alloc/vec/
mod.rs

1//! A contiguous growable array type with heap-allocated contents, written
2//! `Vec<T>`.
3//!
4//! Vectors have *O*(1) indexing, amortized *O*(1) push (to the end) and
5//! *O*(1) pop (from the end).
6//!
7//! Vectors ensure they never allocate more than `isize::MAX` bytes.
8//!
9//! # Examples
10//!
11//! You can explicitly create a [`Vec`] with [`Vec::new`]:
12//!
13//! ```
14//! let v: Vec<i32> = Vec::new();
15//! ```
16//!
17//! ...or by using the [`vec!`] macro:
18//!
19//! ```
20//! let v: Vec<i32> = vec![];
21//!
22//! let v = vec![1, 2, 3, 4, 5];
23//!
24//! let v = vec![0; 10]; // ten zeroes
25//! ```
26//!
27//! You can [`push`] values onto the end of a vector (which will grow the vector
28//! as needed):
29//!
30//! ```
31//! let mut v = vec![1, 2];
32//!
33//! v.push(3);
34//! ```
35//!
36//! Popping values works in much the same way:
37//!
38//! ```
39//! let mut v = vec![1, 2];
40//!
41//! let two = v.pop();
42//! ```
43//!
44//! Vectors also support indexing (through the [`Index`] and [`IndexMut`] traits):
45//!
46//! ```
47//! let mut v = vec![1, 2, 3];
48//! let three = v[2];
49//! v[1] = v[1] + 5;
50//! ```
51//!
52//! [`push`]: Vec::push
53
54#![stable(feature = "rust1", since = "1.0.0")]
55
56#[cfg(not(no_global_oom_handling))]
57use core::cmp;
58use core::cmp::Ordering;
59use core::hash::{Hash, Hasher};
60#[cfg(not(no_global_oom_handling))]
61use core::iter;
62use core::marker::PhantomData;
63use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
64use core::ops::{self, Index, IndexMut, Range, RangeBounds};
65use core::ptr::{self, NonNull};
66use core::slice::{self, SliceIndex};
67use core::{fmt, intrinsics};
68
69#[stable(feature = "extract_if", since = "1.87.0")]
70pub use self::extract_if::ExtractIf;
71use crate::alloc::{Allocator, Global};
72use crate::borrow::{Cow, ToOwned};
73use crate::boxed::Box;
74use crate::collections::TryReserveError;
75use crate::raw_vec::RawVec;
76
77mod extract_if;
78
79#[cfg(not(no_global_oom_handling))]
80#[stable(feature = "vec_splice", since = "1.21.0")]
81pub use self::splice::Splice;
82
83#[cfg(not(no_global_oom_handling))]
84mod splice;
85
86#[stable(feature = "drain", since = "1.6.0")]
87pub use self::drain::Drain;
88
89mod drain;
90
91#[cfg(not(no_global_oom_handling))]
92mod cow;
93
94#[cfg(not(no_global_oom_handling))]
95pub(crate) use self::in_place_collect::AsVecIntoIter;
96#[stable(feature = "rust1", since = "1.0.0")]
97pub use self::into_iter::IntoIter;
98
99mod into_iter;
100
101#[cfg(not(no_global_oom_handling))]
102use self::is_zero::IsZero;
103
104#[cfg(not(no_global_oom_handling))]
105mod is_zero;
106
107#[cfg(not(no_global_oom_handling))]
108mod in_place_collect;
109
110mod partial_eq;
111
112#[unstable(feature = "vec_peek_mut", issue = "122742")]
113pub use self::peek_mut::PeekMut;
114
115mod peek_mut;
116
117#[cfg(not(no_global_oom_handling))]
118use self::spec_from_elem::SpecFromElem;
119
120#[cfg(not(no_global_oom_handling))]
121mod spec_from_elem;
122
123#[cfg(not(no_global_oom_handling))]
124use self::set_len_on_drop::SetLenOnDrop;
125
126#[cfg(not(no_global_oom_handling))]
127mod set_len_on_drop;
128
129#[cfg(not(no_global_oom_handling))]
130use self::in_place_drop::{InPlaceDrop, InPlaceDstDataSrcBufDrop};
131
132#[cfg(not(no_global_oom_handling))]
133mod in_place_drop;
134
135#[cfg(not(no_global_oom_handling))]
136use self::spec_from_iter_nested::SpecFromIterNested;
137
138#[cfg(not(no_global_oom_handling))]
139mod spec_from_iter_nested;
140
141#[cfg(not(no_global_oom_handling))]
142use self::spec_from_iter::SpecFromIter;
143
144#[cfg(not(no_global_oom_handling))]
145mod spec_from_iter;
146
147#[cfg(not(no_global_oom_handling))]
148use self::spec_extend::SpecExtend;
149
150#[cfg(not(no_global_oom_handling))]
151mod spec_extend;
152
153/// A contiguous growable array type, written as `Vec<T>`, short for 'vector'.
154///
155/// # Examples
156///
157/// ```
158/// let mut vec = Vec::new();
159/// vec.push(1);
160/// vec.push(2);
161///
162/// assert_eq!(vec.len(), 2);
163/// assert_eq!(vec[0], 1);
164///
165/// assert_eq!(vec.pop(), Some(2));
166/// assert_eq!(vec.len(), 1);
167///
168/// vec[0] = 7;
169/// assert_eq!(vec[0], 7);
170///
171/// vec.extend([1, 2, 3]);
172///
173/// for x in &vec {
174///     println!("{x}");
175/// }
176/// assert_eq!(vec, [7, 1, 2, 3]);
177/// ```
178///
179/// The [`vec!`] macro is provided for convenient initialization:
180///
181/// ```
182/// let mut vec1 = vec![1, 2, 3];
183/// vec1.push(4);
184/// let vec2 = Vec::from([1, 2, 3, 4]);
185/// assert_eq!(vec1, vec2);
186/// ```
187///
188/// It can also initialize each element of a `Vec<T>` with a given value.
189/// This may be more efficient than performing allocation and initialization
190/// in separate steps, especially when initializing a vector of zeros:
191///
192/// ```
193/// let vec = vec![0; 5];
194/// assert_eq!(vec, [0, 0, 0, 0, 0]);
195///
196/// // The following is equivalent, but potentially slower:
197/// let mut vec = Vec::with_capacity(5);
198/// vec.resize(5, 0);
199/// assert_eq!(vec, [0, 0, 0, 0, 0]);
200/// ```
201///
202/// For more information, see
203/// [Capacity and Reallocation](#capacity-and-reallocation).
204///
205/// Use a `Vec<T>` as an efficient stack:
206///
207/// ```
208/// let mut stack = Vec::new();
209///
210/// stack.push(1);
211/// stack.push(2);
212/// stack.push(3);
213///
214/// while let Some(top) = stack.pop() {
215///     // Prints 3, 2, 1
216///     println!("{top}");
217/// }
218/// ```
219///
220/// # Indexing
221///
222/// The `Vec` type allows access to values by index, because it implements the
223/// [`Index`] trait. An example will be more explicit:
224///
225/// ```
226/// let v = vec![0, 2, 4, 6];
227/// println!("{}", v[1]); // it will display '2'
228/// ```
229///
230/// However be careful: if you try to access an index which isn't in the `Vec`,
231/// your software will panic! You cannot do this:
232///
233/// ```should_panic
234/// let v = vec![0, 2, 4, 6];
235/// println!("{}", v[6]); // it will panic!
236/// ```
237///
238/// Use [`get`] and [`get_mut`] if you want to check whether the index is in
239/// the `Vec`.
240///
241/// # Slicing
242///
243/// A `Vec` can be mutable. On the other hand, slices are read-only objects.
244/// To get a [slice][prim@slice], use [`&`]. Example:
245///
246/// ```
247/// fn read_slice(slice: &[usize]) {
248///     // ...
249/// }
250///
251/// let v = vec![0, 1];
252/// read_slice(&v);
253///
254/// // ... and that's all!
255/// // you can also do it like this:
256/// let u: &[usize] = &v;
257/// // or like this:
258/// let u: &[_] = &v;
259/// ```
260///
261/// In Rust, it's more common to pass slices as arguments rather than vectors
262/// when you just want to provide read access. The same goes for [`String`] and
263/// [`&str`].
264///
265/// # Capacity and reallocation
266///
267/// The capacity of a vector is the amount of space allocated for any future
268/// elements that will be added onto the vector. This is not to be confused with
269/// the *length* of a vector, which specifies the number of actual elements
270/// within the vector. If a vector's length exceeds its capacity, its capacity
271/// will automatically be increased, but its elements will have to be
272/// reallocated.
273///
274/// For example, a vector with capacity 10 and length 0 would be an empty vector
275/// with space for 10 more elements. Pushing 10 or fewer elements onto the
276/// vector will not change its capacity or cause reallocation to occur. However,
277/// if the vector's length is increased to 11, it will have to reallocate, which
278/// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
279/// whenever possible to specify how big the vector is expected to get.
280///
281/// # Guarantees
282///
283/// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
284/// about its design. This ensures that it's as low-overhead as possible in
285/// the general case, and can be correctly manipulated in primitive ways
286/// by unsafe code. Note that these guarantees refer to an unqualified `Vec<T>`.
287/// If additional type parameters are added (e.g., to support custom allocators),
288/// overriding their defaults may change the behavior.
289///
290/// Most fundamentally, `Vec` is and always will be a (pointer, capacity, length)
291/// triplet. No more, no less. The order of these fields is completely
292/// unspecified, and you should use the appropriate methods to modify these.
293/// The pointer will never be null, so this type is null-pointer-optimized.
294///
295/// However, the pointer might not actually point to allocated memory. In particular,
296/// if you construct a `Vec` with capacity 0 via [`Vec::new`], [`vec![]`][`vec!`],
297/// [`Vec::with_capacity(0)`][`Vec::with_capacity`], or by calling [`shrink_to_fit`]
298/// on an empty Vec, it will not allocate memory. Similarly, if you store zero-sized
299/// types inside a `Vec`, it will not allocate space for them. *Note that in this case
300/// the `Vec` might not report a [`capacity`] of 0*. `Vec` will allocate if and only
301/// if <code>[size_of::\<T>]\() * [capacity]\() > 0</code>. In general, `Vec`'s allocation
302/// details are very subtle --- if you intend to allocate memory using a `Vec`
303/// and use it for something else (either to pass to unsafe code, or to build your
304/// own memory-backed collection), be sure to deallocate this memory by using
305/// `from_raw_parts` to recover the `Vec` and then dropping it.
306///
307/// If a `Vec` *has* allocated memory, then the memory it points to is on the heap
308/// (as defined by the allocator Rust is configured to use by default), and its
309/// pointer points to [`len`] initialized, contiguous elements in order (what
310/// you would see if you coerced it to a slice), followed by <code>[capacity] - [len]</code>
311/// logically uninitialized, contiguous elements.
312///
313/// A vector containing the elements `'a'` and `'b'` with capacity 4 can be
314/// visualized as below. The top part is the `Vec` struct, it contains a
315/// pointer to the head of the allocation in the heap, length and capacity.
316/// The bottom part is the allocation on the heap, a contiguous memory block.
317///
318/// ```text
319///             ptr      len  capacity
320///        +--------+--------+--------+
321///        | 0x0123 |      2 |      4 |
322///        +--------+--------+--------+
323///             |
324///             v
325/// Heap   +--------+--------+--------+--------+
326///        |    'a' |    'b' | uninit | uninit |
327///        +--------+--------+--------+--------+
328/// ```
329///
330/// - **uninit** represents memory that is not initialized, see [`MaybeUninit`].
331/// - Note: the ABI is not stable and `Vec` makes no guarantees about its memory
332///   layout (including the order of fields).
333///
334/// `Vec` will never perform a "small optimization" where elements are actually
335/// stored on the stack for two reasons:
336///
337/// * It would make it more difficult for unsafe code to correctly manipulate
338///   a `Vec`. The contents of a `Vec` wouldn't have a stable address if it were
339///   only moved, and it would be more difficult to determine if a `Vec` had
340///   actually allocated memory.
341///
342/// * It would penalize the general case, incurring an additional branch
343///   on every access.
344///
345/// `Vec` will never automatically shrink itself, even if completely empty. This
346/// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
347/// and then filling it back up to the same [`len`] should incur no calls to
348/// the allocator. If you wish to free up unused memory, use
349/// [`shrink_to_fit`] or [`shrink_to`].
350///
351/// [`push`] and [`insert`] will never (re)allocate if the reported capacity is
352/// sufficient. [`push`] and [`insert`] *will* (re)allocate if
353/// <code>[len] == [capacity]</code>. That is, the reported capacity is completely
354/// accurate, and can be relied on. It can even be used to manually free the memory
355/// allocated by a `Vec` if desired. Bulk insertion methods *may* reallocate, even
356/// when not necessary.
357///
358/// `Vec` does not guarantee any particular growth strategy when reallocating
359/// when full, nor when [`reserve`] is called. The current strategy is basic
360/// and it may prove desirable to use a non-constant growth factor. Whatever
361/// strategy is used will of course guarantee *O*(1) amortized [`push`].
362///
363/// It is guaranteed, in order to respect the intentions of the programmer, that
364/// all of `vec![e_1, e_2, ..., e_n]`, `vec![x; n]`, and [`Vec::with_capacity(n)`] produce a `Vec`
365/// that requests an allocation of the exact size needed for precisely `n` elements from the allocator,
366/// and no other size (such as, for example: a size rounded up to the nearest power of 2).
367/// The allocator will return an allocation that is at least as large as requested, but it may be larger.
368///
369/// It is guaranteed that the [`Vec::capacity`] method returns a value that is at least the requested capacity
370/// and not more than the allocated capacity.
371///
372/// The method [`Vec::shrink_to_fit`] will attempt to discard excess capacity an allocator has given to a `Vec`.
373/// If <code>[len] == [capacity]</code>, then a `Vec<T>` can be converted
374/// to and from a [`Box<[T]>`][owned slice] without reallocating or moving the elements.
375/// `Vec` exploits this fact as much as reasonable when implementing common conversions
376/// such as [`into_boxed_slice`].
377///
378/// `Vec` will not specifically overwrite any data that is removed from it,
379/// but also won't specifically preserve it. Its uninitialized memory is
380/// scratch space that it may use however it wants. It will generally just do
381/// whatever is most efficient or otherwise easy to implement. Do not rely on
382/// removed data to be erased for security purposes. Even if you drop a `Vec`, its
383/// buffer may simply be reused by another allocation. Even if you zero a `Vec`'s memory
384/// first, that might not actually happen because the optimizer does not consider
385/// this a side-effect that must be preserved. There is one case which we will
386/// not break, however: using `unsafe` code to write to the excess capacity,
387/// and then increasing the length to match, is always valid.
388///
389/// Currently, `Vec` does not guarantee the order in which elements are dropped.
390/// The order has changed in the past and may change again.
391///
392/// [`get`]: slice::get
393/// [`get_mut`]: slice::get_mut
394/// [`String`]: crate::string::String
395/// [`&str`]: type@str
396/// [`shrink_to_fit`]: Vec::shrink_to_fit
397/// [`shrink_to`]: Vec::shrink_to
398/// [capacity]: Vec::capacity
399/// [`capacity`]: Vec::capacity
400/// [`Vec::capacity`]: Vec::capacity
401/// [size_of::\<T>]: size_of
402/// [len]: Vec::len
403/// [`len`]: Vec::len
404/// [`push`]: Vec::push
405/// [`insert`]: Vec::insert
406/// [`reserve`]: Vec::reserve
407/// [`Vec::with_capacity(n)`]: Vec::with_capacity
408/// [`MaybeUninit`]: core::mem::MaybeUninit
409/// [owned slice]: Box
410/// [`into_boxed_slice`]: Vec::into_boxed_slice
411#[stable(feature = "rust1", since = "1.0.0")]
412#[rustc_diagnostic_item = "Vec"]
413#[rustc_insignificant_dtor]
414pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
415    buf: RawVec<T, A>,
416    len: usize,
417}
418
419////////////////////////////////////////////////////////////////////////////////
420// Inherent methods
421////////////////////////////////////////////////////////////////////////////////
422
423impl<T> Vec<T> {
424    /// Constructs a new, empty `Vec<T>`.
425    ///
426    /// The vector will not allocate until elements are pushed onto it.
427    ///
428    /// # Examples
429    ///
430    /// ```
431    /// # #![allow(unused_mut)]
432    /// let mut vec: Vec<i32> = Vec::new();
433    /// ```
434    #[inline]
435    #[rustc_const_stable(feature = "const_vec_new", since = "1.39.0")]
436    #[rustc_diagnostic_item = "vec_new"]
437    #[stable(feature = "rust1", since = "1.0.0")]
438    #[must_use]
439    pub const fn new() -> Self {
440        Vec { buf: RawVec::new(), len: 0 }
441    }
442
443    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
444    ///
445    /// The vector will be able to hold at least `capacity` elements without
446    /// reallocating. This method is allowed to allocate for more elements than
447    /// `capacity`. If `capacity` is zero, the vector will not allocate.
448    ///
449    /// It is important to note that although the returned vector has the
450    /// minimum *capacity* specified, the vector will have a zero *length*. For
451    /// an explanation of the difference between length and capacity, see
452    /// *[Capacity and reallocation]*.
453    ///
454    /// If it is important to know the exact allocated capacity of a `Vec`,
455    /// always use the [`capacity`] method after construction.
456    ///
457    /// For `Vec<T>` where `T` is a zero-sized type, there will be no allocation
458    /// and the capacity will always be `usize::MAX`.
459    ///
460    /// [Capacity and reallocation]: #capacity-and-reallocation
461    /// [`capacity`]: Vec::capacity
462    ///
463    /// # Panics
464    ///
465    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
466    ///
467    /// # Examples
468    ///
469    /// ```
470    /// let mut vec = Vec::with_capacity(10);
471    ///
472    /// // The vector contains no items, even though it has capacity for more
473    /// assert_eq!(vec.len(), 0);
474    /// assert!(vec.capacity() >= 10);
475    ///
476    /// // These are all done without reallocating...
477    /// for i in 0..10 {
478    ///     vec.push(i);
479    /// }
480    /// assert_eq!(vec.len(), 10);
481    /// assert!(vec.capacity() >= 10);
482    ///
483    /// // ...but this may make the vector reallocate
484    /// vec.push(11);
485    /// assert_eq!(vec.len(), 11);
486    /// assert!(vec.capacity() >= 11);
487    ///
488    /// // A vector of a zero-sized type will always over-allocate, since no
489    /// // allocation is necessary
490    /// let vec_units = Vec::<()>::with_capacity(10);
491    /// assert_eq!(vec_units.capacity(), usize::MAX);
492    /// ```
493    #[cfg(not(no_global_oom_handling))]
494    #[inline]
495    #[stable(feature = "rust1", since = "1.0.0")]
496    #[must_use]
497    #[rustc_diagnostic_item = "vec_with_capacity"]
498    #[track_caller]
499    pub fn with_capacity(capacity: usize) -> Self {
500        Self::with_capacity_in(capacity, Global)
501    }
502
503    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
504    ///
505    /// The vector will be able to hold at least `capacity` elements without
506    /// reallocating. This method is allowed to allocate for more elements than
507    /// `capacity`. If `capacity` is zero, the vector will not allocate.
508    ///
509    /// # Errors
510    ///
511    /// Returns an error if the capacity exceeds `isize::MAX` _bytes_,
512    /// or if the allocator reports allocation failure.
513    #[inline]
514    #[unstable(feature = "try_with_capacity", issue = "91913")]
515    pub fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
516        Self::try_with_capacity_in(capacity, Global)
517    }
518
519    /// Creates a `Vec<T>` directly from a pointer, a length, and a capacity.
