(s: &[T]) -> Vec<T> {
        s.to_vec()
    }
    #[cfg(test)]
    fn from(s: &[T]) -> Vec<T> {
        crate::slice::to_vec(s)
    }
}

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

#[stable(feature = "vec_from_array", since = "1.44.0")]
impl<T, const N: usize> From<[T; N]> for Vec<T> {
    #[cfg(not(test))]
    fn from(s: [T; N]) -> Vec<T> {
        <[T]>::into_vec(box s)
    }
    #[cfg(test)]
    fn from(s: [T; N]) -> Vec<T> {
        crate::slice::into_vec(box 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 From<&str> for Vec<u8> {
    fn from(s: &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`] (provided
/// by the [`IntoIterator`] trait).
#[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 { &mut *self.as_raw_mut_slice() }
    }

    fn as_raw_mut_slice(&mut self) -> *mut [T] {
        ptr::slice_from_raw_parts_mut(self.ptr as *mut T, self.len())
    }
}

#[stable(feature = "vec_intoiter_as_ref", since = "1.46.0")]
impl<T> AsRef<[T]> for IntoIter<T> {
    fn as_ref(&self) -> &[T] {
        self.as_slice()
    }
}

#[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) {
        struct DropGuard<'a, T>(&'a mut IntoIter<T>);

        impl<T> Drop for DropGuard<'_, T> {
            fn drop(&mut self) {
                // RawVec handles deallocation
                let _ = unsafe { RawVec::from_raw_parts(self.0.buf.as_ptr(), self.0.cap) };
            }
        }

        let guard = DropGuard(self);
        // destroy the remaining elements
        unsafe {
            ptr::drop_in_place(guard.0.as_raw_mut_slice());
        }
        // now `guard` will be dropped and do the rest
    }
}

/// A draining iterator for `Vec<T>`.
///
/// This `struct` is created by [`Vec::drain`].
#[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<T: fmt::Debug> fmt::Debug for Drain<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
    }
}

impl<'a, T> Drain<'a, T> {
    /// Returns the remaining items of this iterator as a slice.
    ///
    /// # Examples
    ///
    /// ```
    /// let mut vec = vec!['a', 'b', 'c'];
    /// let mut drain = vec.drain(..);
    /// assert_eq!(drain.as_slice(), &['a', 'b', 'c']);
    /// let _ = drain.next().unwrap();
    /// assert_eq!(drain.as_slice(), &['b', 'c']);
    /// ```
    #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
    pub fn as_slice(&self) -> &[T] {
        self.iter.as_slice()
    }
}

#[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
impl<'a, T> AsRef<[T]> for Drain<'a, T> {
    fn as_ref(&self) -> &[T] {
        self.as_slice()
    }
}

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

#[stable(feature = "drain", since = "1.6.0")]
impl<T> Iterator for Drain<'_, 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<T> DoubleEndedIterator for Drain<'_, 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<T> Drop for Drain<'_, T> {
    fn drop(&mut self) {
        /// Continues dropping the remaining elements in the `Drain`, then moves back the
        /// un-`Drain`ed elements to restore the original `Vec`.
        struct DropGuard<'r, 'a, T>(&'r mut Drain<'a, T>);

        impl<'r, 'a, T> Drop for DropGuard<'r, 'a, T> {
            fn drop(&mut self) {
                // Continue the same loop we have below. If the loop already finished, this does
                // nothing.
                self.0.for_each(drop);

                if self.0.tail_len > 0 {
                    unsafe {
                        let source_vec = self.0.vec.as_mut();
                        // memmove back untouched tail, update to new length
                        let start = source_vec.len();
                        let tail = self.0.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.0.tail_len);
                        }
                        source_vec.set_len(start + self.0.tail_len);
                    }
                }
            }
        }

        // exhaust self first
        while let Some(item) = self.next() {
            let guard = DropGuard(self);
            drop(item);
            mem::forget(guard);
        }

        // Drop a `DropGuard` to move back the non-drained tail of `self`.
        DropGuard(self);
    }
}

