maybe_uninit.rs.html -- source
Description: Source to the Rust file `src/libcore/mem/maybe_uninit.rs`.
Keywords: rust, rustlang, rust-lang

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410
    411
    412
    413
    414
    415
    416
    417
    418
    419
    420
    421
    422
    423
    424
    425
    426
    427
    428
    429
    430
    431
    432
    433
    434
    435
    436
    437
    438
    439
    440
    441
    442
    443
    444
    445
    446
    447
    448
    449
    450
    451
    452
    453
    454
    455
    456
    457
    458
    459
    460
    461
    462
    463
    464
    465
    466
    467
    468
    469
    470
    471
    472
    473
    474
    475
    476
    477
    478
    479
    480
    481
    482
    483
    484
    485
    486
    487
    488
    489
    490
    491
    492
    493
    494
    495
    496
    497
    498
    499
    500
    501
    502
    503
    504
    505
    506
    507
    508
    509
    510
    511
    512
    513
    514
    515
    516
    517
    518
    519
    520
    521
    522
    523
    524
    525
    526
    527
    528
    529
    530
    531
    532
    533
    534
    535
    536
    537
    538
    539
    540
    541
    542
    543
    544
    545
    546
    547
    548
    549
    550
    551
    552
    553
    554
    555
    556
    557
    558
    559
    560
    561
    562
    563
    564
    565
    566
    567
    568
    569
    570
    571
    572
    573
    574
    575
    576
    577
    578
    579
    580
    581
    582
    583
    584
    585
    586
    587
    588
    589
    590
    591
    592
    593
    594
    595
    596
    597
    598
    599
    600
    601
    602
    603
    604
    605
    606
    607
    608
    609
    610
    611
    612
    613
    614
    615
    616
    617
    618
    619
    620
    621
    622
    623
    624
    625
    626
    627
    628
    629
    630
    631
    632
    633
    634
    635
    636
    637
    638
    639
    640
    641
    642
    643
    644
    645
    646
    647
    648
    649
    650
    651
    652
    653
    654
    655
    656
    657
    658
    659
    660
    661
    662
    663
    664
    665
    666
    667
    668
    669
    670
    671
    672
    673
    674
    675
    676
    677
    678
    679
    680
    681
    682
    683
    684
    685
    686
    687
    688
    689
    690
    691
    692
    693
    694
    695
    696
    697
    698
    699
    700
    701
    702
    703
    704
    705
    706
    707
    708
    709
    710
    711
    712
    713
    714
    715
    716
    717
    718
    719
    720
    721
    722
    723
    724
    725
    726
    727
    728
    729
    730
    731
    732
    733
    734
    735
    736
    737
    738
    739
    740
    741
    742
    743
    744
    745
    746
    747
    748
    749
    750
    751
    752
    753
    754
    755
    756
    757
    758
    759
    760
    761
    762
    763
    764
    765
    766
    767
    768
    769
    770
    771
    772
    773
    774
    775
    776
    777
    778
    779
    780
    781
    782
    783
    784
    785
    786
    787
    788
    789
    790
    791
    792
    793
    794
    795
    796
    797

    use crate::any::type_name;
    use crate::fmt;
    use crate::intrinsics;
    use crate::mem::ManuallyDrop;

