lazy.rs - source
Description: Source of the Rust file `library/core/src/lazy.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

    //! Lazy values and one-time initialization of static data.

    use crate::cell::{Cell, UnsafeCell};
    use crate::fmt;
    use crate::mem;
    use crate::ops::Deref;

    /// A cell which can be written to only once.
    ///
    /// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
    /// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell = OnceCell::new();
    /// assert!(cell.get().is_none());
    ///
    /// let value: &String = cell.get_or_init(|| {
    ///     "Hello, World!".to_string()
    /// });
    /// assert_eq!(value, "Hello, World!");
    /// assert!(cell.get().is_some());
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct OnceCell<T> {
        // Invariant: written to at most once.
        inner: UnsafeCell<Option<T>>,
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T> Default for OnceCell<T> {
        fn default() -> Self {
            Self::new()
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self.get() {
                Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
                None => f.write_str("OnceCell(Uninit)"),
            }
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Clone> Clone for OnceCell<T> {
        fn clone(&self) -> OnceCell<T> {
            let res = OnceCell::new();
            if let Some(value) = self.get() {
                match res.set(value.clone()) {
                    Ok(()) => (),
                    Err(_) => unreachable!(),
                }
            }
            res
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: PartialEq> PartialEq for OnceCell<T> {
        fn eq(&self, other: &Self) -> bool {
            self.get() == other.get()
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Eq> Eq for OnceCell<T> {}

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T> From<T> for OnceCell<T> {
        fn from(value: T) -> Self {
            OnceCell { inner: UnsafeCell::new(Some(value)) }
        }
    }

    impl<T> OnceCell<T> {
        /// Creates a new empty cell.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new() -> OnceCell<T> {
            OnceCell { inner: UnsafeCell::new(None) }
        }

        /// Gets the reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get(&self) -> Option<&T> {
            // SAFETY: Safe due to `inner`'s invariant
            unsafe { &*self.inner.get() }.as_ref()
        }

        /// Gets the mutable reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_mut(&mut self) -> Option<&mut T> {
            // SAFETY: Safe because we have unique access
            unsafe { &mut *self.inner.get() }.as_mut()
        }

        /// Sets the contents of the cell to `value`.
        ///
        /// # Errors
        ///
        /// This method returns `Ok(())` if the cell was empty and `Err(value)` if
        /// it was full.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::new();
        /// assert!(cell.get().is_none());
        ///
        /// assert_eq!(cell.set(92), Ok(()));
        /// assert_eq!(cell.set(62), Err(62));
        ///
        /// assert!(cell.get().is_some());
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn set(&self, value: T) -> Result<(), T> {
            // SAFETY: Safe because we cannot have overlapping mutable borrows
            let slot = unsafe { &*self.inner.get() };
            if slot.is_some() {
                return Err(value);
            }

            // SAFETY: This is the only place where we set the slot, no races
            // due to reentrancy/concurrency are possible, and we've
            // checked that slot is currently `None`, so this write
            // maintains the `inner`'s invariant.
            let slot = unsafe { &mut *self.inner.get() };
            *slot = Some(value);
            Ok(())
        }

        /// Gets the contents of the cell, initializing it with `f`
        /// if the cell was empty.
        ///
        /// # Panics
        ///
        /// If `f` panics, the panic is propagated to the caller, and the cell
        /// remains uninitialized.
        ///
        /// It is an error to reentrantly initialize the cell from `f`. Doing
        /// so results in a panic.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::new();
        /// let value = cell.get_or_init(|| 92);
        /// assert_eq!(value, &92);
        /// let value = cell.get_or_init(|| unreachable!());
        /// assert_eq!(value, &92);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_or_init<F>(&self, f: F) -> &T
        where
            F: FnOnce() -> T,
        {
            match self.get_or_try_init(|| Ok::<T, !>(f())) {
                Ok(val) => val,
            }
        }

