       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
     798
     799
     800
     801
     802
     803
     804
     805
     806
     807
     808
     809
     810
     811
     812
     813
     814
     815
     816
     817
     818
     819
     820
     821
     822
     823
     824
     825
     826
     827
     828
     829
     830
     831
     832
     833
     834
     835
     836
     837
     838
     839
     840
     841
     842
     843
     844
     845
     846
     847
     848
     849
     850
     851
     852
     853
     854
     855
     856
     857
     858
     859
     860
     861
     862
     863
     864
     865
     866
     867
     868
     869
     870
     871
     872
     873
     874
     875
     876
     877
     878
     879
     880
     881
     882
     883
     884
     885
     886
     887
     888
     889
     890
     891
     892
     893
     894
     895
     896
     897
     898
     899
     900
     901
     902
     903
     904
     905
     906
     907
     908
     909
     910
     911
     912
     913
     914
     915
     916
     917
     918
     919
     920
     921
     922
     923
     924
     925
     926
     927
     928
     929
     930
     931
     932
     933
     934
     935
     936
     937
     938
     939
     940
     941
     942
     943
     944
     945
     946
     947
     948
     949
     950
     951
     952
     953
     954
     955
     956
     957
     958
     959
     960
     961
     962
     963
     964
     965
     966
     967
     968
     969
     970
     971
     972
     973
     974
     975
     976
     977
     978
     979
     980
     981
     982
     983
     984
     985
     986
     987
     988
     989
     990
     991
     992
     993
     994
     995
     996
     997
     998
     999
    1000
    1001
    1002
    1003
    1004
    1005
    1006
    1007
    1008
    1009
    1010
    1011
    1012
    1013
    1014
    1015
    1016
    1017
    1018
    1019
    1020
    1021
    1022
    1023
    1024
    1025
    1026
    1027
    1028

    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Basic functions for dealing with memory.
    //!
    //! This module contains functions for querying the size and alignment of
    //! types, initializing and manipulating memory.

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

    use clone;
    use cmp;
    use fmt;
    use hash;
    use intrinsics;
    use marker::{Copy, PhantomData, Sized};
    use ptr;
    use ops::{Deref, DerefMut};

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use intrinsics::transmute;