    // ignore-tidy-undocumented-unsafe

    /// A wrapper type to construct uninitialized instances of `T`.
    ///
    /// # Initialization invariant
    ///
    /// The compiler, in general, assumes that a variable is properly initialized
    /// according to the requirements of the variable's type. For example, a variable of
    /// reference type must be aligned and non-NULL. This is an invariant that must
    /// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
    /// variable of reference type causes instantaneous [undefined behavior][ub],
    /// no matter whether that reference ever gets used to access memory:
    ///
    /// ```rust,no_run
    /// # #![allow(invalid_value)]
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
    /// // The equivalent code with `MaybeUninit<&i32>`:
    /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior!
    /// ```
    ///
    /// This is exploited by the compiler for various optimizations, such as eliding
    /// run-time checks and optimizing `enum` layout.
    ///
    /// Similarly, entirely uninitialized memory may have any content, while a `bool` must
    /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
    ///
    /// ```rust,no_run
    /// # #![allow(invalid_value)]
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior!
    /// // The equivalent code with `MaybeUninit<bool>`:
    /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
    /// ```
    ///
    /// Moreover, uninitialized memory is special in that the compiler knows that
    /// it does not have a fixed value. This makes it undefined behavior to have
    /// uninitialized data in a variable even if that variable has an integer type,
    /// which otherwise can hold any *fixed* bit pattern:
    ///
    /// ```rust,no_run
    /// # #![allow(invalid_value)]
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
    /// // The equivalent code with `MaybeUninit<i32>`:
    /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
    /// ```
    /// (Notice that the rules around uninitialized integers are not finalized yet, but
    /// until they are, it is advisable to avoid them.)
    ///
    /// On top of that, remember that most types have additional invariants beyond merely
    /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
    /// is considered initialized (under the current implementation; this does not constitute
    /// a stable guarantee) because the only requirement the compiler knows about it
    /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
    /// *immediate* undefined behavior, but will cause undefined behavior with most
    /// safe operations (including dropping it).
    ///
    /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
    ///
    /// # Examples
    ///
    /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
    /// It is a signal to the compiler indicating that the data here might *not*
    /// be initialized:
    ///
    /// ```rust
    /// use std::mem::MaybeUninit;
    ///
    /// // Create an explicitly uninitialized reference. The compiler knows that data inside
    /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
    /// let mut x = MaybeUninit::<&i32>::uninit();
    /// // Set it to a valid value.
    /// unsafe { x.as_mut_ptr().write(&0); }
    /// // Extract the initialized data -- this is only allowed *after* properly
    /// // initializing `x`!
    /// let x = unsafe { x.assume_init() };
    /// ```
    ///
    /// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
    ///
    /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
    /// any of the run-time tracking and without any of the safety checks.
    ///
    /// ## out-pointers
    ///
    /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
    /// from a function, pass it a pointer to some (uninitialized) memory to put the
    /// result into. This can be useful when it is important for the caller to control
    /// how the memory the result is stored in gets allocated, and you want to avoid
    /// unnecessary moves.
    ///
    /// ```
    /// use std::mem::MaybeUninit;
    ///
    /// unsafe fn make_vec(out: *mut Vec<i32>) {
    ///     // `write` does not drop the old contents, which is important.
    ///     out.write(vec![1, 2, 3]);
    /// }
    ///
    /// let mut v = MaybeUninit::uninit();
    /// unsafe { make_vec(v.as_mut_ptr()); }
    /// // Now we know `v` is initialized! This also makes sure the vector gets
    /// // properly dropped.
    /// let v = unsafe { v.assume_init() };
    /// assert_eq!(&v, &[1, 2, 3]);
    /// ```
    ///
    /// ## Initializing an array element-by-element
    ///
    /// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
    ///
    /// ```
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let data = {
    ///     // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    ///     // safe because the type we are claiming to have initialized here is a
    ///     // bunch of `MaybeUninit`s, which do not require initialization.
    ///     let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
    ///         MaybeUninit::uninit().assume_init()
    ///     };
    ///
    ///     // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
    ///     // assignment instead of `ptr::write` does not cause the old
    ///     // uninitialized value to be dropped. Also if there is a panic during
    ///     // this loop, we have a memory leak, but there is no memory safety
    ///     // issue.
    ///     for elem in &mut data[..] {
    ///         *elem = MaybeUninit::new(vec![42]);
    ///     }
    ///
    ///     // Everything is initialized. Transmute the array to the
    ///     // initialized type.
    ///     unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
    /// };
    ///
    /// assert_eq!(&data[0], &[42]);
    /// ```
    ///
    /// You can also work with partially initialized arrays, which could
    /// be found in low-level datastructures.
    ///
    /// ```
    /// use std::mem::MaybeUninit;
    /// use std::ptr;
    ///
    /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    /// // safe because the type we are claiming to have initialized here is a
    /// // bunch of `MaybeUninit`s, which do not require initialization.
    /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
    /// // Count the number of elements we have assigned.
    /// let mut data_len: usize = 0;
    ///
    /// for elem in &mut data[0..500] {
    ///     *elem = MaybeUninit::new(String::from("hello"));
    ///     data_len += 1;
    /// }
    ///
    /// // For each item in the array, drop if we allocated it.
    /// for elem in &mut data[0..data_len] {
    ///     unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
    /// }
    /// ```
    ///
    /// ## Initializing a struct field-by-field
    ///
    /// There is currently no supported way to create a raw pointer or reference
    /// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible
    /// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
    /// to its fields.
    ///
    /// [ub]: ../../reference/behavior-considered-undefined.html
    ///
    /// # Layout
    ///
    /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
    ///
    /// ```rust
    /// use std::mem::{MaybeUninit, size_of, align_of};
    /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
    /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
    /// ```
    ///
    /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
    /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
    /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
    /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
    /// optimizations, potentially resulting in a larger size:
    ///
    /// ```rust
    /// # use std::mem::{MaybeUninit, size_of};
    /// assert_eq!(size_of::<Option<bool>>(), 1);
    /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
    /// ```
    ///
    /// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
    ///
    /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
    /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
    /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
    /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
    /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
    /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
    /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
    /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
    /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
    /// guarantee may evolve.
    #[allow(missing_debug_implementations)]
    #[stable(feature = "maybe_uninit", since = "1.36.0")]
    // Lang item so we can wrap other types in it. This is useful for generators.
    #[lang = "maybe_uninit"]
    #[derive(Copy)]
    #[repr(transparent)]
    pub union MaybeUninit<T> {
        uninit: (),
        value: ManuallyDrop<T>,
    }