520    ///
521    /// # Safety
522    ///
523    /// This is highly unsafe, due to the number of invariants that aren't
524    /// checked:
525    ///
526    /// * `ptr` must have been allocated using the global allocator, such as via
527    ///   the [`alloc::alloc`] function.
528    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
529    ///   (`T` having a less strict alignment is not sufficient, the alignment really
530    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
531    ///   allocated and deallocated with the same layout.)
532    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
533    ///   to be the same size as the pointer was allocated with. (Because similar to
534    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
535    /// * `length` needs to be less than or equal to `capacity`.
536    /// * The first `length` values must be properly initialized values of type `T`.
537    /// * `capacity` needs to be the capacity that the pointer was allocated with.
538    /// * The allocated size in bytes must be no larger than `isize::MAX`.
539    ///   See the safety documentation of [`pointer::offset`].
540    ///
541    /// These requirements are always upheld by any `ptr` that has been allocated
542    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
543    /// upheld.
544    ///
545    /// Violating these may cause problems like corrupting the allocator's
546    /// internal data structures. For example it is normally **not** safe
547    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
548    /// `size_t`, doing so is only safe if the array was initially allocated by
549    /// a `Vec` or `String`.
550    /// It's also not safe to build one from a `Vec<u16>` and its length, because
551    /// the allocator cares about the alignment, and these two types have different
552    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
553    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
554    /// these issues, it is often preferable to do casting/transmuting using
555    /// [`slice::from_raw_parts`] instead.
556    ///
557    /// The ownership of `ptr` is effectively transferred to the
558    /// `Vec<T>` which may then deallocate, reallocate or change the
559    /// contents of memory pointed to by the pointer at will. Ensure
560    /// that nothing else uses the pointer after calling this
561    /// function.
562    ///
563    /// [`String`]: crate::string::String
564    /// [`alloc::alloc`]: crate::alloc::alloc
565    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
566    ///
567    /// # Examples
568    ///
569    /// ```
570    /// use std::ptr;
571    /// use std::mem;
572    ///
573    /// let v = vec![1, 2, 3];
574    ///
575    // FIXME Update this when vec_into_raw_parts is stabilized
576    /// // Prevent running `v`'s destructor so we are in complete control
577    /// // of the allocation.
578    /// let mut v = mem::ManuallyDrop::new(v);
579    ///
580    /// // Pull out the various important pieces of information about `v`
581    /// let p = v.as_mut_ptr();
582    /// let len = v.len();
583    /// let cap = v.capacity();
584    ///
585    /// unsafe {
586    ///     // Overwrite memory with 4, 5, 6
587    ///     for i in 0..len {
588    ///         ptr::write(p.add(i), 4 + i);
589    ///     }
590    ///
591    ///     // Put everything back together into a Vec
592    ///     let rebuilt = Vec::from_raw_parts(p, len, cap);
593    ///     assert_eq!(rebuilt, [4, 5, 6]);
594    /// }
595    /// ```
596    ///
597    /// Using memory that was allocated elsewhere:
598    ///
599    /// ```rust
600    /// use std::alloc::{alloc, Layout};
601    ///
602    /// fn main() {
603    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
604    ///
605    ///     let vec = unsafe {
606    ///         let mem = alloc(layout).cast::<u32>();
607    ///         if mem.is_null() {
608    ///             return;
609    ///         }
610    ///
611    ///         mem.write(1_000_000);
612    ///
613    ///         Vec::from_raw_parts(mem, 1, 16)
614    ///     };
615    ///
616    ///     assert_eq!(vec, &[1_000_000]);
617    ///     assert_eq!(vec.capacity(), 16);
618    /// }
619    /// ```
620    #[inline]
621    #[stable(feature = "rust1", since = "1.0.0")]
622    pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
623        unsafe { Self::from_raw_parts_in(ptr, length, capacity, Global) }
624    }
625
626    #[doc(alias = "from_non_null_parts")]
627    /// Creates a `Vec<T>` directly from a `NonNull` pointer, a length, and a capacity.
628    ///
629    /// # Safety
630    ///
631    /// This is highly unsafe, due to the number of invariants that aren't
632    /// checked:
633    ///
634    /// * `ptr` must have been allocated using the global allocator, such as via
635    ///   the [`alloc::alloc`] function.
636    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
637    ///   (`T` having a less strict alignment is not sufficient, the alignment really
638    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
639    ///   allocated and deallocated with the same layout.)
640    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
641    ///   to be the same size as the pointer was allocated with. (Because similar to
642    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
643    /// * `length` needs to be less than or equal to `capacity`.
644    /// * The first `length` values must be properly initialized values of type `T`.
645    /// * `capacity` needs to be the capacity that the pointer was allocated with.
646    /// * The allocated size in bytes must be no larger than `isize::MAX`.
647    ///   See the safety documentation of [`pointer::offset`].
648    ///
649    /// These requirements are always upheld by any `ptr` that has been allocated
650    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
651    /// upheld.
652    ///
653    /// Violating these may cause problems like corrupting the allocator's
654    /// internal data structures. For example it is normally **not** safe
655    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
656    /// `size_t`, doing so is only safe if the array was initially allocated by
657    /// a `Vec` or `String`.
658    /// It's also not safe to build one from a `Vec<u16>` and its length, because
659    /// the allocator cares about the alignment, and these two types have different
660    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
661    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
662    /// these issues, it is often preferable to do casting/transmuting using
663    /// [`NonNull::slice_from_raw_parts`] instead.
664    ///
665    /// The ownership of `ptr` is effectively transferred to the
666    /// `Vec<T>` which may then deallocate, reallocate or change the
667    /// contents of memory pointed to by the pointer at will. Ensure
668    /// that nothing else uses the pointer after calling this
669    /// function.
670    ///
671    /// [`String`]: crate::string::String
672    /// [`alloc::alloc`]: crate::alloc::alloc
673    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
674    ///
675    /// # Examples
676    ///
677    /// ```
678    /// #![feature(box_vec_non_null)]
679    ///
680    /// use std::ptr::NonNull;
681    /// use std::mem;
682    ///
683    /// let v = vec![1, 2, 3];
684    ///
685    // FIXME Update this when vec_into_raw_parts is stabilized
686    /// // Prevent running `v`'s destructor so we are in complete control
687    /// // of the allocation.
688    /// let mut v = mem::ManuallyDrop::new(v);
689    ///
690    /// // Pull out the various important pieces of information about `v`
691    /// let p = unsafe { NonNull::new_unchecked(v.as_mut_ptr()) };
692    /// let len = v.len();
693    /// let cap = v.capacity();
694    ///
695    /// unsafe {
696    ///     // Overwrite memory with 4, 5, 6
697    ///     for i in 0..len {
698    ///         p.add(i).write(4 + i);
699    ///     }
700    ///
701    ///     // Put everything back together into a Vec
702    ///     let rebuilt = Vec::from_parts(p, len, cap);
703    ///     assert_eq!(rebuilt, [4, 5, 6]);
704    /// }
705    /// ```
706    ///
707    /// Using memory that was allocated elsewhere:
708    ///
709    /// ```rust
710    /// #![feature(box_vec_non_null)]
711    ///
712    /// use std::alloc::{alloc, Layout};
713    /// use std::ptr::NonNull;
714    ///
715    /// fn main() {
716    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
717    ///
718    ///     let vec = unsafe {
719    ///         let Some(mem) = NonNull::new(alloc(layout).cast::<u32>()) else {
720    ///             return;
721    ///         };
722    ///
723    ///         mem.write(1_000_000);
724    ///
725    ///         Vec::from_parts(mem, 1, 16)
726    ///     };
727    ///
728    ///     assert_eq!(vec, &[1_000_000]);
729    ///     assert_eq!(vec.capacity(), 16);
730    /// }
731    /// ```
732    #[inline]
733    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
734    pub unsafe fn from_parts(ptr: NonNull<T>, length: usize, capacity: usize) -> Self {
735        unsafe { Self::from_parts_in(ptr, length, capacity, Global) }
736    }
737
738    /// Returns a mutable reference to the last item in the vector, or
739    /// `None` if it is empty.
740    ///
741    /// # Examples
742    ///
743    /// Basic usage:
744    ///
745    /// ```
746    /// #![feature(vec_peek_mut)]
747    /// let mut vec = Vec::new();
748    /// assert!(vec.peek_mut().is_none());
749    ///
750    /// vec.push(1);
751    /// vec.push(5);
752    /// vec.push(2);
753    /// assert_eq!(vec.last(), Some(&2));
754    /// if let Some(mut val) = vec.peek_mut() {
755    ///     *val = 0;
756    /// }
757    /// assert_eq!(vec.last(), Some(&0));
758    /// ```
759    #[inline]
760    #[unstable(feature = "vec_peek_mut", issue = "122742")]
761    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
762        PeekMut::new(self)
763    }
764
765    /// Decomposes a `Vec<T>` into its raw components: `(pointer, length, capacity)`.
766    ///
767    /// Returns the raw pointer to the underlying data, the length of
768    /// the vector (in elements), and the allocated capacity of the
769    /// data (in elements). These are the same arguments in the same
770    /// order as the arguments to [`from_raw_parts`].
771    ///
772    /// After calling this function, the caller is responsible for the
773    /// memory previously managed by the `Vec`. The only way to do
774    /// this is to convert the raw pointer, length, and capacity back
775    /// into a `Vec` with the [`from_raw_parts`] function, allowing
776    /// the destructor to perform the cleanup.
777    ///
778    /// [`from_raw_parts`]: Vec::from_raw_parts
779    ///
780    /// # Examples
781    ///
782    /// ```
783    /// #![feature(vec_into_raw_parts)]
784    /// let v: Vec<i32> = vec![-1, 0, 1];
785    ///
786    /// let (ptr, len, cap) = v.into_raw_parts();
787    ///
788    /// let rebuilt = unsafe {
789    ///     // We can now make changes to the components, such as
790    ///     // transmuting the raw pointer to a compatible type.
791    ///     let ptr = ptr as *mut u32;
792    ///
793    ///     Vec::from_raw_parts(ptr, len, cap)
794    /// };
795    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
796    /// ```
797    #[must_use = "losing the pointer will leak memory"]
798    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
799    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
800        let mut me = ManuallyDrop::new(self);
801        (me.as_mut_ptr(), me.len(), me.capacity())
802    }
803
804    #[doc(alias = "into_non_null_parts")]
805    /// Decomposes a `Vec<T>` into its raw components: `(NonNull pointer, length, capacity)`.
806    ///
807    /// Returns the `NonNull` pointer to the underlying data, the length of
808    /// the vector (in elements), and the allocated capacity of the
809    /// data (in elements). These are the same arguments in the same
810    /// order as the arguments to [`from_parts`].
811    ///
812    /// After calling this function, the caller is responsible for the
813    /// memory previously managed by the `Vec`. The only way to do
814    /// this is to convert the `NonNull` pointer, length, and capacity back
815    /// into a `Vec` with the [`from_parts`] function, allowing
816    /// the destructor to perform the cleanup.
817    ///
818    /// [`from_parts`]: Vec::from_parts
819    ///
820    /// # Examples
821    ///
822    /// ```
823    /// #![feature(vec_into_raw_parts, box_vec_non_null)]
824    ///
825    /// let v: Vec<i32> = vec![-1, 0, 1];
826    ///
827    /// let (ptr, len, cap) = v.into_parts();
828    ///
829    /// let rebuilt = unsafe {
830    ///     // We can now make changes to the components, such as
831    ///     // transmuting the raw pointer to a compatible type.
832    ///     let ptr = ptr.cast::<u32>();
833    ///
834    ///     Vec::from_parts(ptr, len, cap)
835    /// };
836    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
837    /// ```
838    #[must_use = "losing the pointer will leak memory"]
839    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
840    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
841    pub fn into_parts(self) -> (NonNull<T>, usize, usize) {
842        let (ptr, len, capacity) = self.into_raw_parts();
843        // SAFETY: A `Vec` always has a non-null pointer.
844        (unsafe { NonNull::new_unchecked(ptr) }, len, capacity)
845    }
846}
847
848impl<T, A: Allocator> Vec<T, A> {
849    /// Constructs a new, empty `Vec<T, A>`.
850    ///
851    /// The vector will not allocate until elements are pushed onto it.
852    ///
853    /// # Examples
854    ///
855    /// ```
856    /// #![feature(allocator_api)]
857    ///
858    /// use std::alloc::System;
859    ///
860    /// # #[allow(unused_mut)]
861    /// let mut vec: Vec<i32, _> = Vec::new_in(System);
862    /// ```
863    #[inline]
864    #[unstable(feature = "allocator_api", issue = "32838")]
865    pub const fn new_in(alloc: A) -> Self {
866        Vec { buf: RawVec::new_in(alloc), len: 0 }
867    }
868
869    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
870    /// with the provided allocator.
871    ///
872    /// The vector will be able to hold at least `capacity` elements without
873    /// reallocating. This method is allowed to allocate for more elements than
874    /// `capacity`. If `capacity` is zero, the vector will not allocate.
875    ///
876    /// It is important to note that although the returned vector has the
877    /// minimum *capacity* specified, the vector will have a zero *length*. For
878    /// an explanation of the difference between length and capacity, see
879    /// *[Capacity and reallocation]*.
880    ///
881    /// If it is important to know the exact allocated capacity of a `Vec`,
882    /// always use the [`capacity`] method after construction.
883    ///
884    /// For `Vec<T, A>` where `T` is a zero-sized type, there will be no allocation
885    /// and the capacity will always be `usize::MAX`.
886    ///
887    /// [Capacity and reallocation]: #capacity-and-reallocation
888    /// [`capacity`]: Vec::capacity
889    ///
890    /// # Panics
891    ///
892    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
893    ///
894    /// # Examples
895    ///
896    /// ```
897    /// #![feature(allocator_api)]
898    ///
899    /// use std::alloc::System;
900    ///
901    /// let mut vec = Vec::with_capacity_in(10, System);
902    ///
903    /// // The vector contains no items, even though it has capacity for more
904    /// assert_eq!(vec.len(), 0);
905    /// assert!(vec.capacity() >= 10);
906    ///
907    /// // These are all done without reallocating...
908    /// for i in 0..10 {
909    ///     vec.push(i);
910    /// }
911    /// assert_eq!(vec.len(), 10);
912    /// assert!(vec.capacity() >= 10);
913    ///
914    /// // ...but this may make the vector reallocate
915    /// vec.push(11);
916    /// assert_eq!(vec.len(), 11);
917    /// assert!(vec.capacity() >= 11);
918    ///
919    /// // A vector of a zero-sized type will always over-allocate, since no
920    /// // allocation is necessary
921    /// let vec_units = Vec::<(), System>::with_capacity_in(10, System);
922    /// assert_eq!(vec_units.capacity(), usize::MAX);
923    /// ```
924    #[cfg(not(no_global_oom_handling))]
925    #[inline]
926    #[unstable(feature = "allocator_api", issue = "32838")]
927    #[track_caller]
928    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
929        Vec { buf: RawVec::with_capacity_in(capacity, alloc), len: 0 }
930    }
931
932    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
933    /// with the provided allocator.
934    ///
935    /// The vector will be able to hold at least `capacity` elements without
936    /// reallocating. This method is allowed to allocate for more elements than
937    /// `capacity`. If `capacity` is zero, the vector will not allocate.
938    ///
939    /// # Errors
940    ///
941    /// Returns an error if the capacity exceeds `isize::MAX` _bytes_,
942    /// or if the allocator reports allocation failure.
943    #[inline]
944    #[unstable(feature = "allocator_api", issue = "32838")]
945    // #[unstable(feature = "try_with_capacity", issue = "91913")]
946    pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
947        Ok(Vec { buf: RawVec::try_with_capacity_in(capacity, alloc)?, len: 0 })
948    }
949
950    /// Creates a `Vec<T, A>` directly from a pointer, a length, a capacity,
951    /// and an allocator.
952    ///
953    /// # Safety
954    ///
955    /// This is highly unsafe, due to the number of invariants that aren't
956    /// checked:
957    ///
958    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
959    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
960    ///   (`T` having a less strict alignment is not sufficient, the alignment really
961    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
962    ///   allocated and deallocated with the same layout.)
963    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
964    ///   to be the same size as the pointer was allocated with. (Because similar to
965    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
966    /// * `length` needs to be less than or equal to `capacity`.
967    /// * The first `length` values must be properly initialized values of type `T`.
968    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
969    /// * The allocated size in bytes must be no larger than `isize::MAX`.
970    ///   See the safety documentation of [`pointer::offset`].
971    ///
972    /// These requirements are always upheld by any `ptr` that has been allocated
973    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
974    /// upheld.
975    ///
976    /// Violating these may cause problems like corrupting the allocator's
977    /// internal data structures. For example it is **not** safe
978    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
979    /// It's also not safe to build one from a `Vec<u16>` and its length, because
980    /// the allocator cares about the alignment, and these two types have different
981    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
982    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
983    ///
984    /// The ownership of `ptr` is effectively transferred to the
985    /// `Vec<T>` which may then deallocate, reallocate or change the
986    /// contents of memory pointed to by the pointer at will. Ensure
987    /// that nothing else uses the pointer after calling this
988    /// function.
989    ///
990    /// [`String`]: crate::string::String
991    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
992    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
993    /// [*fit*]: crate::alloc::Allocator#memory-fitting
994    ///
995    /// # Examples
996    ///
997    /// ```
998    /// #![feature(allocator_api)]
999    ///
1000    /// use std::alloc::System;
1001    ///
1002    /// use std::ptr;
1003    /// use std::mem;
1004    ///
1005    /// let mut v = Vec::with_capacity_in(3, System);
1006    /// v.push(1);
1007    /// v.push(2);
1008    /// v.push(3);
1009    ///
1010    // FIXME Update this when vec_into_raw_parts is stabilized
1011    /// // Prevent running `v`'s destructor so we are in complete control
1012    /// // of the allocation.
1013    /// let mut v = mem::ManuallyDrop::new(v);
1014    ///
1015    /// // Pull out the various important pieces of information about `v`
1016    /// let p = v.as_mut_ptr();
1017    /// let len = v.len();
1018    /// let cap = v.capacity();
1019    /// let alloc = v.allocator();
1020    ///
1021    /// unsafe {
1022    ///     // Overwrite memory with 4, 5, 6
1023    ///     for i in 0..len {
1024    ///         ptr::write(p.add(i), 4 + i);
1025    ///     }
1026    ///
1027    ///     // Put everything back together into a Vec
1028    ///     let rebuilt = Vec::from_raw_parts_in(p, len, cap, alloc.clone());
1029    ///     assert_eq!(rebuilt, [4, 5, 6]);
1030    /// }
1031    /// ```
1032    ///
1033    /// Using memory that was allocated elsewhere:
1034    ///
1035    /// ```rust
1036    /// #![feature(allocator_api)]
1037    ///
1038    /// use std::alloc::{AllocError, Allocator, Global, Layout};
1039    ///
1040    /// fn main() {
1041    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
1042    ///
1043    ///     let vec = unsafe {
1044    ///         let mem = match Global.allocate(layout) {
1045    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
1046    ///             Err(AllocError) => return,
1047    ///         };
1048    ///
1049    ///         mem.write(1_000_000);
1050    ///
1051    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
1052    ///     };
1053    ///
1054    ///     assert_eq!(vec, &[1_000_000]);
1055    ///     assert_eq!(vec.capacity(), 16);
1056    /// }
1057    /// ```
1058    #[inline]
1059    #[unstable(feature = "allocator_api", issue = "32838")]
1060    pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
1061        unsafe { Vec { buf: RawVec::from_raw_parts_in(ptr, capacity, alloc), len: length } }
1062    }
1063
1064    #[doc(alias = "from_non_null_parts_in")]
1065    /// Creates a `Vec<T, A>` directly from a `NonNull` pointer, a length, a capacity,
1066    /// and an allocator.