    /// Leaks a value: takes ownership and "forgets" about the value **without running
    /// its destructor**.
    ///
    /// Any resources the value manages, such as heap memory or a file handle, will linger
    /// forever in an unreachable state.
    ///
    /// If you want to dispose of a value properly, running its destructor, see
    /// [`mem::drop`][drop].
    ///
    /// # Safety
    ///
    /// `forget` is not marked as `unsafe`, because Rust's safety guarantees
    /// do not include a guarantee that destructors will always run. For example,
    /// a program can create a reference cycle using [`Rc`][rc], or call
    /// [`process::exit`][exit] to exit without running destructors. Thus, allowing
    /// `mem::forget` from safe code does not fundamentally change Rust's safety
    /// guarantees.
    ///
    /// That said, leaking resources such as memory or I/O objects is usually undesirable,
    /// so `forget` is only recommended for specialized use cases like those shown below.
    ///
    /// Because forgetting a value is allowed, any `unsafe` code you write must
    /// allow for this possibility. You cannot return a value and expect that the
    /// caller will necessarily run the value's destructor.
    ///
    /// [rc]: ../../std/rc/struct.Rc.html
    /// [exit]: ../../std/process/fn.exit.html
    ///
    /// # Examples
    ///
    /// Leak some heap memory by never deallocating it:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let heap_memory = Box::new(3);
    /// mem::forget(heap_memory);
    /// ```
    ///
    /// Leak an I/O object, never closing the file:
    ///
    /// ```no_run
    /// use std::mem;
    /// use std::fs::File;
    ///
    /// let file = File::open("foo.txt").unwrap();
    /// mem::forget(file);
    /// ```
    ///
    /// The practical use cases for `forget` are rather specialized and mainly come
    /// up in unsafe or FFI code.
    ///
    /// ## Use case 1
    ///
    /// You have created an uninitialized value using [`mem::uninitialized`][uninit].
    /// You must either initialize or `forget` it on every computation path before
    /// Rust drops it automatically, like at the end of a scope or after a panic.
    /// Running the destructor on an uninitialized value would be [undefined behavior][ub].
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// # let some_condition = false;
    /// unsafe {
    ///     let mut uninit_vec: Vec<u32> = mem::uninitialized();
    ///
    ///     if some_condition {
    ///         // Initialize the variable.
    ///         ptr::write(&mut uninit_vec, Vec::new());
    ///     } else {
    ///         // Forget the uninitialized value so its destructor doesn't run.
    ///         mem::forget(uninit_vec);
    ///     }
    /// }
    /// ```
    ///
    /// ## Use case 2
    ///
    /// You have duplicated the bytes making up a value, without doing a proper
    /// [`Clone`][clone]. You need the value's destructor to run only once,
    /// because a double `free` is undefined behavior.
    ///
    /// An example is a possible implementation of [`mem::swap`][swap]:
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// # #[allow(dead_code)]
    /// fn swap<T>(x: &mut T, y: &mut T) {
    ///     unsafe {
    ///         // Give ourselves some scratch space to work with
    ///         let mut t: T = mem::uninitialized();
    ///
    ///         // Perform the swap, `&mut` pointers never alias
    ///         ptr::copy_nonoverlapping(&*x, &mut t, 1);
    ///         ptr::copy_nonoverlapping(&*y, x, 1);
    ///         ptr::copy_nonoverlapping(&t, y, 1);
    ///
    ///         // y and t now point to the same thing, but we need to completely
    ///         // forget `t` because we do not want to run the destructor for `T`
    ///         // on its value, which is still owned somewhere outside this function.
    ///         mem::forget(t);
    ///     }
    /// }
    /// ```
    ///
    /// ## Use case 3
    ///
    /// You are transferring ownership across a [FFI] boundary to code written in
    /// another language. You need to `forget` the value on the Rust side because Rust
    /// code is no longer responsible for it.
    ///
    /// ```no_run
    /// use std::mem;
    ///
    /// extern "C" {
    ///     fn my_c_function(x: *const u32);
    /// }
    ///
    /// let x: Box<u32> = Box::new(3);
    ///
    /// // Transfer ownership into C code.
    /// unsafe {
    ///     my_c_function(&*x);
    /// }
    /// mem::forget(x);
    /// ```
    ///
    /// In this case, C code must call back into Rust to free the object. Calling C's `free`
    /// function on a [`Box`][box] is *not* safe! Also, `Box` provides an [`into_raw`][into_raw]
    /// method which is the preferred way to do this in practice.
    ///
    /// [drop]: fn.drop.html
    /// [uninit]: fn.uninitialized.html
    /// [clone]: ../clone/trait.Clone.html
    /// [swap]: fn.swap.html
    /// [FFI]: ../../book/first-edition/ffi.html
    /// [box]: ../../std/boxed/struct.Box.html
    /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
    /// [ub]: ../../reference/behavior-considered-undefined.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn forget<T>(t: T) {
        ManuallyDrop::new(t);
    }

    /// Returns the size of a type in bytes.
    ///
    /// More specifically, this is the offset in bytes between successive elements
    /// in an array with that item type including alignment padding. Thus, for any
    /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
    ///
    /// In general, the size of a type is not stable across compilations, but
    /// specific types such as primitives are.
    ///
    /// The following table gives the size for primitives.
    ///
    /// Type | size_of::\<Type>()
    /// ---- | ---------------
    /// () | 0
    /// u8 | 1
    /// u16 | 2
    /// u32 | 4
    /// u64 | 8
    /// i8 | 1
    /// i16 | 2
    /// i32 | 4
    /// i64 | 8
    /// f32 | 4
    /// f64 | 8
    /// char | 4
    ///
    /// Furthermore, `usize` and `isize` have the same size.
    ///
    /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
    /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
    ///
    /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
    /// have the same size. Likewise for `*const T` and `*mut T`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// // Some primitives
    /// assert_eq!(4, mem::size_of::<i32>());
    /// assert_eq!(8, mem::size_of::<f64>());
    /// assert_eq!(0, mem::size_of::<()>());
    ///
    /// // Some arrays
    /// assert_eq!(8, mem::size_of::<[i32; 2]>());
    /// assert_eq!(12, mem::size_of::<[i32; 3]>());
    /// assert_eq!(0, mem::size_of::<[i32; 0]>());
    ///
    ///
    /// // Pointer size equality
    /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
    /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
    /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
    /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_size_of"))]
    pub const fn size_of<T>() -> usize {
        unsafe { intrinsics::size_of::<T>() }
    }