    #[stable(feature = "maybe_uninit", since = "1.36.0")]
    impl<T: Copy> Clone for MaybeUninit<T> {
        #[inline(always)]
        fn clone(&self) -> Self {
            // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
            *self
        }
    }

    #[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
    impl<T> fmt::Debug for MaybeUninit<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad(type_name::<Self>())
        }
    }

    impl<T> MaybeUninit<T> {
        /// Creates a new `MaybeUninit<T>` initialized with the given value.
        /// It is safe to call [`assume_init`] on the return value of this function.
        ///
        /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
        /// It is your responsibility to make sure `T` gets dropped if it got initialized.
        ///
        /// [`assume_init`]: #method.assume_init
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
        #[inline(always)]
        pub const fn new(val: T) -> MaybeUninit<T> {
            MaybeUninit { value: ManuallyDrop::new(val) }
        }

        /// Creates a new `MaybeUninit<T>` in an uninitialized state.
        ///
        /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
        /// It is your responsibility to make sure `T` gets dropped if it got initialized.
        ///
        /// See the [type-level documentation][type] for some examples.
        ///
        /// [type]: union.MaybeUninit.html
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
        #[inline(always)]
        #[rustc_diagnostic_item = "maybe_uninit_uninit"]
        pub const fn uninit() -> MaybeUninit<T> {
            MaybeUninit { uninit: () }
        }

        /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
        ///
        /// Note: in a future Rust version this method may become unnecessary
        /// when array literal syntax allows
        /// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147).
        /// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice_assume_init)]
        ///
        /// use std::mem::MaybeUninit;
        ///
        /// extern "C" {
        ///     fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
        /// }
        ///
        /// /// Returns a (possibly smaller) slice of data that was actually read
        /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
        ///     unsafe {
        ///         let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
        ///         MaybeUninit::slice_get_ref(&buf[..len])
        ///     }
        /// }
        ///
        /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
        /// let data = read(&mut buf);
        /// ```
        #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
        #[inline(always)]
        pub fn uninit_array<const LEN: usize>() -> [Self; LEN] {
            unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
        }

        /// A promotable constant, equivalent to `uninit()`.
        #[unstable(
            feature = "internal_uninit_const",
            issue = "none",
            reason = "hack to work around promotability"
        )]
        pub const UNINIT: Self = Self::uninit();

        /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
        /// filled with `0` bytes. It depends on `T` whether that already makes for
        /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
        /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
        /// be null.
        ///
        /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
        /// It is your responsibility to make sure `T` gets dropped if it got initialized.
        ///
        /// # Example
        ///
        /// Correct usage of this function: initializing a struct with zero, where all
        /// fields of the struct can hold the bit-pattern 0 as a valid value.
        ///
        /// ```rust
        /// use std::mem::MaybeUninit;
        ///
        /// let x = MaybeUninit::<(u8, bool)>::zeroed();
        /// let x = unsafe { x.assume_init() };
        /// assert_eq!(x, (0, false));
        /// ```
        ///
        /// *Incorrect* usage of this function: initializing a struct with zero, where some fields
        /// cannot hold 0 as a valid value.
        ///
        /// ```rust,no_run
        /// use std::mem::MaybeUninit;
        ///
        /// enum NotZero { One = 1, Two = 2 };
        ///
        /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
        /// let x = unsafe { x.assume_init() };
        /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
        /// // This is undefined behavior.
        /// ```
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[inline]
        #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
        pub fn zeroed() -> MaybeUninit<T> {
            let mut u = MaybeUninit::<T>::uninit();
            unsafe {
                u.as_mut_ptr().write_bytes(0u8, 1);
            }
            u
        }