1067    ///
1068    /// # Safety
1069    ///
1070    /// This is highly unsafe, due to the number of invariants that aren't
1071    /// checked:
1072    ///
1073    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
1074    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
1075    ///   (`T` having a less strict alignment is not sufficient, the alignment really
1076    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
1077    ///   allocated and deallocated with the same layout.)
1078    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
1079    ///   to be the same size as the pointer was allocated with. (Because similar to
1080    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
1081    /// * `length` needs to be less than or equal to `capacity`.
1082    /// * The first `length` values must be properly initialized values of type `T`.
1083    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
1084    /// * The allocated size in bytes must be no larger than `isize::MAX`.
1085    ///   See the safety documentation of [`pointer::offset`].
1086    ///
1087    /// These requirements are always upheld by any `ptr` that has been allocated
1088    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
1089    /// upheld.
1090    ///
1091    /// Violating these may cause problems like corrupting the allocator's
1092    /// internal data structures. For example it is **not** safe
1093    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
1094    /// It's also not safe to build one from a `Vec<u16>` and its length, because
1095    /// the allocator cares about the alignment, and these two types have different
1096    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
1097    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
1098    ///
1099    /// The ownership of `ptr` is effectively transferred to the
1100    /// `Vec<T>` which may then deallocate, reallocate or change the
1101    /// contents of memory pointed to by the pointer at will. Ensure
1102    /// that nothing else uses the pointer after calling this
1103    /// function.
1104    ///
1105    /// [`String`]: crate::string::String
1106    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
1107    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
1108    /// [*fit*]: crate::alloc::Allocator#memory-fitting
1109    ///
1110    /// # Examples
1111    ///
1112    /// ```
1113    /// #![feature(allocator_api, box_vec_non_null)]
1114    ///
1115    /// use std::alloc::System;
1116    ///
1117    /// use std::ptr::NonNull;
1118    /// use std::mem;
1119    ///
1120    /// let mut v = Vec::with_capacity_in(3, System);
1121    /// v.push(1);
1122    /// v.push(2);
1123    /// v.push(3);
1124    ///
1125    // FIXME Update this when vec_into_raw_parts is stabilized
1126    /// // Prevent running `v`'s destructor so we are in complete control
1127    /// // of the allocation.
1128    /// let mut v = mem::ManuallyDrop::new(v);
1129    ///
1130    /// // Pull out the various important pieces of information about `v`
1131    /// let p = unsafe { NonNull::new_unchecked(v.as_mut_ptr()) };
1132    /// let len = v.len();
1133    /// let cap = v.capacity();
1134    /// let alloc = v.allocator();
1135    ///
1136    /// unsafe {
1137    ///     // Overwrite memory with 4, 5, 6
1138    ///     for i in 0..len {
1139    ///         p.add(i).write(4 + i);
1140    ///     }
1141    ///
1142    ///     // Put everything back together into a Vec
1143    ///     let rebuilt = Vec::from_parts_in(p, len, cap, alloc.clone());
1144    ///     assert_eq!(rebuilt, [4, 5, 6]);
1145    /// }
1146    /// ```
1147    ///
1148    /// Using memory that was allocated elsewhere:
1149    ///
1150    /// ```rust
1151    /// #![feature(allocator_api, box_vec_non_null)]
1152    ///
1153    /// use std::alloc::{AllocError, Allocator, Global, Layout};
1154    ///
1155    /// fn main() {
1156    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
1157    ///
1158    ///     let vec = unsafe {
1159    ///         let mem = match Global.allocate(layout) {
1160    ///             Ok(mem) => mem.cast::<u32>(),
1161    ///             Err(AllocError) => return,
1162    ///         };
1163    ///
1164    ///         mem.write(1_000_000);
1165    ///
1166    ///         Vec::from_parts_in(mem, 1, 16, Global)
1167    ///     };
1168    ///
1169    ///     assert_eq!(vec, &[1_000_000]);
1170    ///     assert_eq!(vec.capacity(), 16);
1171    /// }
1172    /// ```
1173    #[inline]
1174    #[unstable(feature = "allocator_api", reason = "new API", issue = "32838")]
1175    // #[unstable(feature = "box_vec_non_null", issue = "130364")]
1176    pub unsafe fn from_parts_in(ptr: NonNull<T>, length: usize, capacity: usize, alloc: A) -> Self {
1177        unsafe { Vec { buf: RawVec::from_nonnull_in(ptr, capacity, alloc), len: length } }
1178    }
1179
1180    /// Decomposes a `Vec<T>` into its raw components: `(pointer, length, capacity, allocator)`.
1181    ///
1182    /// Returns the raw pointer to the underlying data, the length of the vector (in elements),
1183    /// the allocated capacity of the data (in elements), and the allocator. These are the same
1184    /// arguments in the same order as the arguments to [`from_raw_parts_in`].
1185    ///
1186    /// After calling this function, the caller is responsible for the
1187    /// memory previously managed by the `Vec`. The only way to do
1188    /// this is to convert the raw pointer, length, and capacity back
1189    /// into a `Vec` with the [`from_raw_parts_in`] function, allowing
1190    /// the destructor to perform the cleanup.
1191    ///
1192    /// [`from_raw_parts_in`]: Vec::from_raw_parts_in
1193    ///
1194    /// # Examples
1195    ///
1196    /// ```
1197    /// #![feature(allocator_api, vec_into_raw_parts)]
1198    ///
1199    /// use std::alloc::System;
1200    ///
1201    /// let mut v: Vec<i32, System> = Vec::new_in(System);
1202    /// v.push(-1);
1203    /// v.push(0);
1204    /// v.push(1);
1205    ///
1206    /// let (ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
1207    ///
1208    /// let rebuilt = unsafe {
1209    ///     // We can now make changes to the components, such as
1210    ///     // transmuting the raw pointer to a compatible type.
1211    ///     let ptr = ptr as *mut u32;
1212    ///
1213    ///     Vec::from_raw_parts_in(ptr, len, cap, alloc)
1214    /// };
1215    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
1216    /// ```
1217    #[must_use = "losing the pointer will leak memory"]
1218    #[unstable(feature = "allocator_api", issue = "32838")]
1219    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
1220    pub fn into_raw_parts_with_alloc(self) -> (*mut T, usize, usize, A) {
1221        let mut me = ManuallyDrop::new(self);
1222        let len = me.len();
1223        let capacity = me.capacity();
1224        let ptr = me.as_mut_ptr();
1225        let alloc = unsafe { ptr::read(me.allocator()) };
1226        (ptr, len, capacity, alloc)
1227    }
1228
1229    #[doc(alias = "into_non_null_parts_with_alloc")]
1230    /// Decomposes a `Vec<T>` into its raw components: `(NonNull pointer, length, capacity, allocator)`.
1231    ///
1232    /// Returns the `NonNull` pointer to the underlying data, the length of the vector (in elements),
1233    /// the allocated capacity of the data (in elements), and the allocator. These are the same
1234    /// arguments in the same order as the arguments to [`from_parts_in`].
1235    ///
1236    /// After calling this function, the caller is responsible for the
1237    /// memory previously managed by the `Vec`. The only way to do
1238    /// this is to convert the `NonNull` pointer, length, and capacity back
1239    /// into a `Vec` with the [`from_parts_in`] function, allowing
1240    /// the destructor to perform the cleanup.
1241    ///
1242    /// [`from_parts_in`]: Vec::from_parts_in
1243    ///
1244    /// # Examples
1245    ///
1246    /// ```
1247    /// #![feature(allocator_api, vec_into_raw_parts, box_vec_non_null)]
1248    ///
1249    /// use std::alloc::System;
1250    ///
1251    /// let mut v: Vec<i32, System> = Vec::new_in(System);
1252    /// v.push(-1);
1253    /// v.push(0);
1254    /// v.push(1);
1255    ///
1256    /// let (ptr, len, cap, alloc) = v.into_parts_with_alloc();
1257    ///
1258    /// let rebuilt = unsafe {
1259    ///     // We can now make changes to the components, such as
1260    ///     // transmuting the raw pointer to a compatible type.
1261    ///     let ptr = ptr.cast::<u32>();
1262    ///
1263    ///     Vec::from_parts_in(ptr, len, cap, alloc)
1264    /// };
1265    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
1266    /// ```
1267    #[must_use = "losing the pointer will leak memory"]
1268    #[unstable(feature = "allocator_api", issue = "32838")]
1269    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1270    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
1271    pub fn into_parts_with_alloc(self) -> (NonNull<T>, usize, usize, A) {
1272        let (ptr, len, capacity, alloc) = self.into_raw_parts_with_alloc();
1273        // SAFETY: A `Vec` always has a non-null pointer.
1274        (unsafe { NonNull::new_unchecked(ptr) }, len, capacity, alloc)
1275    }
1276
1277    /// Returns the total number of elements the vector can hold without
1278    /// reallocating.
1279    ///
1280    /// # Examples
1281    ///
1282    /// ```
1283    /// let mut vec: Vec<i32> = Vec::with_capacity(10);
1284    /// vec.push(42);
1285    /// assert!(vec.capacity() >= 10);
1286    /// ```
1287    ///
1288    /// A vector with zero-sized elements will always have a capacity of usize::MAX:
1289    ///
1290    /// ```
1291    /// #[derive(Clone)]
1292    /// struct ZeroSized;
1293    ///
1294    /// fn main() {
1295    ///     assert_eq!(std::mem::size_of::<ZeroSized>(), 0);
1296    ///     let v = vec![ZeroSized; 0];
1297    ///     assert_eq!(v.capacity(), usize::MAX);
1298    /// }
1299    /// ```
1300    #[inline]
1301    #[stable(feature = "rust1", since = "1.0.0")]
1302    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1303    pub const fn capacity(&self) -> usize {
1304        self.buf.capacity()
1305    }
1306
1307    /// Reserves capacity for at least `additional` more elements to be inserted
1308    /// in the given `Vec<T>`. The collection may reserve more space to
1309    /// speculatively avoid frequent reallocations. After calling `reserve`,
1310    /// capacity will be greater than or equal to `self.len() + additional`.
1311    /// Does nothing if capacity is already sufficient.
1312    ///
1313    /// # Panics
1314    ///
1315    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
1316    ///
1317    /// # Examples
1318    ///
1319    /// ```
1320    /// let mut vec = vec![1];
1321    /// vec.reserve(10);
1322    /// assert!(vec.capacity() >= 11);
1323    /// ```
1324    #[cfg(not(no_global_oom_handling))]
1325    #[stable(feature = "rust1", since = "1.0.0")]
1326    #[track_caller]
1327    #[rustc_diagnostic_item = "vec_reserve"]
1328    pub fn reserve(&mut self, additional: usize) {
1329        self.buf.reserve(self.len, additional);
1330    }
1331
1332    /// Reserves the minimum capacity for at least `additional` more elements to
1333    /// be inserted in the given `Vec<T>`. Unlike [`reserve`], this will not
1334    /// deliberately over-allocate to speculatively avoid frequent allocations.
1335    /// After calling `reserve_exact`, capacity will be greater than or equal to
1336    /// `self.len() + additional`. Does nothing if the capacity is already
1337    /// sufficient.
1338    ///
1339    /// Note that the allocator may give the collection more space than it
1340    /// requests. Therefore, capacity can not be relied upon to be precisely
1341    /// minimal. Prefer [`reserve`] if future insertions are expected.
1342    ///
1343    /// [`reserve`]: Vec::reserve
1344    ///
1345    /// # Panics
1346    ///
1347    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
1348    ///
1349    /// # Examples
1350    ///
1351    /// ```
1352    /// let mut vec = vec![1];
1353    /// vec.reserve_exact(10);
1354    /// assert!(vec.capacity() >= 11);
1355    /// ```
1356    #[cfg(not(no_global_oom_handling))]
1357    #[stable(feature = "rust1", since = "1.0.0")]
1358    #[track_caller]
1359    pub fn reserve_exact(&mut self, additional: usize) {
1360        self.buf.reserve_exact(self.len, additional);
1361    }
1362
1363    /// Tries to reserve capacity for at least `additional` more elements to be inserted
1364    /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
1365    /// frequent reallocations. After calling `try_reserve`, capacity will be
1366    /// greater than or equal to `self.len() + additional` if it returns
1367    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
1368    /// preserves the contents even if an error occurs.
1369    ///
1370    /// # Errors
1371    ///
1372    /// If the capacity overflows, or the allocator reports a failure, then an error
1373    /// is returned.
1374    ///
1375    /// # Examples
1376    ///
1377    /// ```
1378    /// use std::collections::TryReserveError;
1379    ///
1380    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
1381    ///     let mut output = Vec::new();
1382    ///
1383    ///     // Pre-reserve the memory, exiting if we can't
1384    ///     output.try_reserve(data.len())?;
1385    ///
1386    ///     // Now we know this can't OOM in the middle of our complex work
1387    ///     output.extend(data.iter().map(|&val| {
1388    ///         val * 2 + 5 // very complicated
1389    ///     }));
1390    ///
1391    ///     Ok(output)
1392    /// }
1393    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
1394    /// ```
1395    #[stable(feature = "try_reserve", since = "1.57.0")]
1396    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
1397        self.buf.try_reserve(self.len, additional)
1398    }
1399
1400    /// Tries to reserve the minimum capacity for at least `additional`
1401    /// elements to be inserted in the given `Vec<T>`. Unlike [`try_reserve`],
1402    /// this will not deliberately over-allocate to speculatively avoid frequent
1403    /// allocations. After calling `try_reserve_exact`, capacity will be greater
1404    /// than or equal to `self.len() + additional` if it returns `Ok(())`.
1405    /// Does nothing if the capacity is already sufficient.
1406    ///
1407    /// Note that the allocator may give the collection more space than it
1408    /// requests. Therefore, capacity can not be relied upon to be precisely
1409    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
1410    ///
1411    /// [`try_reserve`]: Vec::try_reserve
1412    ///
1413    /// # Errors
1414    ///
1415    /// If the capacity overflows, or the allocator reports a failure, then an error
1416    /// is returned.
1417    ///
1418    /// # Examples
1419    ///
1420    /// ```
1421    /// use std::collections::TryReserveError;
1422    ///
1423    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
1424    ///     let mut output = Vec::new();
1425    ///
1426    ///     // Pre-reserve the memory, exiting if we can't
1427    ///     output.try_reserve_exact(data.len())?;
1428    ///
1429    ///     // Now we know this can't OOM in the middle of our complex work
1430    ///     output.extend(data.iter().map(|&val| {
1431    ///         val * 2 + 5 // very complicated
1432    ///     }));
1433    ///
1434    ///     Ok(output)
1435    /// }
1436    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
1437    /// ```
1438    #[stable(feature = "try_reserve", since = "1.57.0")]
1439    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
1440        self.buf.try_reserve_exact(self.len, additional)
1441    }
1442
1443    /// Shrinks the capacity of the vector as much as possible.
1444    ///
1445    /// The behavior of this method depends on the allocator, which may either shrink the vector
1446    /// in-place or reallocate. The resulting vector might still have some excess capacity, just as
1447    /// is the case for [`with_capacity`]. See [`Allocator::shrink`] for more details.
1448    ///
1449    /// [`with_capacity`]: Vec::with_capacity
1450    ///
1451    /// # Examples
1452    ///
1453    /// ```
1454    /// let mut vec = Vec::with_capacity(10);
1455    /// vec.extend([1, 2, 3]);
1456    /// assert!(vec.capacity() >= 10);
1457    /// vec.shrink_to_fit();
1458    /// assert!(vec.capacity() >= 3);
1459    /// ```
1460    #[cfg(not(no_global_oom_handling))]
1461    #[stable(feature = "rust1", since = "1.0.0")]
1462    #[track_caller]
1463    #[inline]
1464    pub fn shrink_to_fit(&mut self) {
1465        // The capacity is never less than the length, and there's nothing to do when
1466        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
1467        // by only calling it with a greater capacity.
1468        if self.capacity() > self.len {
1469            self.buf.shrink_to_fit(self.len);
1470        }
1471    }
1472
1473    /// Shrinks the capacity of the vector with a lower bound.
1474    ///
1475    /// The capacity will remain at least as large as both the length
1476    /// and the supplied value.
1477    ///
1478    /// If the current capacity is less than the lower limit, this is a no-op.
1479    ///
1480    /// # Examples
1481    ///
1482    /// ```
1483    /// let mut vec = Vec::with_capacity(10);
1484    /// vec.extend([1, 2, 3]);
1485    /// assert!(vec.capacity() >= 10);
1486    /// vec.shrink_to(4);
1487    /// assert!(vec.capacity() >= 4);
1488    /// vec.shrink_to(0);
1489    /// assert!(vec.capacity() >= 3);
1490    /// ```
1491    #[cfg(not(no_global_oom_handling))]
1492    #[stable(feature = "shrink_to", since = "1.56.0")]
1493    #[track_caller]
1494    pub fn shrink_to(&mut self, min_capacity: usize) {
1495        if self.capacity() > min_capacity {
1496            self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
1497        }
1498    }
1499
1500    /// Converts the vector into [`Box<[T]>`][owned slice].
1501    ///
1502    /// Before doing the conversion, this method discards excess capacity like [`shrink_to_fit`].
1503    ///
1504    /// [owned slice]: Box
1505    /// [`shrink_to_fit`]: Vec::shrink_to_fit
1506    ///
1507    /// # Examples
1508    ///
1509    /// ```
1510    /// let v = vec![1, 2, 3];
1511    ///
1512    /// let slice = v.into_boxed_slice();
1513    /// ```
1514    ///
1515    /// Any excess capacity is removed:
1516    ///
1517    /// ```
1518    /// let mut vec = Vec::with_capacity(10);
1519    /// vec.extend([1, 2, 3]);
1520    ///
1521    /// assert!(vec.capacity() >= 10);
1522    /// let slice = vec.into_boxed_slice();
1523    /// assert_eq!(slice.into_vec().capacity(), 3);
1524    /// ```
1525    #[cfg(not(no_global_oom_handling))]
1526    #[stable(feature = "rust1", since = "1.0.0")]
1527    #[track_caller]
1528    pub fn into_boxed_slice(mut self) -> Box<[T], A> {
1529        unsafe {
1530            self.shrink_to_fit();
1531            let me = ManuallyDrop::new(self);
1532            let buf = ptr::read(&me.buf);
1533            let len = me.len();
1534            buf.into_box(len).assume_init()
1535        }
1536    }
1537
1538    /// Shortens the vector, keeping the first `len` elements and dropping
1539    /// the rest.
1540    ///
1541    /// If `len` is greater or equal to the vector's current length, this has
1542    /// no effect.
1543    ///
1544    /// The [`drain`] method can emulate `truncate`, but causes the excess
1545    /// elements to be returned instead of dropped.
1546    ///
1547    /// Note that this method has no effect on the allocated capacity
1548    /// of the vector.
1549    ///
1550    /// # Examples
1551    ///
1552    /// Truncating a five element vector to two elements:
1553    ///
1554    /// ```
1555    /// let mut vec = vec![1, 2, 3, 4, 5];
1556    /// vec.truncate(2);
1557    /// assert_eq!(vec, [1, 2]);
1558    /// ```
1559    ///
1560    /// No truncation occurs when `len` is greater than the vector's current
1561    /// length:
1562    ///
1563    /// ```
1564    /// let mut vec = vec![1, 2, 3];
1565    /// vec.truncate(8);
1566    /// assert_eq!(vec, [1, 2, 3]);
1567    /// ```
1568    ///
1569    /// Truncating when `len == 0` is equivalent to calling the [`clear`]
1570    /// method.