    /// Returns the size of the pointed-to value in bytes.
    ///
    /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
    /// statically known size, e.g. a slice [`[T]`][slice] or a [trait object],
    /// then `size_of_val` can be used to get the dynamically-known size.
    ///
    /// [slice]: ../../std/primitive.slice.html
    /// [trait object]: ../../book/first-edition/trait-objects.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::size_of_val(&5i32));
    ///
    /// let x: [u8; 13] = [0; 13];
    /// let y: &[u8] = &x;
    /// assert_eq!(13, mem::size_of_val(y));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::size_of_val(val) }
    }

    /// Returns the [ABI]-required minimum alignment of a type.
    ///
    /// Every reference to a value of the type `T` must be a multiple of this number.
    ///
    /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    ///
    /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(deprecated)]
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::min_align_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
    pub fn min_align_of<T>() -> usize {
        unsafe { intrinsics::min_align_of::<T>() }
    }

    /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
    ///
    /// Every reference to a value of the type `T` must be a multiple of this number.
    ///
    /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(deprecated)]
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::min_align_of_val(&5i32));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
    pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::min_align_of_val(val) }
    }

    /// Returns the [ABI]-required minimum alignment of a type.
    ///
    /// Every reference to a value of the type `T` must be a multiple of this number.
    ///
    /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    ///
    /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::align_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg_attr(not(stage0), rustc_const_unstable(feature = "const_align_of"))]
    pub const fn align_of<T>() -> usize {
        unsafe { intrinsics::min_align_of::<T>() }
    }

    /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
    ///
    /// Every reference to a value of the type `T` must be a multiple of this number.
    ///
    /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::align_of_val(&5i32));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::min_align_of_val(val) }
    }

    /// Returns whether dropping values of type `T` matters.
    ///
    /// This is purely an optimization hint, and may be implemented conservatively.
    /// For instance, always returning `true` would be a valid implementation of
    /// this function.
    ///
    /// Low level implementations of things like collections, which need to manually
    /// drop their data, should use this function to avoid unnecessarily
    /// trying to drop all their contents when they are destroyed. This might not
    /// make a difference in release builds (where a loop that has no side-effects
    /// is easily detected and eliminated), but is often a big win for debug builds.
    ///
    /// Note that `ptr::drop_in_place` already performs this check, so if your workload
    /// can be reduced to some small number of drop_in_place calls, using this is
    /// unnecessary. In particular note that you can drop_in_place a slice, and that
    /// will do a single needs_drop check for all the values.
    ///
    /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
    /// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
    /// values one at a time and should use this API.
    ///
    ///
    /// # Examples
    ///
    /// Here's an example of how a collection might make use of needs_drop:
    ///
    /// ```
    /// use std::{mem, ptr};
    ///
    /// pub struct MyCollection<T> {
    /// #   data: [T; 1],
    ///     /* ... */
    /// }
    /// # impl<T> MyCollection<T> {
    /// #   fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
    /// #   fn free_buffer(&mut self) {}
    /// # }
    ///
    /// impl<T> Drop for MyCollection<T> {
    ///     fn drop(&mut self) {
    ///         unsafe {
    ///             // drop the data
    ///             if mem::needs_drop::<T>() {
    ///                 for x in self.iter_mut() {
    ///                     ptr::drop_in_place(x);
    ///                 }
    ///             }
    ///             self.free_buffer();
    ///         }
    ///     }
    /// }
    /// ```
    #[inline]
    #[stable(feature = "needs_drop", since = "1.22.0")]
    pub fn needs_drop<T>() -> bool {
        unsafe { intrinsics::needs_drop::<T>() }
    }