        /// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value
        /// without dropping it, so be careful not to use this twice unless you want to
        /// skip running the destructor. For your convenience, this also returns a mutable
        /// reference to the (now safely initialized) contents of `self`.
        #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
        #[inline(always)]
        pub fn write(&mut self, val: T) -> &mut T {
            unsafe {
                self.value = ManuallyDrop::new(val);
                self.get_mut()
            }
        }

        /// Gets a pointer to the contained value. Reading from this pointer or turning it
        /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
        /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
        /// (except inside an `UnsafeCell<T>`).
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
        /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
        /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
        /// let x_vec = unsafe { &*x.as_ptr() };
        /// assert_eq!(x_vec.len(), 3);
        /// ```
        ///
        /// *Incorrect* usage of this method:
        ///
        /// ```rust,no_run
        /// use std::mem::MaybeUninit;
        ///
        /// let x = MaybeUninit::<Vec<u32>>::uninit();
        /// let x_vec = unsafe { &*x.as_ptr() };
        /// // We have created a reference to an uninitialized vector! This is undefined behavior.
        /// ```
        ///
        /// (Notice that the rules around references to uninitialized data are not finalized yet, but
        /// until they are, it is advisable to avoid them.)
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[inline(always)]
        pub fn as_ptr(&self) -> *const T {
            unsafe { &*self.value as *const T }
        }

        /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
        /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
        /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
        /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
        /// // This is okay because we initialized it.
        /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
        /// x_vec.push(3);
        /// assert_eq!(x_vec.len(), 4);
        /// ```
        ///
        /// *Incorrect* usage of this method:
        ///
        /// ```rust,no_run
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
        /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
        /// // We have created a reference to an uninitialized vector! This is undefined behavior.
        /// ```
        ///
        /// (Notice that the rules around references to uninitialized data are not finalized yet, but
        /// until they are, it is advisable to avoid them.)
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[inline(always)]
        pub fn as_mut_ptr(&mut self) -> *mut T {
            unsafe { &mut *self.value as *mut T }
        }

        /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
        /// to ensure that the data will get dropped, because the resulting `T` is
        /// subject to the usual drop handling.
        ///
        /// # Safety
        ///
        /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
        /// state. Calling this when the content is not yet fully initialized causes immediate undefined
        /// behavior. The [type-level documentation][inv] contains more information about
        /// this initialization invariant.
        ///
        /// [inv]: #initialization-invariant
        ///
        /// On top of that, remember that most types have additional invariants beyond merely
        /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
        /// is considered initialized (under the current implementation; this does not constitute
        /// a stable guarantee) because the only requirement the compiler knows about it
        /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
        /// *immediate* undefined behavior, but will cause undefined behavior with most
        /// safe operations (including dropping it).
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<bool>::uninit();
        /// unsafe { x.as_mut_ptr().write(true); }
        /// let x_init = unsafe { x.assume_init() };
        /// assert_eq!(x_init, true);
        /// ```
        ///
        /// *Incorrect* usage of this method:
        ///
        /// ```rust,no_run
        /// use std::mem::MaybeUninit;
        ///
        /// let x = MaybeUninit::<Vec<u32>>::uninit();
        /// let x_init = unsafe { x.assume_init() };
        /// // `x` had not been initialized yet, so this last line caused undefined behavior.
        /// ```
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[inline(always)]
        #[rustc_diagnostic_item = "assume_init"]
        pub unsafe fn assume_init(self) -> T {
            #[cfg(bootstrap)]
            intrinsics::panic_if_uninhabited::<T>();
            #[cfg(not(bootstrap))]
            intrinsics::assert_inhabited::<T>();
            ManuallyDrop::into_inner(self.value)
        }

