      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

    //! Types which pin data to its location in memory
    //!
    //! It is sometimes useful to have objects that are guaranteed to not move,
    //! in the sense that their placement in memory does not change, and can thus be relied upon.
    //!
    //! A prime example of such a scenario would be building self-referencial structs,
    //! since moving an object with pointers to itself will invalidate them,
    //! which could cause undefined behavior.
    //!
    //! In order to prevent objects from moving, they must be pinned
    //! by wrapping a pointer to the data in the [`Pin`] type. A pointer wrapped
    //! in a `Pin` is otherwise equivalent to its normal version, e.g. `Pin<Box<T>>`
    //! and `Box<T>` work the same way except that the first is pinning the value
    //! of `T` in place.
    //!
    //! First of all, these are pointer types because pinned data mustn't be passed around by value
    //! (that would change its location in memory).
    //! Secondly, since data can be moved out of `&mut` and `Box` with functions such as [`swap`],
    //! which causes their contents to swap places in memory,
    //! we need dedicated types that prohibit such operations.
    //!
    //! However, these restrictions are usually not necessary,
    //! so most types implement the [`Unpin`] auto-trait,
    //! which indicates that the type can be moved out safely.
    //! Doing so removes the limitations of pinning types,
    //! making them the same as their non-pinning counterparts.
    //!
    //! [`Pin`]: struct.Pin.html
    //! [`Unpin`]: trait.Unpin.html
    //! [`swap`]: ../../std/mem/fn.swap.html
    //! [`Box`]: ../../std/boxed/struct.Box.html
    //!
    //! # Examples
    //!
    //! ```rust
    //! #![feature(pin)]
    //!
    //! use std::pin::Pin;
    //! use std::marker::Pinned;
    //! use std::ptr::NonNull;
    //!
    //! // This is a self referencial struct since the slice field points to the data field.
    //! // We cannot inform the compiler about that with a normal reference,
    //! // since this pattern cannot be described with the usual borrowing rules.
    //! // Instead we use a raw pointer, though one which is known to not be null,
    //! // since we know it's pointing at the string.
    //! struct Unmovable {
    //!     data: String,
    //!     slice: NonNull<String>,
    //!     _pin: Pinned,
    //! }
    //!
    //! impl Unmovable {
    //!     // To ensure the data doesn't move when the function returns,
    //!     // we place it in the heap where it will stay for the lifetime of the object,
    //!     // and the only way to access it would be through a pointer to it.
    //!     fn new(data: String) -> Pin<Box<Self>> {
    //!         let res = Unmovable {
    //!             data,
    //!             // we only create the pointer once the data is in place
    //!             // otherwise it will have already moved before we even started
    //!             slice: NonNull::dangling(),
    //!             _pin: Pinned,
    //!         };
    //!         let mut boxed = Box::pinned(res);
    //!
    //!         let slice = NonNull::from(&boxed.data);
    //!         // we know this is safe because modifying a field doesn't move the whole struct
    //!         unsafe {
    //!             let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed);
    //!             Pin::get_mut_unchecked(mut_ref).slice = slice;
    //!         }
    //!         boxed
    //!     }
    //! }
    //!
    //! let unmoved = Unmovable::new("hello".to_string());
    //! // The pointer should point to the correct location,
    //! // so long as the struct hasn't moved.
    //! // Meanwhile, we are free to move the pointer around.
    //! # #[allow(unused_mut)]
    //! let mut still_unmoved = unmoved;
    //! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data));
    //!
    //! // Since our type doesn't implement Unpin, this will fail to compile:
    //! // let new_unmoved = Unmovable::new("world".to_string());
    //! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved);
    //! ```

    #![unstable(feature = "pin", issue = "49150")]

    use fmt;
    use marker::Sized;
    use ops::{Deref, DerefMut, CoerceUnsized};

    #[doc(inline)]
    pub use marker::Unpin;

    /// A pinned pointer.
    ///
    /// This is a wrapper around a kind of pointer which makes that pointer "pin" its
    /// value in place, preventing the value referenced by that pointer from being moved
    /// unless it implements [`Unpin`].
    ///
    /// See the [`pin` module] documentation for further explanation on pinning.
    ///
    /// [`Unpin`]: ../../std/marker/trait.Unpin.html
    /// [`pin` module]: ../../std/pin/index.html
    //
    // Note: the derives below are allowed because they all only use `&P`, so they
    // cannot move the value behind `pointer`.
    #[unstable(feature = "pin", issue = "49150")]
    #[fundamental]
    #[derive(Copy, Clone, Hash, Eq, PartialEq, Ord, PartialOrd)]
    pub struct Pin<P> {
        pointer: P,
    }

    impl<P: Deref> Pin<P>
    where
        P::Target: Unpin,
    {
        /// Construct a new `Pin` around a pointer to some data of a type that
        /// implements `Unpin`.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn new(pointer: P) -> Pin<P> {
            // Safety: the value pointed to is `Unpin`, and so has no requirements
            // around pinning.
            unsafe { Pin::new_unchecked(pointer) }
        }
    }

    impl<P: Deref> Pin<P> {
        /// Construct a new `Pin` around a reference to some data of a type that
        /// may or may not implement `Unpin`.
        ///
        /// # Safety
        ///
        /// This constructor is unsafe because we cannot guarantee that the data
        /// pointed to by `pointer` is pinned. If the constructed `Pin<P>` does
        /// not guarantee that the data `P` points to is pinned, constructing a
        /// `Pin<P>` is undefined behavior.
        ///
        /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used
        /// instead.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
            Pin { pointer }
        }