1571    ///
1572    /// ```
1573    /// let mut vec = vec![1, 2, 3];
1574    /// vec.truncate(0);
1575    /// assert_eq!(vec, []);
1576    /// ```
1577    ///
1578    /// [`clear`]: Vec::clear
1579    /// [`drain`]: Vec::drain
1580    #[stable(feature = "rust1", since = "1.0.0")]
1581    pub fn truncate(&mut self, len: usize) {
1582        // This is safe because:
1583        //
1584        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
1585        //   case avoids creating an invalid slice, and
1586        // * the `len` of the vector is shrunk before calling `drop_in_place`,
1587        //   such that no value will be dropped twice in case `drop_in_place`
1588        //   were to panic once (if it panics twice, the program aborts).
1589        unsafe {
1590            // Note: It's intentional that this is `>` and not `>=`.
1591            //       Changing it to `>=` has negative performance
1592            //       implications in some cases. See #78884 for more.
1593            if len > self.len {
1594                return;
1595            }
1596            let remaining_len = self.len - len;
1597            let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
1598            self.len = len;
1599            ptr::drop_in_place(s);
1600        }
1601    }
1602
1603    /// Extracts a slice containing the entire vector.
1604    ///
1605    /// Equivalent to `&s[..]`.
1606    ///
1607    /// # Examples
1608    ///
1609    /// ```
1610    /// use std::io::{self, Write};
1611    /// let buffer = vec![1, 2, 3, 5, 8];
1612    /// io::sink().write(buffer.as_slice()).unwrap();
1613    /// ```
1614    #[inline]
1615    #[stable(feature = "vec_as_slice", since = "1.7.0")]
1616    #[rustc_diagnostic_item = "vec_as_slice"]
1617    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1618    pub const fn as_slice(&self) -> &[T] {
1619        // SAFETY: `slice::from_raw_parts` requires pointee is a contiguous, aligned buffer of size
1620        // `len` containing properly-initialized `T`s. Data must not be mutated for the returned
1621        // lifetime. Further, `len * size_of::<T>` <= `isize::MAX`, and allocation does not
1622        // "wrap" through overflowing memory addresses.
1623        //
1624        // * Vec API guarantees that self.buf:
1625        //      * contains only properly-initialized items within 0..len
1626        //      * is aligned, contiguous, and valid for `len` reads
1627        //      * obeys size and address-wrapping constraints
1628        //
1629        // * We only construct `&mut` references to `self.buf` through `&mut self` methods; borrow-
1630        //   check ensures that it is not possible to mutably alias `self.buf` within the
1631        //   returned lifetime.
1632        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
1633    }
1634
1635    /// Extracts a mutable slice of the entire vector.
1636    ///
1637    /// Equivalent to `&mut s[..]`.
1638    ///
1639    /// # Examples
1640    ///
1641    /// ```
1642    /// use std::io::{self, Read};
1643    /// let mut buffer = vec![0; 3];
1644    /// io::repeat(0b101).read_exact(buffer.as_mut_slice()).unwrap();
1645    /// ```
1646    #[inline]
1647    #[stable(feature = "vec_as_slice", since = "1.7.0")]
1648    #[rustc_diagnostic_item = "vec_as_mut_slice"]
1649    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1650    pub const fn as_mut_slice(&mut self) -> &mut [T] {
1651        // SAFETY: `slice::from_raw_parts_mut` requires pointee is a contiguous, aligned buffer of
1652        // size `len` containing properly-initialized `T`s. Data must not be accessed through any
1653        // other pointer for the returned lifetime. Further, `len * size_of::<T>` <=
1654        // `ISIZE::MAX` and allocation does not "wrap" through overflowing memory addresses.
1655        //
1656        // * Vec API guarantees that self.buf:
1657        //      * contains only properly-initialized items within 0..len
1658        //      * is aligned, contiguous, and valid for `len` reads
1659        //      * obeys size and address-wrapping constraints
1660        //
1661        // * We only construct references to `self.buf` through `&self` and `&mut self` methods;
1662        //   borrow-check ensures that it is not possible to construct a reference to `self.buf`
1663        //   within the returned lifetime.
1664        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
1665    }
1666
1667    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
1668    /// valid for zero sized reads if the vector didn't allocate.
1669    ///
1670    /// The caller must ensure that the vector outlives the pointer this
1671    /// function returns, or else it will end up dangling.
1672    /// Modifying the vector may cause its buffer to be reallocated,
1673    /// which would also make any pointers to it invalid.
1674    ///
1675    /// The caller must also ensure that the memory the pointer (non-transitively) points to
1676    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
1677    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
1678    ///
1679    /// This method guarantees that for the purpose of the aliasing model, this method
1680    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1681    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1682    /// and [`as_non_null`].
1683    /// Note that calling other methods that materialize mutable references to the slice,
1684    /// or mutable references to specific elements you are planning on accessing through this pointer,
1685    /// as well as writing to those elements, may still invalidate this pointer.
1686    /// See the second example below for how this guarantee can be used.
1687    ///
1688    ///
1689    /// # Examples
1690    ///
1691    /// ```
1692    /// let x = vec![1, 2, 4];
1693    /// let x_ptr = x.as_ptr();
1694    ///
1695    /// unsafe {
1696    ///     for i in 0..x.len() {
1697    ///         assert_eq!(*x_ptr.add(i), 1 << i);
1698    ///     }
1699    /// }
1700    /// ```
1701    ///
1702    /// Due to the aliasing guarantee, the following code is legal:
1703    ///
1704    /// ```rust
1705    /// unsafe {
1706    ///     let mut v = vec![0, 1, 2];
1707    ///     let ptr1 = v.as_ptr();
1708    ///     let _ = ptr1.read();
1709    ///     let ptr2 = v.as_mut_ptr().offset(2);
1710    ///     ptr2.write(2);
1711    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`
1712    ///     // because it mutated a different element:
1713    ///     let _ = ptr1.read();
1714    /// }
1715    /// ```
1716    ///
1717    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1718    /// [`as_ptr`]: Vec::as_ptr
1719    /// [`as_non_null`]: Vec::as_non_null
1720    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
1721    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1722    #[rustc_never_returns_null_ptr]
1723    #[rustc_as_ptr]
1724    #[inline]
1725    pub const fn as_ptr(&self) -> *const T {
1726        // We shadow the slice method of the same name to avoid going through
1727        // `deref`, which creates an intermediate reference.
1728        self.buf.ptr()
1729    }
1730
1731    /// Returns a raw mutable pointer to the vector's buffer, or a dangling
1732    /// raw pointer valid for zero sized reads if the vector didn't allocate.
1733    ///
1734    /// The caller must ensure that the vector outlives the pointer this
1735    /// function returns, or else it will end up dangling.
1736    /// Modifying the vector may cause its buffer to be reallocated,
1737    /// which would also make any pointers to it invalid.
1738    ///
1739    /// This method guarantees that for the purpose of the aliasing model, this method
1740    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1741    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1742    /// and [`as_non_null`].
1743    /// Note that calling other methods that materialize references to the slice,
1744    /// or references to specific elements you are planning on accessing through this pointer,
1745    /// may still invalidate this pointer.
1746    /// See the second example below for how this guarantee can be used.
1747    ///
1748    /// # Examples
1749    ///
1750    /// ```
1751    /// // Allocate vector big enough for 4 elements.
1752    /// let size = 4;
1753    /// let mut x: Vec<i32> = Vec::with_capacity(size);
1754    /// let x_ptr = x.as_mut_ptr();
1755    ///
1756    /// // Initialize elements via raw pointer writes, then set length.
1757    /// unsafe {
1758    ///     for i in 0..size {
1759    ///         *x_ptr.add(i) = i as i32;
1760    ///     }
1761    ///     x.set_len(size);
1762    /// }
1763    /// assert_eq!(&*x, &[0, 1, 2, 3]);
1764    /// ```
1765    ///
1766    /// Due to the aliasing guarantee, the following code is legal:
1767    ///
1768    /// ```rust
1769    /// unsafe {
1770    ///     let mut v = vec![0];
1771    ///     let ptr1 = v.as_mut_ptr();
1772    ///     ptr1.write(1);
1773    ///     let ptr2 = v.as_mut_ptr();
1774    ///     ptr2.write(2);
1775    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1776    ///     ptr1.write(3);
1777    /// }
1778    /// ```
1779    ///
1780    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1781    /// [`as_ptr`]: Vec::as_ptr
1782    /// [`as_non_null`]: Vec::as_non_null
1783    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
1784    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1785    #[rustc_never_returns_null_ptr]
1786    #[rustc_as_ptr]
1787    #[inline]
1788    pub const fn as_mut_ptr(&mut self) -> *mut T {
1789        // We shadow the slice method of the same name to avoid going through
1790        // `deref_mut`, which creates an intermediate reference.
1791        self.buf.ptr()
1792    }
1793
1794    /// Returns a `NonNull` pointer to the vector's buffer, or a dangling
1795    /// `NonNull` pointer valid for zero sized reads if the vector didn't allocate.
1796    ///
1797    /// The caller must ensure that the vector outlives the pointer this
1798    /// function returns, or else it will end up dangling.
1799    /// Modifying the vector may cause its buffer to be reallocated,
1800    /// which would also make any pointers to it invalid.
1801    ///
1802    /// This method guarantees that for the purpose of the aliasing model, this method
1803    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1804    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1805    /// and [`as_non_null`].
1806    /// Note that calling other methods that materialize references to the slice,
1807    /// or references to specific elements you are planning on accessing through this pointer,
1808    /// may still invalidate this pointer.
1809    /// See the second example below for how this guarantee can be used.
1810    ///
1811    /// # Examples
1812    ///
1813    /// ```
1814    /// #![feature(box_vec_non_null)]
1815    ///
1816    /// // Allocate vector big enough for 4 elements.
1817    /// let size = 4;
1818    /// let mut x: Vec<i32> = Vec::with_capacity(size);
1819    /// let x_ptr = x.as_non_null();
1820    ///
1821    /// // Initialize elements via raw pointer writes, then set length.
1822    /// unsafe {
1823    ///     for i in 0..size {
1824    ///         x_ptr.add(i).write(i as i32);
1825    ///     }
1826    ///     x.set_len(size);
1827    /// }
1828    /// assert_eq!(&*x, &[0, 1, 2, 3]);
1829    /// ```
1830    ///
1831    /// Due to the aliasing guarantee, the following code is legal:
1832    ///
1833    /// ```rust
1834    /// #![feature(box_vec_non_null)]
1835    ///
1836    /// unsafe {
1837    ///     let mut v = vec![0];
1838    ///     let ptr1 = v.as_non_null();
1839    ///     ptr1.write(1);
1840    ///     let ptr2 = v.as_non_null();
1841    ///     ptr2.write(2);
1842    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1843    ///     ptr1.write(3);
1844    /// }
1845    /// ```
1846    ///
1847    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1848    /// [`as_ptr`]: Vec::as_ptr
1849    /// [`as_non_null`]: Vec::as_non_null
1850    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1851    #[rustc_const_unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1852    #[inline]
1853    pub const fn as_non_null(&mut self) -> NonNull<T> {
1854        self.buf.non_null()
1855    }
1856
1857    /// Returns a reference to the underlying allocator.
1858    #[unstable(feature = "allocator_api", issue = "32838")]
1859    #[inline]
1860    pub fn allocator(&self) -> &A {
1861        self.buf.allocator()
1862    }
1863
1864    /// Forces the length of the vector to `new_len`.
1865    ///
1866    /// This is a low-level operation that maintains none of the normal
1867    /// invariants of the type. Normally changing the length of a vector
1868    /// is done using one of the safe operations instead, such as
1869    /// [`truncate`], [`resize`], [`extend`], or [`clear`].
1870    ///
1871    /// [`truncate`]: Vec::truncate
1872    /// [`resize`]: Vec::resize
1873    /// [`extend`]: Extend::extend
1874    /// [`clear`]: Vec::clear
1875    ///
1876    /// # Safety
1877    ///
1878    /// - `new_len` must be less than or equal to [`capacity()`].
1879    /// - The elements at `old_len..new_len` must be initialized.
1880    ///
1881    /// [`capacity()`]: Vec::capacity
1882    ///
1883    /// # Examples
1884    ///
1885    /// See [`spare_capacity_mut()`] for an example with safe
1886    /// initialization of capacity elements and use of this method.
1887    ///
1888    /// `set_len()` can be useful for situations in which the vector
1889    /// is serving as a buffer for other code, particularly over FFI:
1890    ///
1891    /// ```no_run
1892    /// # #![allow(dead_code)]
1893    /// # // This is just a minimal skeleton for the doc example;
1894    /// # // don't use this as a starting point for a real library.
1895    /// # pub struct StreamWrapper { strm: *mut std::ffi::c_void }
1896    /// # const Z_OK: i32 = 0;
1897    /// # unsafe extern "C" {
1898    /// #     fn deflateGetDictionary(
1899    /// #         strm: *mut std::ffi::c_void,
1900    /// #         dictionary: *mut u8,
1901    /// #         dictLength: *mut usize,
1902    /// #     ) -> i32;
1903    /// # }
1904    /// # impl StreamWrapper {
1905    /// pub fn get_dictionary(&self) -> Option<Vec<u8>> {
1906    ///     // Per the FFI method's docs, "32768 bytes is always enough".
1907    ///     let mut dict = Vec::with_capacity(32_768);
1908    ///     let mut dict_length = 0;
1909    ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
1910    ///     // 1. `dict_length` elements were initialized.
1911    ///     // 2. `dict_length` <= the capacity (32_768)
1912    ///     // which makes `set_len` safe to call.
1913    ///     unsafe {
1914    ///         // Make the FFI call...
1915    ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
1916    ///         if r == Z_OK {
1917    ///             // ...and update the length to what was initialized.
1918    ///             dict.set_len(dict_length);
1919    ///             Some(dict)
1920    ///         } else {
1921    ///             None
1922    ///         }
1923    ///     }
1924    /// }
1925    /// # }
1926    /// ```
1927    ///
1928    /// While the following example is sound, there is a memory leak since
1929    /// the inner vectors were not freed prior to the `set_len` call:
1930    ///
1931    /// ```
1932    /// let mut vec = vec![vec![1, 0, 0],
1933    ///                    vec![0, 1, 0],
1934    ///                    vec![0, 0, 1]];
1935    /// // SAFETY:
1936    /// // 1. `old_len..0` is empty so no elements need to be initialized.
1937    /// // 2. `0 <= capacity` always holds whatever `capacity` is.
1938    /// unsafe {
1939    ///     vec.set_len(0);
1940    /// #   // FIXME(https://github.com/rust-lang/miri/issues/3670):
1941    /// #   // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1942    /// #   vec.set_len(3);
1943    /// }
1944    /// ```
1945    ///
1946    /// Normally, here, one would use [`clear`] instead to correctly drop
1947    /// the contents and thus not leak memory.
1948    ///
1949    /// [`spare_capacity_mut()`]: Vec::spare_capacity_mut
1950    #[inline]
1951    #[stable(feature = "rust1", since = "1.0.0")]
1952    pub unsafe fn set_len(&mut self, new_len: usize) {
1953        debug_assert!(new_len <= self.capacity());
1954
1955        self.len = new_len;
1956    }
1957
1958    /// Removes an element from the vector and returns it.
1959    ///
1960    /// The removed element is replaced by the last element of the vector.
1961    ///
1962    /// This does not preserve ordering of the remaining elements, but is *O*(1).
1963    /// If you need to preserve the element order, use [`remove`] instead.
1964    ///
1965    /// [`remove`]: Vec::remove
1966    ///
1967    /// # Panics
1968    ///
1969    /// Panics if `index` is out of bounds.
1970    ///
1971    /// # Examples
1972    ///
1973    /// ```
1974    /// let mut v = vec!["foo", "bar", "baz", "qux"];
1975    ///
1976    /// assert_eq!(v.swap_remove(1), "bar");
1977    /// assert_eq!(v, ["foo", "qux", "baz"]);
1978    ///
1979    /// assert_eq!(v.swap_remove(0), "foo");
1980    /// assert_eq!(v, ["baz", "qux"]);
1981    /// ```
1982    #[inline]
1983    #[stable(feature = "rust1", since = "1.0.0")]
1984    pub fn swap_remove(&mut self, index: usize) -> T {
1985        #[cold]
1986        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
1987        #[track_caller]
1988        #[optimize(size)]
1989        fn assert_failed(index: usize, len: usize) -> ! {
1990            panic!("swap_remove index (is {index}) should be < len (is {len})");
1991        }
1992
1993        let len = self.len();
1994        if index >= len {
1995            assert_failed(index, len);
1996        }
1997        unsafe {
1998            // We replace self[index] with the last element. Note that if the
1999            // bounds check above succeeds there must be a last element (which
2000            // can be self[index] itself).
2001            let value = ptr::read(self.as_ptr().add(index));
2002            let base_ptr = self.as_mut_ptr();
2003            ptr::copy(base_ptr.add(len - 1), base_ptr.add(index), 1);
2004            self.set_len(len - 1);
2005            value
2006        }
2007    }
2008
2009    /// Inserts an element at position `index` within the vector, shifting all
2010    /// elements after it to the right.
2011    ///
2012    /// # Panics
2013    ///
2014    /// Panics if `index > len`.
2015    ///
2016    /// # Examples
2017    ///
2018    /// ```
2019    /// let mut vec = vec!['a', 'b', 'c'];
2020    /// vec.insert(1, 'd');
2021    /// assert_eq!(vec, ['a', 'd', 'b', 'c']);
2022    /// vec.insert(4, 'e');
2023    /// assert_eq!(vec, ['a', 'd', 'b', 'c', 'e']);
2024    /// ```
2025    ///
2026    /// # Time complexity
2027    ///
2028    /// Takes *O*([`Vec::len`]) time. All items after the insertion index must be
2029    /// shifted to the right. In the worst case, all elements are shifted when
2030    /// the insertion index is 0.
2031    #[cfg(not(no_global_oom_handling))]
2032    #[stable(feature = "rust1", since = "1.0.0")]
2033    #[track_caller]
2034    pub fn insert(&mut self, index: usize, element: T) {
2035        #[cold]
2036        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2037        #[track_caller]
2038        #[optimize(size)]
2039        fn assert_failed(index: usize, len: usize) -> ! {
2040            panic!("insertion index (is {index}) should be <= len (is {len})");
2041        }
2042
2043        let len = self.len();
2044        if index > len {
2045            assert_failed(index, len);
2046        }
2047
2048        // space for the new element
2049        if len == self.buf.capacity() {
2050            self.buf.grow_one();
2051        }
2052
2053        unsafe {
2054            // infallible
2055            // The spot to put the new value
2056            {
2057                let p = self.as_mut_ptr().add(index);
2058                if index < len {
2059                    // Shift everything over to make space. (Duplicating the
2060                    // `index`th element into two consecutive places.)
2061                    ptr::copy(p, p.add(1), len - index);
2062                }
2063                // Write it in, overwriting the first copy of the `index`th
2064                // element.
2065                ptr::write(p, element);
2066            }
2067            self.set_len(len + 1);
2068        }
2069    }
2070
2071    /// Removes and returns the element at position `index` within the vector,
2072    /// shifting all elements after it to the left.
2073    ///
2074    /// Note: Because this shifts over the remaining elements, it has a
2075    /// worst-case performance of *O*(*n*). If you don't need the order of elements
2076    /// to be preserved, use [`swap_remove`] instead. If you'd like to remove
2077    /// elements from the beginning of the `Vec`, consider using
2078    /// [`VecDeque::pop_front`] instead.
2079    ///
2080    /// [`swap_remove`]: Vec::swap_remove
2081    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
2082    ///
2083    /// # Panics
2084    ///
2085    /// Panics if `index` is out of bounds.