    /// Creates a value whose bytes are all zero.
    ///
    /// This has the same effect as allocating space with
    /// [`mem::uninitialized`][uninit] and then zeroing it out. It is useful for
    /// [FFI] sometimes, but should generally be avoided.
    ///
    /// There is no guarantee that an all-zero byte-pattern represents a valid value of
    /// some type `T`. If `T` has a destructor and the value is destroyed (due to
    /// a panic or the end of a scope) before being initialized, then the destructor
    /// will run on zeroed data, likely leading to [undefined behavior][ub].
    ///
    /// See also the documentation for [`mem::uninitialized`][uninit], which has
    /// many of the same caveats.
    ///
    /// [uninit]: fn.uninitialized.html
    /// [FFI]: ../../book/first-edition/ffi.html
    /// [ub]: ../../reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let x: i32 = unsafe { mem::zeroed() };
    /// assert_eq!(0, x);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn zeroed<T>() -> T {
        intrinsics::init()
    }

    /// Bypasses Rust's normal memory-initialization checks by pretending to
    /// produce a value of type `T`, while doing nothing at all.
    ///
    /// **This is incredibly dangerous and should not be done lightly. Deeply
    /// consider initializing your memory with a default value instead.**
    ///
    /// This is useful for [FFI] functions and initializing arrays sometimes,
    /// but should generally be avoided.
    ///
    /// [FFI]: ../../book/first-edition/ffi.html
    ///
    /// # Undefined behavior
    ///
    /// It is [undefined behavior][ub] to read uninitialized memory, even just an
    /// uninitialized boolean. For instance, if you branch on the value of such
    /// a boolean, your program may take one, both, or neither of the branches.
    ///
    /// Writing to the uninitialized value is similarly dangerous. Rust believes the
    /// value is initialized, and will therefore try to [`Drop`] the uninitialized
    /// value and its fields if you try to overwrite it in a normal manner. The only way
    /// to safely initialize an uninitialized value is with [`ptr::write`][write],
    /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no].
    ///
    /// If the value does implement [`Drop`], it must be initialized before
    /// it goes out of scope (and therefore would be dropped). Note that this
    /// includes a `panic` occurring and unwinding the stack suddenly.
    ///
    /// # Examples
    ///
    /// Here's how to safely initialize an array of [`Vec`]s.
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// // Only declare the array. This safely leaves it
    /// // uninitialized in a way that Rust will track for us.
    /// // However we can't initialize it element-by-element
    /// // safely, and we can't use the `[value; 1000]`
    /// // constructor because it only works with `Copy` data.
    /// let mut data: [Vec<u32>; 1000];
    ///
    /// unsafe {
    ///     // So we need to do this to initialize it.
    ///     data = mem::uninitialized();
    ///
    ///     // DANGER ZONE: if anything panics or otherwise
    ///     // incorrectly reads the array here, we will have
    ///     // Undefined Behavior.
    ///
    ///     // It's ok to mutably iterate the data, since this
    ///     // doesn't involve reading it at all.
    ///     // (ptr and len are statically known for arrays)
    ///     for elem in &mut data[..] {
    ///         // *elem = Vec::new() would try to drop the
    ///         // uninitialized memory at `elem` -- bad!
    ///         //
    ///         // Vec::new doesn't allocate or do really
    ///         // anything. It's only safe to call here
    ///         // because we know it won't panic.
    ///         ptr::write(elem, Vec::new());
    ///     }
    ///
    ///     // SAFE ZONE: everything is initialized.
    /// }
    ///
    /// println!("{:?}", &data[0]);
    /// ```
    ///
    /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized`
    /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a
    /// value that is only [`Clone`], so the following is semantically equivalent and
    /// vastly less dangerous, as long as you can live with an extra heap
    /// allocation:
    ///
    /// ```
    /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
    /// println!("{:?}", &data[0]);
    /// ```
    ///
    /// [`Vec`]: ../../std/vec/struct.Vec.html
    /// [`vec!`]: ../../std/macro.vec.html
    /// [`Clone`]: ../../std/clone/trait.Clone.html
    /// [ub]: ../../reference/behavior-considered-undefined.html
    /// [write]: ../ptr/fn.write.html
    /// [copy]: ../intrinsics/fn.copy.html
    /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
    /// [`Drop`]: ../ops/trait.Drop.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn uninitialized<T>() -> T {
        intrinsics::uninit()
    }

