lazy.rs

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

#[cfg(test)]
mod tests;

use crate::{
    cell::{Cell, UnsafeCell},
    fmt,
    marker::PhantomData,
    mem::MaybeUninit,
    ops::{Deref, Drop},
    panic::{RefUnwindSafe, UnwindSafe},
    pin::Pin,
    sync::Once,
};

#[doc(inline)]
#[unstable(feature = "once_cell", issue = "74465")]
pub use core::lazy::*;

/// A synchronization primitive which can be written to only once.
///
/// This type is a thread-safe `OnceCell`.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::SyncOnceCell;
///
/// static CELL: SyncOnceCell<String> = SyncOnceCell::new();
/// assert!(CELL.get().is_none());
///
/// std::thread::spawn(|| {
///     let value: &String = CELL.get_or_init(|| {
///         "Hello, World!".to_string()
///     });
///     assert_eq!(value, "Hello, World!");
/// }).join().unwrap();
///
/// let value: Option<&String> = CELL.get();
/// assert!(value.is_some());
/// assert_eq!(value.unwrap().as_str(), "Hello, World!");
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct SyncOnceCell<T> {
    once: Once,
    // Whether or not the value is initialized is tracked by `state_and_queue`.
    value: UnsafeCell<MaybeUninit<T>>,
    /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
    ///
    /// ```compile_fail,E0597
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// struct A<'a>(&'a str);
    ///
    /// impl<'a> Drop for A<'a> {
    ///     fn drop(&mut self) {}
    /// }
    ///
    /// let cell = SyncOnceCell::new();
    /// {
    ///     let s = String::new();
    ///     let _ = cell.set(A(&s));
    /// }
    /// ```
    _marker: PhantomData<T>,
}

// Why do we need `T: Send`?
// Thread A creates a `SyncOnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Sync + Send> Sync for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Send> Send for SyncOnceCell<T> {}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}

#[unstable(feature = "once_cell", issue = "74465")]
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for SyncOnceCell<T> {
    /// Creates a new empty cell.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// fn main() {
    ///     assert_eq!(SyncOnceCell::<()>::new(), SyncOnceCell::default());
    /// }
    /// ```
    fn default() -> SyncOnceCell<T> {
        SyncOnceCell::new()
    }
}

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

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

#[unstable(feature = "once_cell", issue = "74465")]
impl<T> From<T> for SyncOnceCell<T> {
    /// Create a new cell with its contents set to `value`.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// # fn main() -> Result<(), i32> {
    /// let a = SyncOnceCell::from(3);
    /// let b = SyncOnceCell::new();
    /// b.set(3)?;
    /// assert_eq!(a, b);
    /// Ok(())
    /// # }
    /// ```
    fn from(value: T) -> Self {
        let cell = Self::new();
        match cell.set(value) {
            Ok(()) => cell,
            Err(_) => unreachable!(),
        }
    }
}

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

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

impl<T> SyncOnceCell<T> {
    /// Creates a new empty cell.
    #[unstable(feature = "once_cell", issue = "74465")]
    #[must_use]
    pub const fn new() -> SyncOnceCell<T> {
        SyncOnceCell {
            once: Once::new(),
            value: UnsafeCell::new(MaybeUninit::uninit()),
            _marker: PhantomData,
        }
    }

    /// Gets the reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty, or being initialized. This
    /// method never blocks.
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get(&self) -> Option<&T> {
        if self.is_initialized() {
            // Safe b/c checked is_initialized
            Some(unsafe { self.get_unchecked() })
        } else {
            None
        }
    }

    /// Gets the mutable reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty. This method never blocks.
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get_mut(&mut self) -> Option<&mut T> {
        if self.is_initialized() {
            // Safe b/c checked is_initialized and we have a unique access
            Some(unsafe { self.get_unchecked_mut() })
        } else {
            None
        }
    }