        /// Gets the contents of the cell, initializing it with `f` if
        /// the cell was empty. If the cell was empty and `f` failed, an
        /// error is returned.
        ///
        /// # Panics
        ///
        /// If `f` panics, the panic is propagated to the caller, and the cell
        /// remains uninitialized.
        ///
        /// It is an error to reentrantly initialize the cell from `f`. Doing
        /// so results in a panic.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::new();
        /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
        /// assert!(cell.get().is_none());
        /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
        ///     Ok(92)
        /// });
        /// assert_eq!(value, Ok(&92));
        /// assert_eq!(cell.get(), Some(&92))
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            if let Some(val) = self.get() {
                return Ok(val);
            }
            let val = f()?;
            // Note that *some* forms of reentrant initialization might lead to
            // UB (see `reentrant_init` test). I believe that just removing this
            // `assert`, while keeping `set/get` would be sound, but it seems
            // better to panic, rather than to silently use an old value.
            assert!(self.set(val).is_ok(), "reentrant init");
            Ok(self.get().unwrap())
        }

        /// Consumes the cell, returning the wrapped value.
        ///
        /// Returns `None` if the cell was empty.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell: OnceCell<String> = OnceCell::new();
        /// assert_eq!(cell.into_inner(), None);
        ///
        /// let cell = OnceCell::new();
        /// cell.set("hello".to_string()).unwrap();
        /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn into_inner(self) -> Option<T> {
            // Because `into_inner` takes `self` by value, the compiler statically verifies
            // that it is not currently borrowed. So it is safe to move out `Option<T>`.
            self.inner.into_inner()
        }

        /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
        ///
        /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
        ///
        /// Safety is guaranteed by requiring a mutable reference.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let mut cell: OnceCell<String> = OnceCell::new();
        /// assert_eq!(cell.take(), None);
        ///
        /// let mut cell = OnceCell::new();
        /// cell.set("hello".to_string()).unwrap();
        /// assert_eq!(cell.take(), Some("hello".to_string()));
        /// assert_eq!(cell.get(), None);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn take(&mut self) -> Option<T> {
            mem::take(self).into_inner()
        }
    }

    /// A value which is initialized on the first access.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::Lazy;
    ///
    /// let lazy: Lazy<i32> = Lazy::new(|| {
    ///     println!("initializing");
    ///     92
    /// });
    /// println!("ready");
    /// println!("{}", *lazy);
    /// println!("{}", *lazy);
    ///
    /// // Prints:
    /// //   ready
    /// //   initializing
    /// //   92
    /// //   92
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct Lazy<T, F = fn() -> T> {
        cell: OnceCell<T>,
        init: Cell<Option<F>>,
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
        }
    }

    impl<T, F> Lazy<T, F> {
        /// Creates a new lazy value with the given initializing function.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// # fn main() {
        /// use std::lazy::Lazy;
        ///
        /// let hello = "Hello, World!".to_string();
        ///
        /// let lazy = Lazy::new(|| hello.to_uppercase());
        ///
        /// assert_eq!(&*lazy, "HELLO, WORLD!");
        /// # }
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new(init: F) -> Lazy<T, F> {
            Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
        }
    }

    impl<T, F: FnOnce() -> T> Lazy<T, F> {
        /// Forces the evaluation of this lazy value and returns a reference to
        /// the result.
        ///
        /// This is equivalent to the `Deref` impl, but is explicit.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::Lazy;
        ///
        /// let lazy = Lazy::new(|| 92);
        ///
        /// assert_eq!(Lazy::force(&lazy), &92);
        /// assert_eq!(&*lazy, &92);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn force(this: &Lazy<T, F>) -> &T {
            this.cell.get_or_init(|| match this.init.take() {
                Some(f) => f(),
                None => panic!("`Lazy` instance has previously been poisoned"),
            })
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
        type Target = T;
        fn deref(&self) -> &T {
            Lazy::force(self)
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Default> Default for Lazy<T> {
        /// Creates a new lazy value using `Default` as the initializing function.
        fn default() -> Lazy<T> {
            Lazy::new(T::default)
        }
    }