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

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<T> TrustedLen for Drain<'_, T> {}

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

/// A splicing iterator for `Vec`.
///
/// This struct is created by [`Vec::splice()`].
/// See its documentation for more.
#[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<I: Iterator> Iterator for Splice<'_, 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<I: Iterator> DoubleEndedIterator for Splice<'_, I> {
    fn next_back(&mut self) -> Option<Self::Item> {
        self.drain.next_back()
    }
}

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

#[stable(feature = "vec_splice", since = "1.21.0")]
impl<I: Iterator> Drop for Splice<'_, 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<T> Drain<'_, 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.
    /// Returns `true` if 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 = unsafe { self.vec.as_mut() };
        let range_start = vec.len;
        let range_end = self.tail_start;
        let range_slice = unsafe {
            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() {
                unsafe { ptr::write(place, new_item) };
                vec.len += 1;
            } else {
                return false;
            }
        }
        true
    }

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

        let new_tail_start = self.tail_start + additional;
        unsafe {
            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 which uses a closure to determine if an element should be removed.
///
/// This struct is created by [`Vec::drain_filter`].
/// See its documentation for more.
#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
#[derive(Debug)]
pub struct DrainFilter<'a, T, F>
where
    F: FnMut(&mut T) -> bool,
{
    vec: &'a mut Vec<T>,
    /// The index of the item that will be inspected by the next call to `next`.
    idx: usize,
    /// The number of items that have been drained (removed) thus far.
    del: usize,
    /// The original length of `vec` prior to draining.
    old_len: usize,
    /// The filter test predicate.
    pred: F,
    /// A flag that indicates a panic has occurred in the filter test prodicate.
    /// This is used as a hint in the drop implementation to prevent consumption
    /// of the remainder of the `DrainFilter`. Any unprocessed items will be
    /// backshifted in the `vec`, but no further items will be dropped or
    /// tested by the filter predicate.
    panic_flag: bool,
}

#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
impl<T, F> Iterator for DrainFilter<'_, 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;
                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
                self.panic_flag = true;
                let drained = (self.pred)(&mut v[i]);
                self.panic_flag = false;
                // Update the index *after* the predicate is called. If the index
                // is updated prior and the predicate panics, the element at this
                // index would be leaked.
                self.idx += 1;
                if drained {
                    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];
                    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<T, F> Drop for DrainFilter<'_, T, F>
where
    F: FnMut(&mut T) -> bool,
{
    fn drop(&mut self) {
        struct BackshiftOnDrop<'a, 'b, T, F>
        where
            F: FnMut(&mut T) -> bool,
        {
            drain: &'b mut DrainFilter<'a, T, F>,
        }

        impl<'a, 'b, T, F> Drop for BackshiftOnDrop<'a, 'b, T, F>
        where
            F: FnMut(&mut T) -> bool,
        {
            fn drop(&mut self) {
                unsafe {
                    if self.drain.idx < self.drain.old_len && self.drain.del > 0 {
                        // This is a pretty messed up state, and there isn't really an
                        // obviously right thing to do. We don't want to keep trying
                        // to execute `pred`, so we just backshift all the unprocessed
                        // elements and tell the vec that they still exist. The backshift
                        // is required to prevent a double-drop of the last successfully
                        // drained item prior to a panic in the predicate.
                        let ptr = self.drain.vec.as_mut_ptr();
                        let src = ptr.add(self.drain.idx);
                        let dst = src.sub(self.drain.del);
                        let tail_len = self.drain.old_len - self.drain.idx;
                        src.copy_to(dst, tail_len);
                    }
                    self.drain.vec.set_len(self.drain.old_len - self.drain.del);
                }
            }
        }

        let backshift = BackshiftOnDrop { drain: self };

        // Attempt to consume any remaining elements if the filter predicate
        // has not yet panicked. We'll backshift any remaining elements
        // whether we've already panicked or if the consumption here panics.
        if !backshift.drain.panic_flag {
            backshift.drain.for_each(drop);
        }
    }
}