    /// Sets the contents of this cell to `value`.
    ///
    /// May block if another thread is currently attempting to initialize the cell. The cell is
    /// guaranteed to contain a value when set returns, though not necessarily the one provided.
    ///
    /// Returns `Ok(())` if the cell's value was set by this call.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// static CELL: SyncOnceCell<i32> = SyncOnceCell::new();
    ///
    /// fn main() {
    ///     assert!(CELL.get().is_none());
    ///
    ///     std::thread::spawn(|| {
    ///         assert_eq!(CELL.set(92), Ok(()));
    ///     }).join().unwrap();
    ///
    ///     assert_eq!(CELL.set(62), Err(62));
    ///     assert_eq!(CELL.get(), Some(&92));
    /// }
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn set(&self, value: T) -> Result<(), T> {
        let mut value = Some(value);
        self.get_or_init(|| value.take().unwrap());
        match value {
            None => Ok(()),
            Some(value) => Err(value),
        }
    }

    /// Gets the contents of the cell, initializing it with `f` if the cell
    /// was empty.
    ///
    /// Many threads may call `get_or_init` concurrently with different
    /// initializing functions, but it is guaranteed that only one function
    /// will be executed.
    ///
    /// # 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`. The
    /// exact outcome is unspecified. Current implementation deadlocks, but
    /// this may be changed to a panic in the future.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// let cell = SyncOnceCell::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`.
    /// The exact outcome is unspecified. Current implementation
    /// deadlocks, but this may be changed to a panic in the future.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// let cell = SyncOnceCell::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>,
    {
        // Fast path check
        // NOTE: We need to perform an acquire on the state in this method
        // in order to correctly synchronize `SyncLazy::force`. This is
        // currently done by calling `self.get()`, which in turn calls
        // `self.is_initialized()`, which in turn performs the acquire.
        if let Some(value) = self.get() {
            return Ok(value);
        }
        self.initialize(f)?;

        debug_assert!(self.is_initialized());

        // SAFETY: The inner value has been initialized
        Ok(unsafe { self.get_unchecked() })
    }

    /// Internal-only API that gets the contents of the cell, initializing it
    /// in two steps with `f` and `g` if the cell was empty.
    ///
    /// `f` is called to construct the value, which is then moved into the cell
    /// and given as a (pinned) mutable reference to `g` to finish
    /// initialization.
    ///
    /// This allows `g` to inspect an manipulate the value after it has been
    /// moved into its final place in the cell, but before the cell is
    /// considered initialized.
    ///
    /// # Panics
    ///
    /// If `f` or `g` panics, the panic is propagated to the caller, and the
    /// cell remains uninitialized.
    ///
    /// With the current implementation, if `g` panics, the value from `f` will
    /// not be dropped. This should probably be fixed if this is ever used for
    /// a type where this matters.
    ///
    /// It is an error to reentrantly initialize the cell from `f`. The exact
    /// outcome is unspecified. Current implementation deadlocks, but this may
    /// be changed to a panic in the future.
    pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
    where
        F: FnOnce() -> T,
        G: FnOnce(Pin<&mut T>),
    {
        if let Some(value) = self.get_ref().get() {
            // SAFETY: The inner value was already initialized, and will not be
            // moved anymore.
            return unsafe { Pin::new_unchecked(value) };
        }

        let slot = &self.value;

        // Ignore poisoning from other threads
        // If another thread panics, then we'll be able to run our closure
        self.once.call_once_force(|_| {
            let value = f();
            // SAFETY: We use the Once (self.once) to guarantee unique access
            // to the UnsafeCell (slot).
            let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
            // SAFETY: The value has been written to its final place in
            // self.value. We do not to move it anymore, which we promise here
            // with a Pin<&mut T>.
            g(unsafe { Pin::new_unchecked(value) });
        });

        // SAFETY: The inner value has been initialized, and will not be moved
        // anymore.
        unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
    }

    /// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
    /// `None` if the cell was empty.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// let cell: SyncOnceCell<String> = SyncOnceCell::new();
    /// assert_eq!(cell.into_inner(), None);
    ///
    /// let cell = SyncOnceCell::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(mut self) -> Option<T> {
        self.take()
    }