        /// Get a pinned shared reference from this pinned pointer.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn as_ref(self: &Pin<P>) -> Pin<&P::Target> {
            unsafe { Pin::new_unchecked(&*self.pointer) }
        }
    }

    impl<P: DerefMut> Pin<P> {
        /// Get a pinned mutable reference from this pinned pointer.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn as_mut(self: &mut Pin<P>) -> Pin<&mut P::Target> {
            unsafe { Pin::new_unchecked(&mut *self.pointer) }
        }

        /// Assign a new value to the memory behind the pinned reference.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn set(mut self: Pin<P>, value: P::Target)
        where
            P::Target: Sized,
        {
            *self.pointer = value;
        }
    }

    impl<'a, T: ?Sized> Pin<&'a T> {
        /// Construct a new pin by mapping the interior value.
        ///
        /// For example, if you  wanted to get a `Pin` of a field of something,
        /// you could use this to get access to that field in one line of code.
        ///
        /// # Safety
        ///
        /// This function is unsafe. You must guarantee that the data you return
        /// will not move so long as the argument value does not move (for example,
        /// because it is one of the fields of that value), and also that you do
        /// not move out of the argument you receive to the interior function.
        #[unstable(feature = "pin", issue = "49150")]
        pub unsafe fn map_unchecked<U, F>(this: Pin<&'a T>, func: F) -> Pin<&'a U> where
            F: FnOnce(&T) -> &U,
        {
            let pointer = &*this.pointer;
            let new_pointer = func(pointer);
            Pin::new_unchecked(new_pointer)
        }

        /// Get a shared reference out of a pin.
        ///
        /// Note: `Pin` also implements `Deref` to the target, which can be used
        /// to access the inner value. However, `Deref` only provides a reference
        /// that lives for as long as the borrow of the `Pin`, not the lifetime of
        /// the `Pin` itself. This method allows turning the `Pin` into a reference
        /// with the same lifetime as the original `Pin`.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn get_ref(this: Pin<&'a T>) -> &'a T {
            this.pointer
        }
    }

    impl<'a, T: ?Sized> Pin<&'a mut T> {
        /// Convert this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn into_ref(this: Pin<&'a mut T>) -> Pin<&'a T> {
            Pin { pointer: this.pointer }
        }

        /// Get a mutable reference to the data inside of this `Pin`.
        ///
        /// This requires that the data inside this `Pin` is `Unpin`.
        ///
        /// Note: `Pin` also implements `DerefMut` to the data, which can be used
        /// to access the inner value. However, `DerefMut` only provides a reference
        /// that lives for as long as the borrow of the `Pin`, not the lifetime of
        /// the `Pin` itself. This method allows turning the `Pin` into a reference
        /// with the same lifetime as the original `Pin`.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub fn get_mut(this: Pin<&'a mut T>) -> &'a mut T
            where T: Unpin,
        {
            this.pointer
        }

        /// Get a mutable reference to the data inside of this `Pin`.
        ///
        /// # Safety
        ///
        /// This function is unsafe. You must guarantee that you will never move
        /// the data out of the mutable reference you receive when you call this
        /// function, so that the invariants on the `Pin` type can be upheld.
        ///
        /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
        /// instead.
        #[unstable(feature = "pin", issue = "49150")]
        #[inline(always)]
        pub unsafe fn get_mut_unchecked(this: Pin<&'a mut T>) -> &'a mut T {
            this.pointer
        }

        /// Construct a new pin by mapping the interior value.
        ///
        /// For example, if you  wanted to get a `Pin` of a field of something,
        /// you could use this to get access to that field in one line of code.
        ///
        /// # Safety
        ///
        /// This function is unsafe. You must guarantee that the data you return
        /// will not move so long as the argument value does not move (for example,
        /// because it is one of the fields of that value), and also that you do
        /// not move out of the argument you receive to the interior function.
        #[unstable(feature = "pin", issue = "49150")]
        pub unsafe fn map_unchecked_mut<U, F>(this: Pin<&'a mut T>, func: F) -> Pin<&'a mut U> where
            F: FnOnce(&mut T) -> &mut U,
        {
            let pointer = Pin::get_mut_unchecked(this);
            let new_pointer = func(pointer);
            Pin::new_unchecked(new_pointer)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<P: Deref> Deref for Pin<P> {
        type Target = P::Target;
        fn deref(&self) -> &P::Target {
            Pin::get_ref(Pin::as_ref(self))
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<P: DerefMut> DerefMut for Pin<P>
    where
        P::Target: Unpin
    {
        fn deref_mut(&mut self) -> &mut P::Target {
            Pin::get_mut(Pin::as_mut(self))
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<P: fmt::Debug> fmt::Debug for Pin<P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&self.pointer, f)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<P: fmt::Display> fmt::Display for Pin<P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&self.pointer, f)
        }
    }

    #[unstable(feature = "pin", issue = "49150")]
    impl<P: fmt::Pointer> fmt::Pointer for Pin<P> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Pointer::fmt(&self.pointer, f)
        }
    }

    // Note: this means that any impl of `CoerceUnsized` that allows coercing from
    // a type that impls `Deref<Target=impl !Unpin>` to a type that impls
    // `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound
    // for other reasons, though, so we just need to take care not to allow such
    // impls to land in std.
    #[unstable(feature = "pin", issue = "49150")]
    impl<P, U> CoerceUnsized<Pin<U>> for Pin<P>
    where
        P: CoerceUnsized<U>,
    {}

    #[unstable(feature = "pin", issue = "49150")]
    impl<P> Unpin for Pin<P> {}

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse 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`)

Search multiple things at once by splitting your query with comma (e.g. `str,u8` or `String,struct:Vec,test`)