        /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
        /// to the usual drop handling.
        ///
        /// Whenever possible, it is preferable to use [`assume_init`] instead, which
        /// prevents duplicating the content of the `MaybeUninit<T>`.
        ///
        /// # Safety
        ///
        /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
        /// state. Calling this when the content is not yet fully initialized causes undefined
        /// behavior. The [type-level documentation][inv] contains more information about
        /// this initialization invariant.
        ///
        /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using
        /// multiple copies of the data (by calling `read` multiple times, or first
        /// calling `read` and then [`assume_init`]), it is your responsibility
        /// to ensure that that data may indeed be duplicated.
        ///
        /// [inv]: #initialization-invariant
        /// [`assume_init`]: #method.assume_init
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// #![feature(maybe_uninit_extra)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<u32>::uninit();
        /// x.write(13);
        /// let x1 = unsafe { x.read() };
        /// // `u32` is `Copy`, so we may read multiple times.
        /// let x2 = unsafe { x.read() };
        /// assert_eq!(x1, x2);
        ///
        /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
        /// x.write(None);
        /// let x1 = unsafe { x.read() };
        /// // Duplicating a `None` value is okay, so we may read multiple times.
        /// let x2 = unsafe { x.read() };
        /// assert_eq!(x1, x2);
        /// ```
        ///
        /// *Incorrect* usage of this method:
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_extra)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
        /// x.write(Some(vec![0,1,2]));
        /// let x1 = unsafe { x.read() };
        /// let x2 = unsafe { x.read() };
        /// // We now created two copies of the same vector, leading to a double-free when
        /// // they both get dropped!
        /// ```
        #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
        #[inline(always)]
        pub unsafe fn read(&self) -> T {
            #[cfg(bootstrap)]
            intrinsics::panic_if_uninhabited::<T>();
            #[cfg(not(bootstrap))]
            intrinsics::assert_inhabited::<T>();
            self.as_ptr().read()
        }

        /// Gets a shared reference to the contained value.
        ///
        /// This can be useful when we want to access a `MaybeUninit` that has been
        /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
        /// of `.assume_init()`).
        ///
        /// # Safety
        ///
        /// Calling this when the content is not yet fully initialized causes undefined
        /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
        /// is in an initialized state.
        ///
        /// # Examples
        ///
        /// ### Correct usage of this method:
        ///
        /// ```rust
        /// #![feature(maybe_uninit_ref)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
        /// // Initialize `x`:
        /// unsafe { x.as_mut_ptr().write(vec![1, 2, 3]); }
        /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
        /// // create a shared reference to it:
        /// let x: &Vec<u32> = unsafe {
        ///     // Safety: `x` has been initialized.
        ///     x.get_ref()
        /// };
        /// assert_eq!(x, &vec![1, 2, 3]);
        /// ```
        ///
        /// ### *Incorrect* usages of this method:
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_ref)]
        /// use std::mem::MaybeUninit;
        ///
        /// let x = MaybeUninit::<Vec<u32>>::uninit();
        /// let x_vec: &Vec<u32> = unsafe { x.get_ref() };
        /// // We have created a reference to an uninitialized vector! This is undefined behavior.
        /// ```
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_ref)]
        /// use std::{cell::Cell, mem::MaybeUninit};
        ///
        /// let b = MaybeUninit::<Cell<bool>>::uninit();
        /// // Initialize the `MaybeUninit` using `Cell::set`:
        /// unsafe {
        ///     b.get_ref().set(true);
        ///  // ^^^^^^^^^^^
        ///  // Reference to an uninitialized `Cell<bool>`: UB!
        /// }
        /// ```
        #[unstable(feature = "maybe_uninit_ref", issue = "63568")]
        #[inline(always)]
        pub unsafe fn get_ref(&self) -> &T {
            #[cfg(bootstrap)]
            intrinsics::panic_if_uninhabited::<T>();
            #[cfg(not(bootstrap))]
            intrinsics::assert_inhabited::<T>();
            &*self.value
        }