    /// Swaps the values at two mutable locations, without deinitializing either one.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let mut x = 5;
    /// let mut y = 42;
    ///
    /// mem::swap(&mut x, &mut y);
    ///
    /// assert_eq!(42, x);
    /// assert_eq!(5, y);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn swap<T>(x: &mut T, y: &mut T) {
        unsafe {
            ptr::swap_nonoverlapping(x, y, 1);
        }
    }

    /// Replaces the value at a mutable location with a new one, returning the old value, without
    /// deinitializing either one.
    ///
    /// # Examples
    ///
    /// A simple example:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let mut v: Vec<i32> = vec![1, 2];
    ///
    /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
    /// assert_eq!(2, old_v.len());
    /// assert_eq!(3, v.len());
    /// ```
    ///
    /// `replace` allows consumption of a struct field by replacing it with another value.
    /// Without `replace` you can run into issues like these:
    ///
    /// ```compile_fail,E0507
    /// struct Buffer<T> { buf: Vec<T> }
    ///
    /// impl<T> Buffer<T> {
    ///     fn get_and_reset(&mut self) -> Vec<T> {
    ///         // error: cannot move out of dereference of `&mut`-pointer
    ///         let buf = self.buf;
    ///         self.buf = Vec::new();
    ///         buf
    ///     }
    /// }
    /// ```
    ///
    /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
    /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
    /// `self`, allowing it to be returned:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// use std::mem;
    ///
    /// # struct Buffer<T> { buf: Vec<T> }
    /// impl<T> Buffer<T> {
    ///     fn get_and_reset(&mut self) -> Vec<T> {
    ///         mem::replace(&mut self.buf, Vec::new())
    ///     }
    /// }
    /// ```
    ///
    /// [`Clone`]: ../../std/clone/trait.Clone.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn replace<T>(dest: &mut T, mut src: T) -> T {
        swap(dest, &mut src);
        src
    }

    /// Disposes of a value.
    ///
    /// While this does call the argument's implementation of [`Drop`][drop],
    /// it will not release any borrows, as borrows are based on lexical scope.
    ///
    /// This effectively does nothing for
    /// [types which implement `Copy`](../../book/first-edition/ownership.html#copy-types),
    /// e.g. integers. Such values are copied and _then_ moved into the function,
    /// so the value persists after this function call.
    ///
    /// This function is not magic; it is literally defined as
    ///
    /// ```
    /// pub fn drop<T>(_x: T) { }
    /// ```
    ///
    /// Because `_x` is moved into the function, it is automatically dropped before
    /// the function returns.
    ///
    /// [drop]: ../ops/trait.Drop.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v = vec![1, 2, 3];
    ///
    /// drop(v); // explicitly drop the vector
    /// ```
    ///
    /// Borrows are based on lexical scope, so this produces an error:
    ///
    /// ```compile_fail,E0502
    /// let mut v = vec![1, 2, 3];
    /// let x = &v[0];
    ///
    /// drop(x); // explicitly drop the reference, but the borrow still exists
    ///
    /// v.push(4); // error: cannot borrow `v` as mutable because it is also
    ///            // borrowed as immutable
    /// ```
    ///
    /// An inner scope is needed to fix this:
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    ///
    /// {
    ///     let x = &v[0];
    ///
    ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
    /// }
    ///
    /// v.push(4); // no problems
    /// ```
    ///
    /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
    /// release a [`RefCell`] borrow:
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let x = RefCell::new(1);
    ///
    /// let mut mutable_borrow = x.borrow_mut();
    /// *mutable_borrow = 1;
    ///
    /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
    ///
    /// let borrow = x.borrow();
    /// println!("{}", *borrow);
    /// ```
    ///
    /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
    ///
    /// ```
    /// #[derive(Copy, Clone)]
    /// struct Foo(u8);
    ///
    /// let x = 1;
    /// let y = Foo(2);
    /// drop(x); // a copy of `x` is moved and dropped
    /// drop(y); // a copy of `y` is moved and dropped
    ///
    /// println!("x: {}, y: {}", x, y.0); // still available
    /// ```
    ///
    /// [`RefCell`]: ../../std/cell/struct.RefCell.html
    /// [`Copy`]: ../../std/marker/trait.Copy.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn drop<T>(_x: T) { }