2086    ///
2087    /// # Examples
2088    ///
2089    /// ```
2090    /// let mut v = vec!['a', 'b', 'c'];
2091    /// assert_eq!(v.remove(1), 'b');
2092    /// assert_eq!(v, ['a', 'c']);
2093    /// ```
2094    #[stable(feature = "rust1", since = "1.0.0")]
2095    #[track_caller]
2096    #[rustc_confusables("delete", "take")]
2097    pub fn remove(&mut self, index: usize) -> T {
2098        #[cold]
2099        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2100        #[track_caller]
2101        #[optimize(size)]
2102        fn assert_failed(index: usize, len: usize) -> ! {
2103            panic!("removal index (is {index}) should be < len (is {len})");
2104        }
2105
2106        let len = self.len();
2107        if index >= len {
2108            assert_failed(index, len);
2109        }
2110        unsafe {
2111            // infallible
2112            let ret;
2113            {
2114                // the place we are taking from.
2115                let ptr = self.as_mut_ptr().add(index);
2116                // copy it out, unsafely having a copy of the value on
2117                // the stack and in the vector at the same time.
2118                ret = ptr::read(ptr);
2119
2120                // Shift everything down to fill in that spot.
2121                ptr::copy(ptr.add(1), ptr, len - index - 1);
2122            }
2123            self.set_len(len - 1);
2124            ret
2125        }
2126    }
2127
2128    /// Retains only the elements specified by the predicate.
2129    ///
2130    /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
2131    /// This method operates in place, visiting each element exactly once in the
2132    /// original order, and preserves the order of the retained elements.
2133    ///
2134    /// # Examples
2135    ///
2136    /// ```
2137    /// let mut vec = vec![1, 2, 3, 4];
2138    /// vec.retain(|&x| x % 2 == 0);
2139    /// assert_eq!(vec, [2, 4]);
2140    /// ```
2141    ///
2142    /// Because the elements are visited exactly once in the original order,
2143    /// external state may be used to decide which elements to keep.
2144    ///
2145    /// ```
2146    /// let mut vec = vec![1, 2, 3, 4, 5];
2147    /// let keep = [false, true, true, false, true];
2148    /// let mut iter = keep.iter();
2149    /// vec.retain(|_| *iter.next().unwrap());
2150    /// assert_eq!(vec, [2, 3, 5]);
2151    /// ```
2152    #[stable(feature = "rust1", since = "1.0.0")]
2153    pub fn retain<F>(&mut self, mut f: F)
2154    where
2155        F: FnMut(&T) -> bool,
2156    {
2157        self.retain_mut(|elem| f(elem));
2158    }
2159
2160    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
2161    ///
2162    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
2163    /// This method operates in place, visiting each element exactly once in the
2164    /// original order, and preserves the order of the retained elements.
2165    ///
2166    /// # Examples
2167    ///
2168    /// ```
2169    /// let mut vec = vec![1, 2, 3, 4];
2170    /// vec.retain_mut(|x| if *x <= 3 {
2171    ///     *x += 1;
2172    ///     true
2173    /// } else {
2174    ///     false
2175    /// });
2176    /// assert_eq!(vec, [2, 3, 4]);
2177    /// ```
2178    #[stable(feature = "vec_retain_mut", since = "1.61.0")]
2179    pub fn retain_mut<F>(&mut self, mut f: F)
2180    where
2181        F: FnMut(&mut T) -> bool,
2182    {
2183        let original_len = self.len();
2184
2185        if original_len == 0 {
2186            // Empty case: explicit return allows better optimization, vs letting compiler infer it
2187            return;
2188        }
2189
2190        // Avoid double drop if the drop guard is not executed,
2191        // since we may make some holes during the process.
2192        unsafe { self.set_len(0) };
2193
2194        // Vec: [Kept, Kept, Hole, Hole, Hole, Hole, Unchecked, Unchecked]
2195        //      |<-              processed len   ->| ^- next to check
2196        //                  |<-  deleted cnt     ->|
2197        //      |<-              original_len                          ->|
2198        // Kept: Elements which predicate returns true on.
2199        // Hole: Moved or dropped element slot.
2200        // Unchecked: Unchecked valid elements.
2201        //
2202        // This drop guard will be invoked when predicate or `drop` of element panicked.
2203        // It shifts unchecked elements to cover holes and `set_len` to the correct length.
2204        // In cases when predicate and `drop` never panick, it will be optimized out.
2205        struct BackshiftOnDrop<'a, T, A: Allocator> {
2206            v: &'a mut Vec<T, A>,
2207            processed_len: usize,
2208            deleted_cnt: usize,
2209            original_len: usize,
2210        }
2211
2212        impl<T, A: Allocator> Drop for BackshiftOnDrop<'_, T, A> {
2213            fn drop(&mut self) {
2214                if self.deleted_cnt > 0 {
2215                    // SAFETY: Trailing unchecked items must be valid since we never touch them.
2216                    unsafe {
2217                        ptr::copy(
2218                            self.v.as_ptr().add(self.processed_len),
2219                            self.v.as_mut_ptr().add(self.processed_len - self.deleted_cnt),
2220                            self.original_len - self.processed_len,
2221                        );
2222                    }
2223                }
2224                // SAFETY: After filling holes, all items are in contiguous memory.
2225                unsafe {
2226                    self.v.set_len(self.original_len - self.deleted_cnt);
2227                }
2228            }
2229        }
2230
2231        let mut g = BackshiftOnDrop { v: self, processed_len: 0, deleted_cnt: 0, original_len };
2232
2233        fn process_loop<F, T, A: Allocator, const DELETED: bool>(
2234            original_len: usize,
2235            f: &mut F,
2236            g: &mut BackshiftOnDrop<'_, T, A>,
2237        ) where
2238            F: FnMut(&mut T) -> bool,
2239        {
2240            while g.processed_len != original_len {
2241                // SAFETY: Unchecked element must be valid.
2242                let cur = unsafe { &mut *g.v.as_mut_ptr().add(g.processed_len) };
2243                if !f(cur) {
2244                    // Advance early to avoid double drop if `drop_in_place` panicked.
2245                    g.processed_len += 1;
2246                    g.deleted_cnt += 1;
2247                    // SAFETY: We never touch this element again after dropped.
2248                    unsafe { ptr::drop_in_place(cur) };
2249                    // We already advanced the counter.
2250                    if DELETED {
2251                        continue;
2252                    } else {
2253                        break;
2254                    }
2255                }
2256                if DELETED {
2257                    // SAFETY: `deleted_cnt` > 0, so the hole slot must not overlap with current element.
2258                    // We use copy for move, and never touch this element again.
2259                    unsafe {
2260                        let hole_slot = g.v.as_mut_ptr().add(g.processed_len - g.deleted_cnt);
2261                        ptr::copy_nonoverlapping(cur, hole_slot, 1);
2262                    }
2263                }
2264                g.processed_len += 1;
2265            }
2266        }
2267
2268        // Stage 1: Nothing was deleted.
2269        process_loop::<F, T, A, false>(original_len, &mut f, &mut g);
2270
2271        // Stage 2: Some elements were deleted.
2272        process_loop::<F, T, A, true>(original_len, &mut f, &mut g);
2273
2274        // All item are processed. This can be optimized to `set_len` by LLVM.
2275        drop(g);
2276    }
2277
2278    /// Removes all but the first of consecutive elements in the vector that resolve to the same
2279    /// key.
2280    ///
2281    /// If the vector is sorted, this removes all duplicates.
2282    ///
2283    /// # Examples
2284    ///
2285    /// ```
2286    /// let mut vec = vec![10, 20, 21, 30, 20];
2287    ///
2288    /// vec.dedup_by_key(|i| *i / 10);
2289    ///
2290    /// assert_eq!(vec, [10, 20, 30, 20]);
2291    /// ```
2292    #[stable(feature = "dedup_by", since = "1.16.0")]
2293    #[inline]
2294    pub fn dedup_by_key<F, K>(&mut self, mut key: F)
2295    where
2296        F: FnMut(&mut T) -> K,
2297        K: PartialEq,
2298    {
2299        self.dedup_by(|a, b| key(a) == key(b))
2300    }
2301
2302    /// Removes all but the first of consecutive elements in the vector satisfying a given equality
2303    /// relation.
2304    ///
2305    /// The `same_bucket` function is passed references to two elements from the vector and
2306    /// must determine if the elements compare equal. The elements are passed in opposite order
2307    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
2308    ///
2309    /// If the vector is sorted, this removes all duplicates.
2310    ///
2311    /// # Examples
2312    ///
2313    /// ```
2314    /// let mut vec = vec!["foo", "bar", "Bar", "baz", "bar"];
2315    ///
2316    /// vec.dedup_by(|a, b| a.eq_ignore_ascii_case(b));
2317    ///
2318    /// assert_eq!(vec, ["foo", "bar", "baz", "bar"]);
2319    /// ```
2320    #[stable(feature = "dedup_by", since = "1.16.0")]
2321    pub fn dedup_by<F>(&mut self, mut same_bucket: F)
2322    where
2323        F: FnMut(&mut T, &mut T) -> bool,
2324    {
2325        let len = self.len();
2326        if len <= 1 {
2327            return;
2328        }
2329
2330        // Check if we ever want to remove anything.
2331        // This allows to use copy_non_overlapping in next cycle.
2332        // And avoids any memory writes if we don't need to remove anything.
2333        let mut first_duplicate_idx: usize = 1;
2334        let start = self.as_mut_ptr();
2335        while first_duplicate_idx != len {
2336            let found_duplicate = unsafe {
2337                // SAFETY: first_duplicate always in range [1..len)
2338                // Note that we start iteration from 1 so we never overflow.
2339                let prev = start.add(first_duplicate_idx.wrapping_sub(1));
2340                let current = start.add(first_duplicate_idx);
2341                // We explicitly say in docs that references are reversed.
2342                same_bucket(&mut *current, &mut *prev)
2343            };
2344            if found_duplicate {
2345                break;
2346            }
2347            first_duplicate_idx += 1;
2348        }
2349        // Don't need to remove anything.
2350        // We cannot get bigger than len.
2351        if first_duplicate_idx == len {
2352            return;
2353        }
2354
2355        /* INVARIANT: vec.len() > read > write > write-1 >= 0 */
2356        struct FillGapOnDrop<'a, T, A: core::alloc::Allocator> {
2357            /* Offset of the element we want to check if it is duplicate */
2358            read: usize,
2359
2360            /* Offset of the place where we want to place the non-duplicate
2361             * when we find it. */
2362            write: usize,
2363
2364            /* The Vec that would need correction if `same_bucket` panicked */
2365            vec: &'a mut Vec<T, A>,
2366        }
2367
2368        impl<'a, T, A: core::alloc::Allocator> Drop for FillGapOnDrop<'a, T, A> {
2369            fn drop(&mut self) {
2370                /* This code gets executed when `same_bucket` panics */
2371
2372                /* SAFETY: invariant guarantees that `read - write`
2373                 * and `len - read` never overflow and that the copy is always
2374                 * in-bounds. */
2375                unsafe {
2376                    let ptr = self.vec.as_mut_ptr();
2377                    let len = self.vec.len();
2378
2379                    /* How many items were left when `same_bucket` panicked.
2380                     * Basically vec[read..].len() */
2381                    let items_left = len.wrapping_sub(self.read);
2382
2383                    /* Pointer to first item in vec[write..write+items_left] slice */
2384                    let dropped_ptr = ptr.add(self.write);
2385                    /* Pointer to first item in vec[read..] slice */
2386                    let valid_ptr = ptr.add(self.read);
2387
2388                    /* Copy `vec[read..]` to `vec[write..write+items_left]`.
2389                     * The slices can overlap, so `copy_nonoverlapping` cannot be used */
2390                    ptr::copy(valid_ptr, dropped_ptr, items_left);
2391
2392                    /* How many items have been already dropped
2393                     * Basically vec[read..write].len() */
2394                    let dropped = self.read.wrapping_sub(self.write);
2395
2396                    self.vec.set_len(len - dropped);
2397                }
2398            }
2399        }
2400
2401        /* Drop items while going through Vec, it should be more efficient than
2402         * doing slice partition_dedup + truncate */
2403
2404        // Construct gap first and then drop item to avoid memory corruption if `T::drop` panics.
2405        let mut gap =
2406            FillGapOnDrop { read: first_duplicate_idx + 1, write: first_duplicate_idx, vec: self };
2407        unsafe {
2408            // SAFETY: we checked that first_duplicate_idx in bounds before.
2409            // If drop panics, `gap` would remove this item without drop.
2410            ptr::drop_in_place(start.add(first_duplicate_idx));
2411        }
2412
2413        /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
2414         * are always in-bounds and read_ptr never aliases prev_ptr */
2415        unsafe {
2416            while gap.read < len {
2417                let read_ptr = start.add(gap.read);
2418                let prev_ptr = start.add(gap.write.wrapping_sub(1));
2419
2420                // We explicitly say in docs that references are reversed.
2421                let found_duplicate = same_bucket(&mut *read_ptr, &mut *prev_ptr);
2422                if found_duplicate {
2423                    // Increase `gap.read` now since the drop may panic.
2424                    gap.read += 1;
2425                    /* We have found duplicate, drop it in-place */
2426                    ptr::drop_in_place(read_ptr);
2427                } else {
2428                    let write_ptr = start.add(gap.write);
2429
2430                    /* read_ptr cannot be equal to write_ptr because at this point
2431                     * we guaranteed to skip at least one element (before loop starts).
2432                     */
2433                    ptr::copy_nonoverlapping(read_ptr, write_ptr, 1);
2434
2435                    /* We have filled that place, so go further */
2436                    gap.write += 1;
2437                    gap.read += 1;
2438                }
2439            }
2440
2441            /* Technically we could let `gap` clean up with its Drop, but
2442             * when `same_bucket` is guaranteed to not panic, this bloats a little
2443             * the codegen, so we just do it manually */
2444            gap.vec.set_len(gap.write);
2445            mem::forget(gap);
2446        }
2447    }
2448
2449    /// Appends an element to the back of a collection.
2450    ///
2451    /// # Panics
2452    ///
2453    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2454    ///
2455    /// # Examples
2456    ///
2457    /// ```
2458    /// let mut vec = vec![1, 2];
2459    /// vec.push(3);
2460    /// assert_eq!(vec, [1, 2, 3]);
2461    /// ```
2462    ///
2463    /// # Time complexity
2464    ///
2465    /// Takes amortized *O*(1) time. If the vector's length would exceed its
2466    /// capacity after the push, *O*(*capacity*) time is taken to copy the
2467    /// vector's elements to a larger allocation. This expensive operation is
2468    /// offset by the *capacity* *O*(1) insertions it allows.
2469    #[cfg(not(no_global_oom_handling))]
2470    #[inline]
2471    #[stable(feature = "rust1", since = "1.0.0")]
2472    #[rustc_confusables("push_back", "put", "append")]
2473    #[track_caller]
2474    pub fn push(&mut self, value: T) {
2475        // Inform codegen that the length does not change across grow_one().
2476        let len = self.len;
2477        // This will panic or abort if we would allocate > isize::MAX bytes
2478        // or if the length increment would overflow for zero-sized types.
2479        if len == self.buf.capacity() {
2480            self.buf.grow_one();
2481        }
2482        unsafe {
2483            let end = self.as_mut_ptr().add(len);
2484            ptr::write(end, value);
2485            self.len = len + 1;
2486        }
2487    }
2488
2489    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
2490    /// with the element.
2491    ///
2492    /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
2493    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
2494    ///
2495    /// [`push`]: Vec::push
2496    /// [`reserve`]: Vec::reserve
2497    /// [`try_reserve`]: Vec::try_reserve
2498    ///
2499    /// # Examples
2500    ///
2501    /// A manual, panic-free alternative to [`FromIterator`]:
2502    ///
2503    /// ```
2504    /// #![feature(vec_push_within_capacity)]
2505    ///
2506    /// use std::collections::TryReserveError;
2507    /// fn from_iter_fallible<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
2508    ///     let mut vec = Vec::new();
2509    ///     for value in iter {
2510    ///         if let Err(value) = vec.push_within_capacity(value) {
2511    ///             vec.try_reserve(1)?;
2512    ///             // this cannot fail, the previous line either returned or added at least 1 free slot
2513    ///             let _ = vec.push_within_capacity(value);
2514    ///         }
2515    ///     }
2516    ///     Ok(vec)
2517    /// }
2518    /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
2519    /// ```
2520    ///
2521    /// # Time complexity
2522    ///
2523    /// Takes *O*(1) time.
2524    #[inline]
2525    #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
2526    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
2527        if self.len == self.buf.capacity() {
2528            return Err(value);
2529        }
2530        unsafe {
2531            let end = self.as_mut_ptr().add(self.len);
2532            ptr::write(end, value);
2533            self.len += 1;
2534        }
2535        Ok(())
2536    }
2537
2538    /// Removes the last element from a vector and returns it, or [`None`] if it
2539    /// is empty.
2540    ///
2541    /// If you'd like to pop the first element, consider using
2542    /// [`VecDeque::pop_front`] instead.
2543    ///
2544    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
2545    ///
2546    /// # Examples
2547    ///
2548    /// ```
2549    /// let mut vec = vec![1, 2, 3];
2550    /// assert_eq!(vec.pop(), Some(3));
2551    /// assert_eq!(vec, [1, 2]);
2552    /// ```
2553    ///
2554    /// # Time complexity
2555    ///
2556    /// Takes *O*(1) time.
2557    #[inline]
2558    #[stable(feature = "rust1", since = "1.0.0")]
2559    #[rustc_diagnostic_item = "vec_pop"]
2560    pub fn pop(&mut self) -> Option<T> {
2561        if self.len == 0 {
2562            None
2563        } else {
2564            unsafe {
2565                self.len -= 1;
2566                core::hint::assert_unchecked(self.len < self.capacity());
2567                Some(ptr::read(self.as_ptr().add(self.len())))
2568            }
2569        }
2570    }
2571
2572    /// Removes and returns the last element from a vector if the predicate
2573    /// returns `true`, or [`None`] if the predicate returns false or the vector
2574    /// is empty (the predicate will not be called in that case).
2575    ///
2576    /// # Examples
2577    ///
2578    /// ```
2579    /// let mut vec = vec![1, 2, 3, 4];
2580    /// let pred = |x: &mut i32| *x % 2 == 0;
2581    ///
2582    /// assert_eq!(vec.pop_if(pred), Some(4));
2583    /// assert_eq!(vec, [1, 2, 3]);
2584    /// assert_eq!(vec.pop_if(pred), None);
2585    /// ```
2586    #[stable(feature = "vec_pop_if", since = "1.86.0")]
2587    pub fn pop_if(&mut self, predicate: impl FnOnce(&mut T) -> bool) -> Option<T> {
2588        let last = self.last_mut()?;
2589        if predicate(last) { self.pop() } else { None }
2590    }
2591
2592    /// Moves all the elements of `other` into `self`, leaving `other` empty.
2593    ///
2594    /// # Panics
2595    ///
2596    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2597    ///
2598    /// # Examples
2599    ///
2600    /// ```
2601    /// let mut vec = vec![1, 2, 3];
2602    /// let mut vec2 = vec![4, 5, 6];
2603    /// vec.append(&mut vec2);
2604    /// assert_eq!(vec, [1, 2, 3, 4, 5, 6]);
2605    /// assert_eq!(vec2, []);
2606    /// ```
2607    #[cfg(not(no_global_oom_handling))]
2608    #[inline]
2609    #[stable(feature = "append", since = "1.4.0")]
2610    #[track_caller]
2611    pub fn append(&mut self, other: &mut Self) {
2612        unsafe {
2613            self.append_elements(other.as_slice() as _);
2614            other.set_len(0);
2615        }
2616    }
2617
2618    /// Appends elements to `self` from other buffer.