        /// Gets a mutable (unique) reference to the contained value.
        ///
        /// This can be useful when we want to access a `MaybeUninit` that has been
        /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
        /// of `.assume_init()`).
        ///
        /// # Safety
        ///
        /// Calling this when the content is not yet fully initialized causes undefined
        /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
        /// is in an initialized state. For instance, `.get_mut()` cannot be used to
        /// initialize a `MaybeUninit`.
        ///
        /// # Examples
        ///
        /// ### Correct usage of this method:
        ///
        /// ```rust
        /// #![feature(maybe_uninit_ref)]
        /// use std::mem::MaybeUninit;
        ///
        /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 2048]) { *buf = [0; 2048] }
        /// # #[cfg(FALSE)]
        /// extern "C" {
        ///     /// Initializes *all* the bytes of the input buffer.
        ///     fn initialize_buffer(buf: *mut [u8; 2048]);
        /// }
        ///
        /// let mut buf = MaybeUninit::<[u8; 2048]>::uninit();
        ///
        /// // Initialize `buf`:
        /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
        /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
        /// // However, using `.assume_init()` may trigger a `memcpy` of the 2048 bytes.
        /// // To assert our buffer has been initialized without copying it, we upgrade
        /// // the `&mut MaybeUninit<[u8; 2048]>` to a `&mut [u8; 2048]`:
        /// let buf: &mut [u8; 2048] = unsafe {
        ///     // Safety: `buf` has been initialized.
        ///     buf.get_mut()
        /// };
        ///
        /// // Now we can use `buf` as a normal slice:
        /// buf.sort_unstable();
        /// assert!(
        ///     buf.windows(2).all(|pair| pair[0] <= pair[1]),
        ///     "buffer is sorted",
        /// );
        /// ```
        ///
        /// ### *Incorrect* usages of this method:
        ///
        /// You cannot use `.get_mut()` to initialize a value:
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_ref)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut b = MaybeUninit::<bool>::uninit();
        /// unsafe {
        ///     *b.get_mut() = true;
        ///     // We have created a (mutable) reference to an uninitialized `bool`!
        ///     // This is undefined behavior.
        /// }
        /// ```
        ///
        /// For instance, you cannot [`Read`] into an uninitialized buffer:
        ///
        /// [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_ref)]
        /// use std::{io, mem::MaybeUninit};
        ///
        /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
        /// {
        ///     let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
        ///     reader.read_exact(unsafe { buffer.get_mut() })?;
        ///                             // ^^^^^^^^^^^^^^^^
        ///                             // (mutable) reference to uninitialized memory!
        ///                             // This is undefined behavior.
        ///     Ok(unsafe { buffer.assume_init() })
        /// }
        /// ```
        ///
        /// Nor can you use direct field access to do field-by-field gradual initialization:
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_ref)]
        /// use std::{mem::MaybeUninit, ptr};
        ///
        /// struct Foo {
        ///     a: u32,
        ///     b: u8,
        /// }
        ///
        /// let foo: Foo = unsafe {
        ///     let mut foo = MaybeUninit::<Foo>::uninit();
        ///     ptr::write(&mut foo.get_mut().a as *mut u32, 1337);
        ///                  // ^^^^^^^^^^^^^
        ///                  // (mutable) reference to uninitialized memory!
        ///                  // This is undefined behavior.
        ///     ptr::write(&mut foo.get_mut().b as *mut u8, 42);
        ///                  // ^^^^^^^^^^^^^
        ///                  // (mutable) reference to uninitialized memory!
        ///                  // This is undefined behavior.
        ///     foo.assume_init()
        /// };
        /// ```
        // FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references
        // to uninitialized data (e.g., in `libcore/fmt/float.rs`).  We should make
        // a final decision about the rules before stabilization.
        #[unstable(feature = "maybe_uninit_ref", issue = "63568")]
        #[inline(always)]
        pub unsafe fn get_mut(&mut self) -> &mut T {
            #[cfg(bootstrap)]
            intrinsics::panic_if_uninhabited::<T>();
            #[cfg(not(bootstrap))]
            intrinsics::assert_inhabited::<T>();
            &mut *self.value
        }

        /// Assuming all the elements are initialized, get a slice to them.
        ///
        /// # Safety
        ///
        /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
        /// really are in an initialized state.
        /// Calling this when the content is not yet fully initialized causes undefined behavior.
        #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
        #[inline(always)]
        pub unsafe fn slice_get_ref(slice: &[Self]) -> &[T] {
            &*(slice as *const [Self] as *const [T])
        }

        /// Assuming all the elements are initialized, get a mutable slice to them.
        ///
        /// # Safety
        ///
        /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
        /// really are in an initialized state.
        /// Calling this when the content is not yet fully initialized causes undefined behavior.
        #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
        #[inline(always)]
        pub unsafe fn slice_get_mut(slice: &mut [Self]) -> &mut [T] {
            &mut *(slice as *mut [Self] as *mut [T])
        }

        /// Gets a pointer to the first element of the array.
        #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
        #[inline(always)]
        pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T {
            this as *const [MaybeUninit<T>] as *const T
        }

        /// Gets a mutable pointer to the first element of the array.
        #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
        #[inline(always)]
        pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T {
            this as *mut [MaybeUninit<T>] as *mut T
        }
    }