    /// Interprets `src` as having type `&U`, and then reads `src` without moving
    /// the contained value.
    ///
    /// This function will unsafely assume the pointer `src` is valid for
    /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
    /// the `&U`. It will also unsafely create a copy of the contained value instead of
    /// moving out of `src`.
    ///
    /// It is not a compile-time error if `T` and `U` have different sizes, but it
    /// is highly encouraged to only invoke this function where `T` and `U` have the
    /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
    /// `T`.
    ///
    /// [ub]: ../../reference/behavior-considered-undefined.html
    /// [size_of]: fn.size_of.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// #[repr(packed)]
    /// struct Foo {
    ///     bar: u8,
    /// }
    ///
    /// let foo_slice = [10u8];
    ///
    /// unsafe {
    ///     // Copy the data from 'foo_slice' and treat it as a 'Foo'
    ///     let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
    ///     assert_eq!(foo_struct.bar, 10);
    ///
    ///     // Modify the copied data
    ///     foo_struct.bar = 20;
    ///     assert_eq!(foo_struct.bar, 20);
    /// }
    ///
    /// // The contents of 'foo_slice' should not have changed
    /// assert_eq!(foo_slice, [10]);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
        ptr::read(src as *const T as *const U)
    }

    /// Opaque type representing the discriminant of an enum.
    ///
    /// See the `discriminant` function in this module for more information.
    #[stable(feature = "discriminant_value", since = "1.21.0")]
    pub struct Discriminant<T>(u64, PhantomData<*const T>);

    // N.B. These trait implementations cannot be derived because we don't want any bounds on T.

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> Copy for Discriminant<T> {}

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> clone::Clone for Discriminant<T> {
        fn clone(&self) -> Self {
            *self
        }
    }

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> cmp::PartialEq for Discriminant<T> {
        fn eq(&self, rhs: &Self) -> bool {
            self.0 == rhs.0
        }
    }

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> cmp::Eq for Discriminant<T> {}

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> hash::Hash for Discriminant<T> {
        fn hash<H: hash::Hasher>(&self, state: &mut H) {
            self.0.hash(state);
        }
    }

    #[stable(feature = "discriminant_value", since = "1.21.0")]
    impl<T> fmt::Debug for Discriminant<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_tuple("Discriminant")
               .field(&self.0)
               .finish()
        }
    }

    /// Returns a value uniquely identifying the enum variant in `v`.
    ///
    /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
    /// return value is unspecified.
    ///
    /// # Stability
    ///
    /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
    /// of some variant will not change between compilations with the same compiler.
    ///
    /// # Examples
    ///
    /// This can be used to compare enums that carry data, while disregarding
    /// the actual data:
    ///
    /// ```
    /// use std::mem;
    ///
    /// enum Foo { A(&'static str), B(i32), C(i32) }
    ///
    /// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
    /// assert!(mem::discriminant(&Foo::B(1))     == mem::discriminant(&Foo::B(2)));
    /// assert!(mem::discriminant(&Foo::B(3))     != mem::discriminant(&Foo::C(3)));
    /// ```
    #[stable(feature = "discriminant_value", since = "1.21.0")]
    pub fn discriminant<T>(v: &T) -> Discriminant<T> {
        unsafe {
            Discriminant(intrinsics::discriminant_value(v), PhantomData)
        }
    }

    /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
    ///
    /// This wrapper is 0-cost.
    ///
    /// # Examples
    ///
    /// This wrapper helps with explicitly documenting the drop order dependencies between fields of
    /// the type:
    ///
    /// ```rust
    /// use std::mem::ManuallyDrop;
    /// struct Peach;
    /// struct Banana;
    /// struct Melon;
    /// struct FruitBox {
    ///     // Immediately clear there’s something non-trivial going on with these fields.
    ///     peach: ManuallyDrop<Peach>,
    ///     melon: Melon, // Field that’s independent of the other two.
    ///     banana: ManuallyDrop<Banana>,
    /// }
    ///
    /// impl Drop for FruitBox {
    ///     fn drop(&mut self) {
    ///         unsafe {
    ///             // Explicit ordering in which field destructors are run specified in the intuitive
    ///             // location – the destructor of the structure containing the fields.
    ///             // Moreover, one can now reorder fields within the struct however much they want.
    ///             ManuallyDrop::drop(&mut self.peach);
    ///             ManuallyDrop::drop(&mut self.banana);
    ///         }
    ///         // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
    ///         // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
    ///     }
    /// }
    /// ```
    #[stable(feature = "manually_drop", since = "1.20.0")]
    #[allow(unions_with_drop_fields)]
    #[derive(Copy)]
    pub union ManuallyDrop<T>{ value: T }