2619    #[cfg(not(no_global_oom_handling))]
2620    #[inline]
2621    #[track_caller]
2622    unsafe fn append_elements(&mut self, other: *const [T]) {
2623        let count = other.len();
2624        self.reserve(count);
2625        let len = self.len();
2626        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
2627        self.len += count;
2628    }
2629
2630    /// Removes the subslice indicated by the given range from the vector,
2631    /// returning a double-ended iterator over the removed subslice.
2632    ///
2633    /// If the iterator is dropped before being fully consumed,
2634    /// it drops the remaining removed elements.
2635    ///
2636    /// The returned iterator keeps a mutable borrow on the vector to optimize
2637    /// its implementation.
2638    ///
2639    /// # Panics
2640    ///
2641    /// Panics if the starting point is greater than the end point or if
2642    /// the end point is greater than the length of the vector.
2643    ///
2644    /// # Leaking
2645    ///
2646    /// If the returned iterator goes out of scope without being dropped (due to
2647    /// [`mem::forget`], for example), the vector may have lost and leaked
2648    /// elements arbitrarily, including elements outside the range.
2649    ///
2650    /// # Examples
2651    ///
2652    /// ```
2653    /// let mut v = vec![1, 2, 3];
2654    /// let u: Vec<_> = v.drain(1..).collect();
2655    /// assert_eq!(v, &[1]);
2656    /// assert_eq!(u, &[2, 3]);
2657    ///
2658    /// // A full range clears the vector, like `clear()` does
2659    /// v.drain(..);
2660    /// assert_eq!(v, &[]);
2661    /// ```
2662    #[stable(feature = "drain", since = "1.6.0")]
2663    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
2664    where
2665        R: RangeBounds<usize>,
2666    {
2667        // Memory safety
2668        //
2669        // When the Drain is first created, it shortens the length of
2670        // the source vector to make sure no uninitialized or moved-from elements
2671        // are accessible at all if the Drain's destructor never gets to run.
2672        //
2673        // Drain will ptr::read out the values to remove.
2674        // When finished, remaining tail of the vec is copied back to cover
2675        // the hole, and the vector length is restored to the new length.
2676        //
2677        let len = self.len();
2678        let Range { start, end } = slice::range(range, ..len);
2679
2680        unsafe {
2681            // set self.vec length's to start, to be safe in case Drain is leaked
2682            self.set_len(start);
2683            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
2684            Drain {
2685                tail_start: end,
2686                tail_len: len - end,
2687                iter: range_slice.iter(),
2688                vec: NonNull::from(self),
2689            }
2690        }
2691    }
2692
2693    /// Clears the vector, removing all values.
2694    ///
2695    /// Note that this method has no effect on the allocated capacity
2696    /// of the vector.
2697    ///
2698    /// # Examples
2699    ///
2700    /// ```
2701    /// let mut v = vec![1, 2, 3];
2702    ///
2703    /// v.clear();
2704    ///
2705    /// assert!(v.is_empty());
2706    /// ```
2707    #[inline]
2708    #[stable(feature = "rust1", since = "1.0.0")]
2709    pub fn clear(&mut self) {
2710        let elems: *mut [T] = self.as_mut_slice();
2711
2712        // SAFETY:
2713        // - `elems` comes directly from `as_mut_slice` and is therefore valid.
2714        // - Setting `self.len` before calling `drop_in_place` means that,
2715        //   if an element's `Drop` impl panics, the vector's `Drop` impl will
2716        //   do nothing (leaking the rest of the elements) instead of dropping
2717        //   some twice.
2718        unsafe {
2719            self.len = 0;
2720            ptr::drop_in_place(elems);
2721        }
2722    }
2723
2724    /// Returns the number of elements in the vector, also referred to
2725    /// as its 'length'.
2726    ///
2727    /// # Examples
2728    ///
2729    /// ```
2730    /// let a = vec![1, 2, 3];
2731    /// assert_eq!(a.len(), 3);
2732    /// ```
2733    #[inline]
2734    #[stable(feature = "rust1", since = "1.0.0")]
2735    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
2736    #[rustc_confusables("length", "size")]
2737    pub const fn len(&self) -> usize {
2738        let len = self.len;
2739
2740        // SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so the maximum value can
2741        // be returned is `usize::checked_div(size_of::<T>()).unwrap_or(usize::MAX)`, which
2742        // matches the definition of `T::MAX_SLICE_LEN`.
2743        unsafe { intrinsics::assume(len <= T::MAX_SLICE_LEN) };
2744
2745        len
2746    }
2747
2748    /// Returns `true` if the vector contains no elements.
2749    ///
2750    /// # Examples
2751    ///
2752    /// ```
2753    /// let mut v = Vec::new();
2754    /// assert!(v.is_empty());
2755    ///
2756    /// v.push(1);
2757    /// assert!(!v.is_empty());
2758    /// ```
2759    #[stable(feature = "rust1", since = "1.0.0")]
2760    #[rustc_diagnostic_item = "vec_is_empty"]
2761    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
2762    pub const fn is_empty(&self) -> bool {
2763        self.len() == 0
2764    }
2765
2766    /// Splits the collection into two at the given index.
2767    ///
2768    /// Returns a newly allocated vector containing the elements in the range
2769    /// `[at, len)`. After the call, the original vector will be left containing
2770    /// the elements `[0, at)` with its previous capacity unchanged.
2771    ///
2772    /// - If you want to take ownership of the entire contents and capacity of
2773    ///   the vector, see [`mem::take`] or [`mem::replace`].
2774    /// - If you don't need the returned vector at all, see [`Vec::truncate`].
2775    /// - If you want to take ownership of an arbitrary subslice, or you don't
2776    ///   necessarily want to store the removed items in a vector, see [`Vec::drain`].
2777    ///
2778    /// # Panics
2779    ///
2780    /// Panics if `at > len`.
2781    ///
2782    /// # Examples
2783    ///
2784    /// ```
2785    /// let mut vec = vec!['a', 'b', 'c'];
2786    /// let vec2 = vec.split_off(1);
2787    /// assert_eq!(vec, ['a']);
2788    /// assert_eq!(vec2, ['b', 'c']);
2789    /// ```
2790    #[cfg(not(no_global_oom_handling))]
2791    #[inline]
2792    #[must_use = "use `.truncate()` if you don't need the other half"]
2793    #[stable(feature = "split_off", since = "1.4.0")]
2794    #[track_caller]
2795    pub fn split_off(&mut self, at: usize) -> Self
2796    where
2797        A: Clone,
2798    {
2799        #[cold]
2800        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2801        #[track_caller]
2802        #[optimize(size)]
2803        fn assert_failed(at: usize, len: usize) -> ! {
2804            panic!("`at` split index (is {at}) should be <= len (is {len})");
2805        }
2806
2807        if at > self.len() {
2808            assert_failed(at, self.len());
2809        }
2810
2811        let other_len = self.len - at;
2812        let mut other = Vec::with_capacity_in(other_len, self.allocator().clone());
2813
2814        // Unsafely `set_len` and copy items to `other`.
2815        unsafe {
2816            self.set_len(at);
2817            other.set_len(other_len);
2818
2819            ptr::copy_nonoverlapping(self.as_ptr().add(at), other.as_mut_ptr(), other.len());
2820        }
2821        other
2822    }
2823
2824    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
2825    ///
2826    /// If `new_len` is greater than `len`, the `Vec` is extended by the
2827    /// difference, with each additional slot filled with the result of
2828    /// calling the closure `f`. The return values from `f` will end up
2829    /// in the `Vec` in the order they have been generated.
2830    ///
2831    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
2832    ///
2833    /// This method uses a closure to create new values on every push. If
2834    /// you'd rather [`Clone`] a given value, use [`Vec::resize`]. If you
2835    /// want to use the [`Default`] trait to generate values, you can
2836    /// pass [`Default::default`] as the second argument.
2837    ///
2838    /// # Panics
2839    ///
2840    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2841    ///
2842    /// # Examples
2843    ///
2844    /// ```
2845    /// let mut vec = vec![1, 2, 3];
2846    /// vec.resize_with(5, Default::default);
2847    /// assert_eq!(vec, [1, 2, 3, 0, 0]);
2848    ///
2849    /// let mut vec = vec![];
2850    /// let mut p = 1;
2851    /// vec.resize_with(4, || { p *= 2; p });
2852    /// assert_eq!(vec, [2, 4, 8, 16]);
2853    /// ```
2854    #[cfg(not(no_global_oom_handling))]
2855    #[stable(feature = "vec_resize_with", since = "1.33.0")]
2856    #[track_caller]
2857    pub fn resize_with<F>(&mut self, new_len: usize, f: F)
2858    where
2859        F: FnMut() -> T,
2860    {
2861        let len = self.len();
2862        if new_len > len {
2863            self.extend_trusted(iter::repeat_with(f).take(new_len - len));
2864        } else {
2865            self.truncate(new_len);
2866        }
2867    }
2868
2869    /// Consumes and leaks the `Vec`, returning a mutable reference to the contents,
2870    /// `&'a mut [T]`.
2871    ///
2872    /// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
2873    /// has only static references, or none at all, then this may be chosen to be
2874    /// `'static`.
2875    ///
2876    /// As of Rust 1.57, this method does not reallocate or shrink the `Vec`,
2877    /// so the leaked allocation may include unused capacity that is not part
2878    /// of the returned slice.
2879    ///
2880    /// This function is mainly useful for data that lives for the remainder of
2881    /// the program's life. Dropping the returned reference will cause a memory
2882    /// leak.
2883    ///
2884    /// # Examples
2885    ///
2886    /// Simple usage:
2887    ///
2888    /// ```
2889    /// let x = vec![1, 2, 3];
2890    /// let static_ref: &'static mut [usize] = x.leak();
2891    /// static_ref[0] += 1;
2892    /// assert_eq!(static_ref, &[2, 2, 3]);
2893    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
2894    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
2895    /// # drop(unsafe { Box::from_raw(static_ref) });
2896    /// ```
2897    #[stable(feature = "vec_leak", since = "1.47.0")]
2898    #[inline]
2899    pub fn leak<'a>(self) -> &'a mut [T]
2900    where
2901        A: 'a,
2902    {
2903        let mut me = ManuallyDrop::new(self);
2904        unsafe { slice::from_raw_parts_mut(me.as_mut_ptr(), me.len) }
2905    }
2906
2907    /// Returns the remaining spare capacity of the vector as a slice of
2908    /// `MaybeUninit<T>`.
2909    ///
2910    /// The returned slice can be used to fill the vector with data (e.g. by
2911    /// reading from a file) before marking the data as initialized using the
2912    /// [`set_len`] method.
2913    ///
2914    /// [`set_len`]: Vec::set_len
2915    ///
2916    /// # Examples
2917    ///
2918    /// ```
2919    /// // Allocate vector big enough for 10 elements.
2920    /// let mut v = Vec::with_capacity(10);
2921    ///
2922    /// // Fill in the first 3 elements.
2923    /// let uninit = v.spare_capacity_mut();
2924    /// uninit[0].write(0);
2925    /// uninit[1].write(1);
2926    /// uninit[2].write(2);
2927    ///
2928    /// // Mark the first 3 elements of the vector as being initialized.
2929    /// unsafe {
2930    ///     v.set_len(3);
2931    /// }
2932    ///
2933    /// assert_eq!(&v, &[0, 1, 2]);
2934    /// ```
2935    #[stable(feature = "vec_spare_capacity", since = "1.60.0")]
2936    #[inline]
2937    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
2938        // Note:
2939        // This method is not implemented in terms of `split_at_spare_mut`,
2940        // to prevent invalidation of pointers to the buffer.
2941        unsafe {
2942            slice::from_raw_parts_mut(
2943                self.as_mut_ptr().add(self.len) as *mut MaybeUninit<T>,
2944                self.buf.capacity() - self.len,
2945            )
2946        }
2947    }
2948
2949    /// Returns vector content as a slice of `T`, along with the remaining spare
2950    /// capacity of the vector as a slice of `MaybeUninit<T>`.
2951    ///
2952    /// The returned spare capacity slice can be used to fill the vector with data
2953    /// (e.g. by reading from a file) before marking the data as initialized using
2954    /// the [`set_len`] method.
2955    ///
2956    /// [`set_len`]: Vec::set_len
2957    ///
2958    /// Note that this is a low-level API, which should be used with care for
2959    /// optimization purposes. If you need to append data to a `Vec`
2960    /// you can use [`push`], [`extend`], [`extend_from_slice`],
2961    /// [`extend_from_within`], [`insert`], [`append`], [`resize`] or
2962    /// [`resize_with`], depending on your exact needs.
2963    ///
2964    /// [`push`]: Vec::push
2965    /// [`extend`]: Vec::extend
2966    /// [`extend_from_slice`]: Vec::extend_from_slice
2967    /// [`extend_from_within`]: Vec::extend_from_within
2968    /// [`insert`]: Vec::insert
2969    /// [`append`]: Vec::append
2970    /// [`resize`]: Vec::resize
2971    /// [`resize_with`]: Vec::resize_with
2972    ///
2973    /// # Examples
2974    ///
2975    /// ```
2976    /// #![feature(vec_split_at_spare)]
2977    ///
2978    /// let mut v = vec![1, 1, 2];
2979    ///
2980    /// // Reserve additional space big enough for 10 elements.
2981    /// v.reserve(10);
2982    ///
2983    /// let (init, uninit) = v.split_at_spare_mut();
2984    /// let sum = init.iter().copied().sum::<u32>();
2985    ///
2986    /// // Fill in the next 4 elements.
2987    /// uninit[0].write(sum);
2988    /// uninit[1].write(sum * 2);
2989    /// uninit[2].write(sum * 3);
2990    /// uninit[3].write(sum * 4);
2991    ///
2992    /// // Mark the 4 elements of the vector as being initialized.
2993    /// unsafe {
2994    ///     let len = v.len();
2995    ///     v.set_len(len + 4);
2996    /// }
2997    ///
2998    /// assert_eq!(&v, &[1, 1, 2, 4, 8, 12, 16]);
2999    /// ```
3000    #[unstable(feature = "vec_split_at_spare", issue = "81944")]
3001    #[inline]
3002    pub fn split_at_spare_mut(&mut self) -> (&mut [T], &mut [MaybeUninit<T>]) {
3003        // SAFETY:
3004        // - len is ignored and so never changed
3005        let (init, spare, _) = unsafe { self.split_at_spare_mut_with_len() };
3006        (init, spare)
3007    }
3008
3009    /// Safety: changing returned .2 (&mut usize) is considered the same as calling `.set_len(_)`.
3010    ///
3011    /// This method provides unique access to all vec parts at once in `extend_from_within`.
3012    unsafe fn split_at_spare_mut_with_len(
3013        &mut self,
3014    ) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
3015        let ptr = self.as_mut_ptr();
3016        // SAFETY:
3017        // - `ptr` is guaranteed to be valid for `self.len` elements
3018        // - but the allocation extends out to `self.buf.capacity()` elements, possibly
3019        // uninitialized
3020        let spare_ptr = unsafe { ptr.add(self.len) };
3021        let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
3022        let spare_len = self.buf.capacity() - self.len;
3023
3024        // SAFETY:
3025        // - `ptr` is guaranteed to be valid for `self.len` elements
3026        // - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
3027        unsafe {
3028            let initialized = slice::from_raw_parts_mut(ptr, self.len);
3029            let spare = slice::from_raw_parts_mut(spare_ptr, spare_len);
3030
3031            (initialized, spare, &mut self.len)
3032        }
3033    }
3034
3035    /// Groups every `N` elements in the `Vec<T>` into chunks to produce a `Vec<[T; N]>`, dropping
3036    /// elements in the remainder. `N` must be greater than zero.
3037    ///
3038    /// If the capacity is not a multiple of the chunk size, the buffer will shrink down to the
3039    /// nearest multiple with a reallocation or deallocation.
3040    ///
3041    /// This function can be used to reverse [`Vec::into_flattened`].
3042    ///
3043    /// # Examples
3044    ///
3045    /// ```
3046    /// #![feature(vec_into_chunks)]
3047    ///
3048    /// let vec = vec![0, 1, 2, 3, 4, 5, 6, 7];
3049    /// assert_eq!(vec.into_chunks::<3>(), [[0, 1, 2], [3, 4, 5]]);
3050    ///
3051    /// let vec = vec![0, 1, 2, 3];
3052    /// let chunks: Vec<[u8; 10]> = vec.into_chunks();
3053    /// assert!(chunks.is_empty());
3054    ///
3055    /// let flat = vec![0; 8 * 8 * 8];
3056    /// let reshaped: Vec<[[[u8; 8]; 8]; 8]> = flat.into_chunks().into_chunks().into_chunks();
3057    /// assert_eq!(reshaped.len(), 1);
3058    /// ```
3059    #[cfg(not(no_global_oom_handling))]
3060    #[unstable(feature = "vec_into_chunks", issue = "142137")]
3061    pub fn into_chunks<const N: usize>(mut self) -> Vec<[T; N], A> {
3062        const {
3063            assert!(N != 0, "chunk size must be greater than zero");
3064        }
3065
3066        let (len, cap) = (self.len(), self.capacity());
3067
3068        let len_remainder = len % N;
3069        if len_remainder != 0 {
3070            self.truncate(len - len_remainder);
3071        }
3072
3073        let cap_remainder = cap % N;
3074        if !T::IS_ZST && cap_remainder != 0 {
3075            self.buf.shrink_to_fit(cap - cap_remainder);
3076        }
3077
3078        let (ptr, _, _, alloc) = self.into_raw_parts_with_alloc();
3079
3080        // SAFETY:
3081        // - `ptr` and `alloc` were just returned from `self.into_raw_parts_with_alloc()`
3082        // - `[T; N]` has the same alignment as `T`
3083        // - `size_of::<[T; N]>() * cap / N == size_of::<T>() * cap`
3084        // - `len / N <= cap / N` because `len <= cap`
3085        // - the allocated memory consists of `len / N` valid values of type `[T; N]`
3086        // - `cap / N` fits the size of the allocated memory after shrinking
3087        unsafe { Vec::from_raw_parts_in(ptr.cast(), len / N, cap / N, alloc) }
3088    }
3089}
3090
3091impl<T: Clone, A: Allocator> Vec<T, A> {
3092    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
3093    ///
3094    /// If `new_len` is greater than `len`, the `Vec` is extended by the
3095    /// difference, with each additional slot filled with `value`.
3096    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
3097    ///
3098    /// This method requires `T` to implement [`Clone`],
3099    /// in order to be able to clone the passed value.
3100    /// If you need more flexibility (or want to rely on [`Default`] instead of
3101    /// [`Clone`]), use [`Vec::resize_with`].
3102    /// If you only need to resize to a smaller size, use [`Vec::truncate`].
3103    ///
3104    /// # Panics
3105    ///
3106    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
3107    ///
3108    /// # Examples
3109    ///
3110    /// ```
3111    /// let mut vec = vec!["hello"];
3112    /// vec.resize(3, "world");
3113    /// assert_eq!(vec, ["hello", "world", "world"]);
3114    ///
3115    /// let mut vec = vec!['a', 'b', 'c', 'd'];
3116    /// vec.resize(2, '_');
3117    /// assert_eq!(vec, ['a', 'b']);
3118    /// ```
3119    #[cfg(not(no_global_oom_handling))]
3120    #[stable(feature = "vec_resize", since = "1.5.0")]
3121    #[track_caller]
3122    pub fn resize(&mut self, new_len: usize, value: T) {
3123        let len = self.len();
3124
3125        if new_len > len {
3126            self.extend_with(new_len - len, value)
3127        } else {
3128            self.truncate(new_len);
3129        }
3130    }
3131
3132    /// Clones and appends all elements in a slice to the `Vec`.
3133    ///
3134    /// Iterates over the slice `other`, clones each element, and then appends
3135    /// it to this `Vec`. The `other` slice is traversed in-order.
3136    ///
3137    /// Note that this function is the same as [`extend`],
3138    /// except that it also works with slice elements that are Clone but not Copy.
3139    /// If Rust gets specialization this function may be deprecated.
3140    ///
3141    /// # Examples
3142    ///
3143    /// ```
3144    /// let mut vec = vec![1];
3145    /// vec.extend_from_slice(&[2, 3, 4]);
3146    /// assert_eq!(vec, [1, 2, 3, 4]);
3147    /// ```
3148    ///
3149    /// [`extend`]: Vec::extend
3150    #[cfg(not(no_global_oom_handling))]
3151    #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
3152    #[track_caller]
3153    pub fn extend_from_slice(&mut self, other: &[T]) {
3154        self.spec_extend(other.iter())
3155    }
3156
3157    /// Given a range `src`, clones a slice of elements in that range and appends it to the end.
3158    ///
3159    /// `src` must be a range that can form a valid subslice of the `Vec`.
3160    ///
3161    /// # Panics
3162    ///
3163    /// Panics if starting index is greater than the end index
3164    /// or if the index is greater than the length of the vector.
3165    ///
3166    /// # Examples
3167    ///
3168    /// ```
3169    /// let mut characters = vec!['a', 'b', 'c', 'd', 'e'];
3170    /// characters.extend_from_within(2..);
3171    /// assert_eq!(characters, ['a', 'b', 'c', 'd', 'e', 'c', 'd', 'e']);
3172    ///
3173    /// let mut numbers = vec![0, 1, 2, 3, 4];
3174    /// numbers.extend_from_within(..2);
3175    /// assert_eq!(numbers, [0, 1, 2, 3, 4, 0, 1]);
3176    ///
3177    /// let mut strings = vec![String::from("hello"), String::from("world"), String::from("!")];
3178    /// strings.extend_from_within(1..=2);
3179    /// assert_eq!(strings, ["hello", "world", "!", "world", "!"]);
3180    /// ```
3181    #[cfg(not(no_global_oom_handling))]
3182    #[stable(feature = "vec_extend_from_within", since = "1.53.0")]
3183    #[track_caller]
3184    pub fn extend_from_within<R>(&mut self, src: R)
3185    where
3186        R: RangeBounds<usize>,
3187    {
3188        let range = slice::range(src, ..self.len());
3189        self.reserve(range.len());
3190
3191        // SAFETY:
3192        // - `slice::range` guarantees that the given range is valid for indexing self
3193        unsafe {
3194            self.spec_extend_from_within(range);
3195        }
3196    }
3197}
3198
3199impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
3200    /// Takes a `Vec<[T; N]>` and flattens it into a `Vec<T>`.
3201    ///
3202    /// # Panics
3203    ///
3204    /// Panics if the length of the resulting vector would overflow a `usize`.
3205    ///
3206    /// This is only possible when flattening a vector of arrays of zero-sized
3207    /// types, and thus tends to be irrelevant in practice. If
3208    /// `size_of::<T>() > 0`, this will never panic.
3209    ///
3210    /// # Examples
3211    ///
3212    /// ```
3213    /// let mut vec = vec![[1, 2, 3], [4, 5, 6], [7, 8, 9]];
3214    /// assert_eq!(vec.pop(), Some([7, 8, 9]));
3215    ///
3216    /// let mut flattened = vec.into_flattened();
3217    /// assert_eq!(flattened.pop(), Some(6));
3218    /// ```
3219    #[stable(feature = "slice_flatten", since = "1.80.0")]
3220    pub fn into_flattened(self) -> Vec<T, A> {
3221        let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
3222        let (new_len, new_cap) = if T::IS_ZST {
3223            (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
3224        } else {
3225            // SAFETY:
3226            // - `cap * N` cannot overflow because the allocation is already in
3227            // the address space.
3228            // - Each `[T; N]` has `N` valid elements, so there are `len * N`
3229            // valid elements in the allocation.
3230            unsafe { (len.unchecked_mul(N), cap.unchecked_mul(N)) }
3231        };
3232        // SAFETY:
3233        // - `ptr` was allocated by `self`
3234        // - `ptr` is well-aligned because `[T; N]` has the same alignment as `T`.
3235        // - `new_cap` refers to the same sized allocation as `cap` because
3236        // `new_cap * size_of::<T>()` == `cap * size_of::<[T; N]>()`
3237        // - `len` <= `cap`, so `len * N` <= `cap * N`.
3238        unsafe { Vec::<T, A>::from_raw_parts_in(ptr.cast(), new_len, new_cap, alloc) }
3239    }
3240}
3241
3242impl<T: Clone, A: Allocator> Vec<T, A> {
3243    #[cfg(not(no_global_oom_handling))]
3244    #[track_caller]
3245    /// Extend the vector by `n` clones of value.
3246    fn extend_with(&mut self, n: usize, value: T) {
3247        self.reserve(n);
3248
3249        unsafe {
3250            let mut ptr = self.as_mut_ptr().add(self.len());
3251            // Use SetLenOnDrop to work around bug where compiler
3252            // might not realize the store through `ptr` through self.set_len()
3253            // don't alias.
3254            let mut local_len = SetLenOnDrop::new(&mut self.len);
3255
3256            // Write all elements except the last one
3257            for _ in 1..n {
3258                ptr::write(ptr, value.clone());
3259                ptr = ptr.add(1);
3260                // Increment the length in every step in case clone() panics
3261                local_len.increment_len(1);
3262            }
3263
3264            if n > 0 {
3265                // We can write the last element directly without cloning needlessly
3266                ptr::write(ptr, value);
3267                local_len.increment_len(1);
3268            }
3269
3270            // len set by scope guard
3271        }
3272    }
3273}
3274
3275impl<T: PartialEq, A: Allocator> Vec<T, A> {
3276    /// Removes consecutive repeated elements in the vector according to the
3277    /// [`PartialEq`] trait implementation.
3278    ///
3279    /// If the vector is sorted, this removes all duplicates.
3280    ///
3281    /// # Examples
3282    ///
3283    /// ```
3284    /// let mut vec = vec![1, 2, 2, 3, 2];
3285    ///
3286    /// vec.dedup();
3287    ///
3288    /// assert_eq!(vec, [1, 2, 3, 2]);
3289    /// ```
3290    #[stable(feature = "rust1", since = "1.0.0")]
3291    #[inline]
3292    pub fn dedup(&mut self) {
3293        self.dedup_by(|a, b| a == b)
3294    }
3295}
3296
3297////////////////////////////////////////////////////////////////////////////////
3298// Internal methods and functions
3299////////////////////////////////////////////////////////////////////////////////
3300
3301#[doc(hidden)]
3302#[cfg(not(no_global_oom_handling))]
3303#[stable(feature = "rust1", since = "1.0.0")]
3304#[rustc_diagnostic_item = "vec_from_elem"]
3305#[track_caller]
3306pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> {
3307    <T as SpecFromElem>::from_elem(elem, n, Global)
3308}
3309
3310#[doc(hidden)]
3311#[cfg(not(no_global_oom_handling))]
3312#[unstable(feature = "allocator_api", issue = "32838")]
3313#[track_caller]
3314pub fn from_elem_in<T: Clone, A: Allocator>(elem: T, n: usize, alloc: A) -> Vec<T, A> {
3315    <T as SpecFromElem>::from_elem(elem, n, alloc)
3316}
3317
3318#[cfg(not(no_global_oom_handling))]
3319trait ExtendFromWithinSpec {
3320    /// # Safety
3321    ///
3322    /// - `src` needs to be valid index
3323    /// - `self.capacity() - self.len()` must be `>= src.len()`
3324    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>);
3325}
3326
3327#[cfg(not(no_global_oom_handling))]
3328impl<T: Clone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
3329    default unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
3330        // SAFETY:
3331        // - len is increased only after initializing elements
3332        let (this, spare, len) = unsafe { self.split_at_spare_mut_with_len() };
3333
3334        // SAFETY:
3335        // - caller guarantees that src is a valid index
3336        let to_clone = unsafe { this.get_unchecked(src) };
3337
3338        iter::zip(to_clone, spare)
3339            .map(|(src, dst)| dst.write(src.clone()))
3340            // Note:
3341            // - Element was just initialized with `MaybeUninit::write`, so it's ok to increase len
3342            // - len is increased after each element to prevent leaks (see issue #82533)
3343            .for_each(|_| *len += 1);
3344    }
3345}
3346
3347#[cfg(not(no_global_oom_handling))]
3348impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
3349    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
3350        let count = src.len();
3351        {
3352            let (init, spare) = self.split_at_spare_mut();
3353
3354            // SAFETY:
3355            // - caller guarantees that `src` is a valid index
3356            let source = unsafe { init.get_unchecked(src) };
3357
3358            // SAFETY:
3359            // - Both pointers are created from unique slice references (`&mut [_]`)
3360            //   so they are valid and do not overlap.
3361            // - Elements are :Copy so it's OK to copy them, without doing
3362            //   anything with the original values
3363            // - `count` is equal to the len of `source`, so source is valid for
3364            //   `count` reads
3365            // - `.reserve(count)` guarantees that `spare.len() >= count` so spare
3366            //   is valid for `count` writes
3367            unsafe { ptr::copy_nonoverlapping(source.as_ptr(), spare.as_mut_ptr() as _, count) };
3368        }
3369
3370        // SAFETY:
3371        // - The elements were just initialized by `copy_nonoverlapping`
3372        self.len += count;
3373    }
3374}
3375
3376////////////////////////////////////////////////////////////////////////////////
3377// Common trait implementations for Vec
3378////////////////////////////////////////////////////////////////////////////////
3379
3380#[stable(feature = "rust1", since = "1.0.0")]
3381impl<T, A: Allocator> ops::Deref for Vec<T, A> {
3382    type Target = [T];
3383
3384    #[inline]
3385    fn deref(&self) -> &[T] {
3386        self.as_slice()
3387    }
3388}
3389
3390#[stable(feature = "rust1", since = "1.0.0")]
3391impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
3392    #[inline]
3393    fn deref_mut(&mut self) -> &mut [T] {
3394        self.as_mut_slice()
3395    }
3396}
3397
3398#[unstable(feature = "deref_pure_trait", issue = "87121")]
3399unsafe impl<T, A: Allocator> ops::DerefPure for Vec<T, A> {}
3400
3401#[cfg(not(no_global_oom_handling))]
3402#[stable(feature = "rust1", since = "1.0.0")]
3403impl<T: Clone, A: Allocator + Clone> Clone for Vec<T, A> {
3404    #[track_caller]
3405    fn clone(&self) -> Self {
3406        let alloc = self.allocator().clone();
3407        <[T]>::to_vec_in(&**self, alloc)
3408    }
3409
3410    /// Overwrites the contents of `self` with a clone of the contents of `source`.
3411    ///
3412    /// This method is preferred over simply assigning `source.clone()` to `self`,
3413    /// as it avoids reallocation if possible. Additionally, if the element type
3414    /// `T` overrides `clone_from()`, this will reuse the resources of `self`'s
3415    /// elements as well.
3416    ///
3417    /// # Examples
3418    ///
3419    /// ```
3420    /// let x = vec![5, 6, 7];
3421    /// let mut y = vec![8, 9, 10];
3422    /// let yp: *const i32 = y.as_ptr();
3423    ///
3424    /// y.clone_from(&x);
3425    ///
3426    /// // The value is the same
3427    /// assert_eq!(x, y);
3428    ///
3429    /// // And no reallocation occurred
3430    /// assert_eq!(yp, y.as_ptr());
3431    /// ```
3432    #[track_caller]
3433    fn clone_from(&mut self, source: &Self) {
3434        crate::slice::SpecCloneIntoVec::clone_into(source.as_slice(), self);
3435    }
3436}
3437
3438/// The hash of a vector is the same as that of the corresponding slice,
3439/// as required by the `core::borrow::Borrow` implementation.
3440///
3441/// ```
3442/// use std::hash::BuildHasher;
3443///
3444/// let b = std::hash::RandomState::new();
3445/// let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
3446/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
3447/// assert_eq!(b.hash_one(v), b.hash_one(s));
3448/// ```
3449#[stable(feature = "rust1", since = "1.0.0")]
3450impl<T: Hash, A: Allocator> Hash for Vec<T, A> {
3451    #[inline]
3452    fn hash<H: Hasher>(&self, state: &mut H) {
3453        Hash::hash(&**self, state)
3454    }
3455}
3456
3457#[stable(feature = "rust1", since = "1.0.0")]
3458impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
3459    type Output = I::Output;
3460
3461    #[inline]
3462    fn index(&self, index: I) -> &Self::Output {
3463        Index::index(&**self, index)
3464    }
3465}
3466
3467#[stable(feature = "rust1", since = "1.0.0")]
3468impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
3469    #[inline]
3470    fn index_mut(&mut self, index: I) -> &mut Self::Output {
3471        IndexMut::index_mut(&mut **self, index)
3472    }
3473}
3474
3475/// Collects an iterator into a Vec, commonly called via [`Iterator::collect()`]
3476///
3477/// # Allocation behavior
3478///
3479/// In general `Vec` does not guarantee any particular growth or allocation strategy.
3480/// That also applies to this trait impl.
3481///
3482/// **Note:** This section covers implementation details and is therefore exempt from
3483/// stability guarantees.
3484///
3485/// Vec may use any or none of the following strategies,
3486/// depending on the supplied iterator:
3487///
3488/// * preallocate based on [`Iterator::size_hint()`]
3489///   * and panic if the number of items is outside the provided lower/upper bounds
3490/// * use an amortized growth strategy similar to `pushing` one item at a time
3491/// * perform the iteration in-place on the original allocation backing the iterator
3492///
3493/// The last case warrants some attention. It is an optimization that in many cases reduces peak memory
3494/// consumption and improves cache locality. But when big, short-lived allocations are created,
3495/// only a small fraction of their items get collected, no further use is made of the spare capacity
3496/// and the resulting `Vec` is moved into a longer-lived structure, then this can lead to the large
3497/// allocations having their lifetimes unnecessarily extended which can result in increased memory
3498/// footprint.
3499///
3500/// In cases where this is an issue, the excess capacity can be discarded with [`Vec::shrink_to()`],
3501/// [`Vec::shrink_to_fit()`] or by collecting into [`Box<[T]>`][owned slice] instead, which additionally reduces
3502/// the size of the long-lived struct.
3503///
3504/// [owned slice]: Box
3505///
3506/// ```rust
3507/// # use std::sync::Mutex;
3508/// static LONG_LIVED: Mutex<Vec<Vec<u16>>> = Mutex::new(Vec::new());
3509///
3510/// for i in 0..10 {
3511///     let big_temporary: Vec<u16> = (0..1024).collect();
3512///     // discard most items
3513///     let mut result: Vec<_> = big_temporary.into_iter().filter(|i| i % 100 == 0).collect();
3514///     // without this a lot of unused capacity might be moved into the global
3515///     result.shrink_to_fit();
3516///     LONG_LIVED.lock().unwrap().push(result);
3517/// }
3518/// ```
3519#[cfg(not(no_global_oom_handling))]
3520#[stable(feature = "rust1", since = "1.0.0")]
3521impl<T> FromIterator<T> for Vec<T> {
3522    #[inline]
3523    #[track_caller]
3524    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
3525        <Self as SpecFromIter<T, I::IntoIter>>::from_iter(iter.into_iter())
3526    }
3527}
3528
3529#[stable(feature = "rust1", since = "1.0.0")]
3530impl<T, A: Allocator> IntoIterator for Vec<T, A> {
3531    type Item = T;
3532    type IntoIter = IntoIter<T, A>;
3533
3534    /// Creates a consuming iterator, that is, one that moves each value out of
3535    /// the vector (from start to end). The vector cannot be used after calling
3536    /// this.
3537    ///
3538    /// # Examples
3539    ///
3540    /// ```
3541    /// let v = vec!["a".to_string(), "b".to_string()];
3542    /// let mut v_iter = v.into_iter();
3543    ///
3544    /// let first_element: Option<String> = v_iter.next();
3545    ///
3546    /// assert_eq!(first_element, Some("a".to_string()));
3547    /// assert_eq!(v_iter.next(), Some("b".to_string()));
3548    /// assert_eq!(v_iter.next(), None);
3549    /// ```
3550    #[inline]
3551    fn into_iter(self) -> Self::IntoIter {
3552        unsafe {
3553            let me = ManuallyDrop::new(self);
3554            let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
3555            let buf = me.buf.non_null();
3556            let begin = buf.as_ptr();
3557            let end = if T::IS_ZST {
3558                begin.wrapping_byte_add(me.len())
3559            } else {
3560                begin.add(me.len()) as *const T
3561            };
3562            let cap = me.buf.capacity();
3563            IntoIter { buf, phantom: PhantomData, cap, alloc, ptr: buf, end }
3564        }
3565    }
3566}
3567
3568#[stable(feature = "rust1", since = "1.0.0")]
3569impl<'a, T, A: Allocator> IntoIterator for &'a Vec<T, A> {
3570    type Item = &'a T;
3571    type IntoIter = slice::Iter<'a, T>;
3572
3573    fn into_iter(self) -> Self::IntoIter {
3574        self.iter()
3575    }
3576}
3577
3578#[stable(feature = "rust1", since = "1.0.0")]
3579impl<'a, T, A: Allocator> IntoIterator for &'a mut Vec<T, A> {
3580    type Item = &'a mut T;
3581    type IntoIter = slice::IterMut<'a, T>;
3582
3583    fn into_iter(self) -> Self::IntoIter {
3584        self.iter_mut()
3585    }
3586}
3587
3588#[cfg(not(no_global_oom_handling))]
3589#[stable(feature = "rust1", since = "1.0.0")]
3590impl<T, A: Allocator> Extend<T> for Vec<T, A> {
3591    #[inline]
3592    #[track_caller]
3593    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
3594        <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
3595    }
3596
3597    #[inline]
3598    #[track_caller]
3599    fn extend_one(&mut self, item: T) {
3600        self.push(item);
3601    }
3602
3603    #[inline]
3604    #[track_caller]
3605    fn extend_reserve(&mut self, additional: usize) {
3606        self.reserve(additional);
3607    }
3608
3609    #[inline]
3610    unsafe fn extend_one_unchecked(&mut self, item: T) {
3611        // SAFETY: Our preconditions ensure the space has been reserved, and `extend_reserve` is implemented correctly.
3612        unsafe {
3613            let len = self.len();
3614            ptr::write(self.as_mut_ptr().add(len), item);
3615            self.set_len(len + 1);
3616        }
3617    }
3618}
3619
3620impl<T, A: Allocator> Vec<T, A> {
3621    // leaf method to which various SpecFrom/SpecExtend implementations delegate when
3622    // they have no further optimizations to apply
3623    #[cfg(not(no_global_oom_handling))]
3624    #[track_caller]
3625    fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
3626        // This is the case for a general iterator.
3627        //
3628        // This function should be the moral equivalent of:
3629        //
3630        //      for item in iterator {
3631        //          self.push(item);
3632        //      }
3633        while let Some(element) = iterator.next() {
3634            let len = self.len();
3635            if len == self.capacity() {
3636                let (lower, _) = iterator.size_hint();
3637                self.reserve(lower.saturating_add(1));
3638            }
3639            unsafe {
3640                ptr::write(self.as_mut_ptr().add(len), element);
3641                // Since next() executes user code which can panic we have to bump the length
3642                // after each step.