    impl<T> ManuallyDrop<T> {
        /// Wrap a value to be manually dropped.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::mem::ManuallyDrop;
        /// ManuallyDrop::new(Box::new(()));
        /// ```
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub fn new(value: T) -> ManuallyDrop<T> {
            ManuallyDrop { value: value }
        }

        /// Extract the value from the ManuallyDrop container.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::mem::ManuallyDrop;
        /// let x = ManuallyDrop::new(Box::new(()));
        /// let _: Box<()> = ManuallyDrop::into_inner(x);
        /// ```
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub fn into_inner(slot: ManuallyDrop<T>) -> T {
            unsafe {
                slot.value
            }
        }

        /// Manually drops the contained value.
        ///
        /// # Safety
        ///
        /// This function runs the destructor of the contained value and thus the wrapped value
        /// now represents uninitialized data. It is up to the user of this method to ensure the
        /// uninitialized data is not actually used.
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
            ptr::drop_in_place(&mut slot.value)
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T> Deref for ManuallyDrop<T> {
        type Target = T;
        #[inline]
        fn deref(&self) -> &Self::Target {
            unsafe {
                &self.value
            }
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T> DerefMut for ManuallyDrop<T> {
        #[inline]
        fn deref_mut(&mut self) -> &mut Self::Target {
            unsafe {
                &mut self.value
            }
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: ::fmt::Debug> ::fmt::Debug for ManuallyDrop<T> {
        fn fmt(&self, fmt: &mut ::fmt::Formatter) -> ::fmt::Result {
            unsafe {
                fmt.debug_tuple("ManuallyDrop").field(&self.value).finish()
            }
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: Clone> Clone for ManuallyDrop<T> {
        fn clone(&self) -> Self {
            ManuallyDrop::new(self.deref().clone())
        }

        fn clone_from(&mut self, source: &Self) {
            self.deref_mut().clone_from(source);
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: Default> Default for ManuallyDrop<T> {
        fn default() -> Self {
            ManuallyDrop::new(Default::default())
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: PartialEq> PartialEq for ManuallyDrop<T> {
        fn eq(&self, other: &Self) -> bool {
            self.deref().eq(other)
        }

        fn ne(&self, other: &Self) -> bool {
            self.deref().ne(other)
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: Eq> Eq for ManuallyDrop<T> {}

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: PartialOrd> PartialOrd for ManuallyDrop<T> {
        fn partial_cmp(&self, other: &Self) -> Option<::cmp::Ordering> {
            self.deref().partial_cmp(other)
        }

        fn lt(&self, other: &Self) -> bool {
            self.deref().lt(other)
        }

        fn le(&self, other: &Self) -> bool {
            self.deref().le(other)
        }

        fn gt(&self, other: &Self) -> bool {
            self.deref().gt(other)
        }

        fn ge(&self, other: &Self) -> bool {
            self.deref().ge(other)
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: Ord> Ord for ManuallyDrop<T> {
        fn cmp(&self, other: &Self) -> ::cmp::Ordering {
            self.deref().cmp(other)
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: ::hash::Hash> ::hash::Hash for ManuallyDrop<T> {
        fn hash<H: ::hash::Hasher>(&self, state: &mut H) {
            self.deref().hash(state);
        }
    }

    /// Tells LLVM that this point in the code is not reachable, enabling further
    /// optimizations.
    ///
    /// NB: This is very different from the `unreachable!()` macro: Unlike the
    /// macro, which panics when it is executed, it is *undefined behavior* to
    /// reach code marked with this function.
    #[inline]
    #[unstable(feature = "unreachable", issue = "43751")]
    pub unsafe fn unreachable() -> ! {
        intrinsics::unreachable()
    }

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)