3643                // NB can't overflow since we would have had to alloc the address space
3644                self.set_len(len + 1);
3645            }
3646        }
3647    }
3648
3649    // specific extend for `TrustedLen` iterators, called both by the specializations
3650    // and internal places where resolving specialization makes compilation slower
3651    #[cfg(not(no_global_oom_handling))]
3652    #[track_caller]
3653    fn extend_trusted(&mut self, iterator: impl iter::TrustedLen<Item = T>) {
3654        let (low, high) = iterator.size_hint();
3655        if let Some(additional) = high {
3656            debug_assert_eq!(
3657                low,
3658                additional,
3659                "TrustedLen iterator's size hint is not exact: {:?}",
3660                (low, high)
3661            );
3662            self.reserve(additional);
3663            unsafe {
3664                let ptr = self.as_mut_ptr();
3665                let mut local_len = SetLenOnDrop::new(&mut self.len);
3666                iterator.for_each(move |element| {
3667                    ptr::write(ptr.add(local_len.current_len()), element);
3668                    // Since the loop executes user code which can panic we have to update
3669                    // the length every step to correctly drop what we've written.
3670                    // NB can't overflow since we would have had to alloc the address space
3671                    local_len.increment_len(1);
3672                });
3673            }
3674        } else {
3675            // Per TrustedLen contract a `None` upper bound means that the iterator length
3676            // truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
3677            // Since the other branch already panics eagerly (via `reserve()`) we do the same here.
3678            // This avoids additional codegen for a fallback code path which would eventually
3679            // panic anyway.
3680            panic!("capacity overflow");
3681        }
3682    }
3683
3684    /// Creates a splicing iterator that replaces the specified range in the vector
3685    /// with the given `replace_with` iterator and yields the removed items.
3686    /// `replace_with` does not need to be the same length as `range`.
3687    ///
3688    /// `range` is removed even if the `Splice` iterator is not consumed before it is dropped.
3689    ///
3690    /// It is unspecified how many elements are removed from the vector
3691    /// if the `Splice` value is leaked.
3692    ///
3693    /// The input iterator `replace_with` is only consumed when the `Splice` value is dropped.
3694    ///
3695    /// This is optimal if:
3696    ///
3697    /// * The tail (elements in the vector after `range`) is empty,
3698    /// * or `replace_with` yields fewer or equal elements than `range`’s length
3699    /// * or the lower bound of its `size_hint()` is exact.
3700    ///
3701    /// Otherwise, a temporary vector is allocated and the tail is moved twice.
3702    ///
3703    /// # Panics
3704    ///
3705    /// Panics if the starting point is greater than the end point or if
3706    /// the end point is greater than the length of the vector.
3707    ///
3708    /// # Examples
3709    ///
3710    /// ```
3711    /// let mut v = vec![1, 2, 3, 4];
3712    /// let new = [7, 8, 9];
3713    /// let u: Vec<_> = v.splice(1..3, new).collect();
3714    /// assert_eq!(v, [1, 7, 8, 9, 4]);
3715    /// assert_eq!(u, [2, 3]);
3716    /// ```
3717    ///
3718    /// Using `splice` to insert new items into a vector efficiently at a specific position
3719    /// indicated by an empty range:
3720    ///
3721    /// ```
3722    /// let mut v = vec![1, 5];
3723    /// let new = [2, 3, 4];
3724    /// v.splice(1..1, new);
3725    /// assert_eq!(v, [1, 2, 3, 4, 5]);
3726    /// ```
3727    #[cfg(not(no_global_oom_handling))]
3728    #[inline]
3729    #[stable(feature = "vec_splice", since = "1.21.0")]
3730    pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<'_, I::IntoIter, A>
3731    where
3732        R: RangeBounds<usize>,
3733        I: IntoIterator<Item = T>,
3734    {
3735        Splice { drain: self.drain(range), replace_with: replace_with.into_iter() }
3736    }
3737
3738    /// Creates an iterator which uses a closure to determine if an element in the range should be removed.
3739    ///
3740    /// If the closure returns `true`, the element is removed from the vector
3741    /// and yielded. If the closure returns `false`, or panics, the element
3742    /// remains in the vector and will not be yielded.
3743    ///
3744    /// Only elements that fall in the provided range are considered for extraction, but any elements
3745    /// after the range will still have to be moved if any element has been extracted.
3746    ///
3747    /// If the returned `ExtractIf` is not exhausted, e.g. because it is dropped without iterating
3748    /// or the iteration short-circuits, then the remaining elements will be retained.
3749    /// Use [`retain_mut`] with a negated predicate if you do not need the returned iterator.
3750    ///
3751    /// [`retain_mut`]: Vec::retain_mut
3752    ///
3753    /// Using this method is equivalent to the following code:
3754    ///
3755    /// ```
3756    /// # let some_predicate = |x: &mut i32| { *x % 2 == 1 };
3757    /// # let mut vec = vec![0, 1, 2, 3, 4, 5, 6];
3758    /// # let mut vec2 = vec.clone();
3759    /// # let range = 1..5;
3760    /// let mut i = range.start;
3761    /// let end_items = vec.len() - range.end;
3762    /// # let mut extracted = vec![];
3763    ///
3764    /// while i < vec.len() - end_items {
3765    ///     if some_predicate(&mut vec[i]) {
3766    ///         let val = vec.remove(i);
3767    /// #         extracted.push(val);
3768    ///         // your code here
3769    ///     } else {
3770    ///         i += 1;
3771    ///     }
3772    /// }
3773    ///
3774    /// # let extracted2: Vec<_> = vec2.extract_if(range, some_predicate).collect();
3775    /// # assert_eq!(vec, vec2);
3776    /// # assert_eq!(extracted, extracted2);
3777    /// ```
3778    ///
3779    /// But `extract_if` is easier to use. `extract_if` is also more efficient,
3780    /// because it can backshift the elements of the array in bulk.
3781    ///
3782    /// The iterator also lets you mutate the value of each element in the
3783    /// closure, regardless of whether you choose to keep or remove it.
3784    ///
3785    /// # Panics
3786    ///
3787    /// If `range` is out of bounds.
3788    ///
3789    /// # Examples
3790    ///
3791    /// Splitting a vector into even and odd values, reusing the original vector:
3792    ///
3793    /// ```
3794    /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
3795    ///
3796    /// let evens = numbers.extract_if(.., |x| *x % 2 == 0).collect::<Vec<_>>();
3797    /// let odds = numbers;
3798    ///
3799    /// assert_eq!(evens, vec![2, 4, 6, 8, 14]);
3800    /// assert_eq!(odds, vec![1, 3, 5, 9, 11, 13, 15]);
3801    /// ```
3802    ///
3803    /// Using the range argument to only process a part of the vector:
3804    ///
3805    /// ```
3806    /// let mut items = vec![0, 0, 0, 0, 0, 0, 0, 1, 2, 1, 2, 1, 2];
3807    /// let ones = items.extract_if(7.., |x| *x == 1).collect::<Vec<_>>();
3808    /// assert_eq!(items, vec![0, 0, 0, 0, 0, 0, 0, 2, 2, 2]);
3809    /// assert_eq!(ones.len(), 3);
3810    /// ```
3811    #[stable(feature = "extract_if", since = "1.87.0")]
3812    pub fn extract_if<F, R>(&mut self, range: R, filter: F) -> ExtractIf<'_, T, F, A>
3813    where
3814        F: FnMut(&mut T) -> bool,
3815        R: RangeBounds<usize>,
3816    {
3817        ExtractIf::new(self, filter, range)
3818    }
3819}
3820
3821/// Extend implementation that copies elements out of references before pushing them onto the Vec.
3822///
3823/// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
3824/// append the entire slice at once.
3825///
3826/// [`copy_from_slice`]: slice::copy_from_slice
3827#[cfg(not(no_global_oom_handling))]
3828#[stable(feature = "extend_ref", since = "1.2.0")]
3829impl<'a, T: Copy + 'a, A: Allocator> Extend<&'a T> for Vec<T, A> {
3830    #[track_caller]
3831    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
3832        self.spec_extend(iter.into_iter())
3833    }
3834
3835    #[inline]
3836    #[track_caller]
3837    fn extend_one(&mut self, &item: &'a T) {
3838        self.push(item);
3839    }
3840
3841    #[inline]
3842    #[track_caller]
3843    fn extend_reserve(&mut self, additional: usize) {
3844        self.reserve(additional);
3845    }
3846
3847    #[inline]
3848    unsafe fn extend_one_unchecked(&mut self, &item: &'a T) {
3849        // SAFETY: Our preconditions ensure the space has been reserved, and `extend_reserve` is implemented correctly.
3850        unsafe {
3851            let len = self.len();
3852            ptr::write(self.as_mut_ptr().add(len), item);
3853            self.set_len(len + 1);
3854        }
3855    }
3856}
3857
3858/// Implements comparison of vectors, [lexicographically](Ord#lexicographical-comparison).
3859#[stable(feature = "rust1", since = "1.0.0")]
3860impl<T, A1, A2> PartialOrd<Vec<T, A2>> for Vec<T, A1>
3861where
3862    T: PartialOrd,
3863    A1: Allocator,
3864    A2: Allocator,
3865{
3866    #[inline]
3867    fn partial_cmp(&self, other: &Vec<T, A2>) -> Option<Ordering> {
3868        PartialOrd::partial_cmp(&**self, &**other)
3869    }
3870}
3871
3872#[stable(feature = "rust1", since = "1.0.0")]
3873impl<T: Eq, A: Allocator> Eq for Vec<T, A> {}
3874
3875/// Implements ordering of vectors, [lexicographically](Ord#lexicographical-comparison).
3876#[stable(feature = "rust1", since = "1.0.0")]
3877impl<T: Ord, A: Allocator> Ord for Vec<T, A> {
3878    #[inline]
3879    fn cmp(&self, other: &Self) -> Ordering {
3880        Ord::cmp(&**self, &**other)
3881    }
3882}
3883
3884#[stable(feature = "rust1", since = "1.0.0")]
3885unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
3886    fn drop(&mut self) {
3887        unsafe {
3888            // use drop for [T]
3889            // use a raw slice to refer to the elements of the vector as weakest necessary type;
3890            // could avoid questions of validity in certain cases
3891            ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
3892        }
3893        // RawVec handles deallocation
3894    }
3895}
3896
3897#[stable(feature = "rust1", since = "1.0.0")]
3898#[rustc_const_unstable(feature = "const_default", issue = "67792")]
3899impl<T> const Default for Vec<T> {
3900    /// Creates an empty `Vec<T>`.
3901    ///
3902    /// The vector will not allocate until elements are pushed onto it.
3903    fn default() -> Vec<T> {
3904        Vec::new()
3905    }
3906}
3907
3908#[stable(feature = "rust1", since = "1.0.0")]
3909impl<T: fmt::Debug, A: Allocator> fmt::Debug for Vec<T, A> {
3910    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3911        fmt::Debug::fmt(&**self, f)
3912    }
3913}
3914
3915#[stable(feature = "rust1", since = "1.0.0")]
3916impl<T, A: Allocator> AsRef<Vec<T, A>> for Vec<T, A> {
3917    fn as_ref(&self) -> &Vec<T, A> {
3918        self
3919    }
3920}
3921
3922#[stable(feature = "vec_as_mut", since = "1.5.0")]
3923impl<T, A: Allocator> AsMut<Vec<T, A>> for Vec<T, A> {
3924    fn as_mut(&mut self) -> &mut Vec<T, A> {
3925        self
3926    }
3927}
3928
3929#[stable(feature = "rust1", since = "1.0.0")]
3930impl<T, A: Allocator> AsRef<[T]> for Vec<T, A> {
3931    fn as_ref(&self) -> &[T] {
3932        self
3933    }
3934}
3935
3936#[stable(feature = "vec_as_mut", since = "1.5.0")]
3937impl<T, A: Allocator> AsMut<[T]> for Vec<T, A> {
3938    fn as_mut(&mut self) -> &mut [T] {
3939        self
3940    }
3941}
3942
3943#[cfg(not(no_global_oom_handling))]
3944#[stable(feature = "rust1", since = "1.0.0")]
3945impl<T: Clone> From<&[T]> for Vec<T> {
3946    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
3947    ///
3948    /// # Examples
3949    ///
3950    /// ```
3951    /// assert_eq!(Vec::from(&[1, 2, 3][..]), vec![1, 2, 3]);
3952    /// ```
3953    #[track_caller]
3954    fn from(s: &[T]) -> Vec<T> {
3955        s.to_vec()
3956    }
3957}
3958
3959#[cfg(not(no_global_oom_handling))]
3960#[stable(feature = "vec_from_mut", since = "1.19.0")]
3961impl<T: Clone> From<&mut [T]> for Vec<T> {
3962    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
3963    ///
3964    /// # Examples
3965    ///
3966    /// ```
3967    /// assert_eq!(Vec::from(&mut [1, 2, 3][..]), vec![1, 2, 3]);
3968    /// ```
3969    #[track_caller]
3970    fn from(s: &mut [T]) -> Vec<T> {
3971        s.to_vec()
3972    }
3973}
3974
3975#[cfg(not(no_global_oom_handling))]
3976#[stable(feature = "vec_from_array_ref", since = "1.74.0")]
3977impl<T: Clone, const N: usize> From<&[T; N]> for Vec<T> {
3978    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
3979    ///
3980    /// # Examples
3981    ///
3982    /// ```
3983    /// assert_eq!(Vec::from(&[1, 2, 3]), vec![1, 2, 3]);
3984    /// ```
3985    #[track_caller]
3986    fn from(s: &[T; N]) -> Vec<T> {
3987        Self::from(s.as_slice())
3988    }
3989}
3990
3991#[cfg(not(no_global_oom_handling))]
3992#[stable(feature = "vec_from_array_ref", since = "1.74.0")]
3993impl<T: Clone, const N: usize> From<&mut [T; N]> for Vec<T> {
3994    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
3995    ///
3996    /// # Examples
3997    ///
3998    /// ```
3999    /// assert_eq!(Vec::from(&mut [1, 2, 3]), vec![1, 2, 3]);
4000    /// ```
4001    #[track_caller]
4002    fn from(s: &mut [T; N]) -> Vec<T> {
4003        Self::from(s.as_mut_slice())
4004    }
4005}
4006
4007#[cfg(not(no_global_oom_handling))]
4008#[stable(feature = "vec_from_array", since = "1.44.0")]
4009impl<T, const N: usize> From<[T; N]> for Vec<T> {
4010    /// Allocates a `Vec<T>` and moves `s`'s items into it.
4011    ///
4012    /// # Examples
4013    ///
4014    /// ```
4015    /// assert_eq!(Vec::from([1, 2, 3]), vec![1, 2, 3]);
4016    /// ```
4017    #[track_caller]
4018    fn from(s: [T; N]) -> Vec<T> {
4019        <[T]>::into_vec(Box::new(s))
4020    }
4021}
4022
4023#[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
4024impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
4025where
4026    [T]: ToOwned<Owned = Vec<T>>,
4027{
4028    /// Converts a clone-on-write slice into a vector.
4029    ///
4030    /// If `s` already owns a `Vec<T>`, it will be returned directly.
4031    /// If `s` is borrowing a slice, a new `Vec<T>` will be allocated and
4032    /// filled by cloning `s`'s items into it.
4033    ///
4034    /// # Examples
4035    ///
4036    /// ```
4037    /// # use std::borrow::Cow;
4038    /// let o: Cow<'_, [i32]> = Cow::Owned(vec![1, 2, 3]);
4039    /// let b: Cow<'_, [i32]> = Cow::Borrowed(&[1, 2, 3]);
4040    /// assert_eq!(Vec::from(o), Vec::from(b));
4041    /// ```
4042    #[track_caller]
4043    fn from(s: Cow<'a, [T]>) -> Vec<T> {
4044        s.into_owned()
4045    }
4046}
4047
4048// note: test pulls in std, which causes errors here
4049#[stable(feature = "vec_from_box", since = "1.18.0")]
4050impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
4051    /// Converts a boxed slice into a vector by transferring ownership of
4052    /// the existing heap allocation.
4053    ///
4054    /// # Examples
4055    ///
4056    /// ```
4057    /// let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
4058    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
4059    /// ```
4060    fn from(s: Box<[T], A>) -> Self {
4061        s.into_vec()
4062    }
4063}
4064
4065// note: test pulls in std, which causes errors here
4066#[cfg(not(no_global_oom_handling))]
4067#[stable(feature = "box_from_vec", since = "1.20.0")]
4068impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
4069    /// Converts a vector into a boxed slice.
4070    ///
4071    /// Before doing the conversion, this method discards excess capacity like [`Vec::shrink_to_fit`].
4072    ///
4073    /// [owned slice]: Box
4074    /// [`Vec::shrink_to_fit`]: Vec::shrink_to_fit
4075    ///
4076    /// # Examples
4077    ///
4078    /// ```
4079    /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
4080    /// ```
4081    ///
4082    /// Any excess capacity is removed:
4083    /// ```
4084    /// let mut vec = Vec::with_capacity(10);
4085    /// vec.extend([1, 2, 3]);
4086    ///
4087    /// assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());
4088    /// ```
4089    #[track_caller]
4090    fn from(v: Vec<T, A>) -> Self {
4091        v.into_boxed_slice()
4092    }
4093}
4094
4095#[cfg(not(no_global_oom_handling))]
4096#[stable(feature = "rust1", since = "1.0.0")]
4097impl From<&str> for Vec<u8> {
4098    /// Allocates a `Vec<u8>` and fills it with a UTF-8 string.
4099    ///
4100    /// # Examples
4101    ///
4102    /// ```
4103    /// assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);
4104    /// ```
4105    #[track_caller]
4106    fn from(s: &str) -> Vec<u8> {
4107        From::from(s.as_bytes())
4108    }
4109}
4110
4111#[stable(feature = "array_try_from_vec", since = "1.48.0")]
4112impl<T, A: Allocator, const N: usize> TryFrom<Vec<T, A>> for [T; N] {
4113    type Error = Vec<T, A>;
4114
4115    /// Gets the entire contents of the `Vec<T>` as an array,
4116    /// if its size exactly matches that of the requested array.
4117    ///
4118    /// # Examples
4119    ///
4120    /// ```
4121    /// assert_eq!(vec![1, 2, 3].try_into(), Ok([1, 2, 3]));
4122    /// assert_eq!(<Vec<i32>>::new().try_into(), Ok([]));
4123    /// ```
4124    ///
4125    /// If the length doesn't match, the input comes back in `Err`:
4126    /// ```
4127    /// let r: Result<[i32; 4], _> = (0..10).collect::<Vec<_>>().try_into();
4128    /// assert_eq!(r, Err(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
4129    /// ```
4130    ///
4131    /// If you're fine with just getting a prefix of the `Vec<T>`,
4132    /// you can call [`.truncate(N)`](Vec::truncate) first.
4133    /// ```
4134    /// let mut v = String::from("hello world").into_bytes();
4135    /// v.sort();
4136    /// v.truncate(2);
4137    /// let [a, b]: [_; 2] = v.try_into().unwrap();
4138    /// assert_eq!(a, b' ');
4139    /// assert_eq!(b, b'd');
4140    /// ```
4141    fn try_from(mut vec: Vec<T, A>) -> Result<[T; N], Vec<T, A>> {
4142        if vec.len() != N {
4143            return Err(vec);
4144        }
4145
4146        // SAFETY: `.set_len(0)` is always sound.
4147        unsafe { vec.set_len(0) };
4148
4149        // SAFETY: A `Vec`'s pointer is always aligned properly, and
4150        // the alignment the array needs is the same as the items.
4151        // We checked earlier that we have sufficient items.
4152        // The items will not double-drop as the `set_len`
4153        // tells the `Vec` not to also drop them.
4154        let array = unsafe { ptr::read(vec.as_ptr() as *const [T; N]) };
4155        Ok(array)
4156    }
4157}