    /// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
    ///
    /// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.
    ///
    /// Safety is guaranteed by requiring a mutable reference.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
    /// assert_eq!(cell.take(), None);
    ///
    /// let mut cell = SyncOnceCell::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> {
        if self.is_initialized() {
            self.once = Once::new();
            // SAFETY: `self.value` is initialized and contains a valid `T`.
            // `self.once` is reset, so `is_initialized()` will be false again
            // which prevents the value from being read twice.
            unsafe { Some((&mut *self.value.get()).assume_init_read()) }
        } else {
            None
        }
    }

    #[inline]
    fn is_initialized(&self) -> bool {
        self.once.is_completed()
    }

    #[cold]
    fn initialize<F, E>(&self, f: F) -> Result<(), E>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let mut res: Result<(), E> = Ok(());
        let slot = &self.value;

        // Ignore poisoning from other threads
        // If another thread panics, then we'll be able to run our closure
        self.once.call_once_force(|p| {
            match f() {
                Ok(value) => {
                    unsafe { (&mut *slot.get()).write(value) };
                }
                Err(e) => {
                    res = Err(e);

                    // Treat the underlying `Once` as poisoned since we
                    // failed to initialize our value. Calls
                    p.poison();
                }
            }
        });
        res
    }

    /// # Safety
    ///
    /// The value must be initialized
    unsafe fn get_unchecked(&self) -> &T {
        debug_assert!(self.is_initialized());
        (&*self.value.get()).assume_init_ref()
    }

    /// # Safety
    ///
    /// The value must be initialized
    unsafe fn get_unchecked_mut(&mut self) -> &mut T {
        debug_assert!(self.is_initialized());
        (&mut *self.value.get()).assume_init_mut()
    }
}

unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
    fn drop(&mut self) {
        if self.is_initialized() {
            // SAFETY: The cell is initialized and being dropped, so it can't
            // be accessed again. We also don't touch the `T` other than
            // dropping it, which validates our usage of #[may_dangle].
            unsafe { (&mut *self.value.get()).assume_init_drop() };
        }
    }
}

/// A value which is initialized on the first access.
///
/// This type is a thread-safe `Lazy`, and can be used in statics.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::collections::HashMap;
///
/// use std::lazy::SyncLazy;
///
/// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| {
///     println!("initializing");
///     let mut m = HashMap::new();
///     m.insert(13, "Spica".to_string());
///     m.insert(74, "Hoyten".to_string());
///     m
/// });
///
/// fn main() {
///     println!("ready");
///     std::thread::spawn(|| {
///         println!("{:?}", HASHMAP.get(&13));
///     }).join().unwrap();
///     println!("{:?}", HASHMAP.get(&74));
///
///     // Prints:
///     //   ready
///     //   initializing
///     //   Some("Spica")
///     //   Some("Hoyten")
/// }
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct SyncLazy<T, F = fn() -> T> {
    cell: SyncOnceCell<T>,
    init: Cell<Option<F>>,
}

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

// We never create a `&F` from a `&SyncLazy<T, F>` so it is fine
// to not impl `Sync` for `F`
// we do create a `&mut Option<F>` in `force`, but this is
// properly synchronized, so it only happens once
// so it also does not contribute to this impl.
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {}
// auto-derived `Send` impl is OK.

#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: UnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: UnwindSafe> UnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: UnwindSafe {}

impl<T, F> SyncLazy<T, F> {
    /// Creates a new lazy value with the given initializing
    /// function.
    #[unstable(feature = "once_cell", issue = "74465")]
    pub const fn new(f: F) -> SyncLazy<T, F> {
        SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) }
    }
}

impl<T, F: FnOnce() -> T> SyncLazy<T, F> {
    /// Forces the evaluation of this lazy value and
    /// returns a reference to result. This is equivalent
    /// to the `Deref` impl, but is explicit.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncLazy;
    ///
    /// let lazy = SyncLazy::new(|| 92);
    ///
    /// assert_eq!(SyncLazy::force(&lazy), &92);
    /// assert_eq!(&*lazy, &92);
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn force(this: &SyncLazy<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 SyncLazy<T, F> {
    type Target = T;
    fn deref(&self) -> &T {
        SyncLazy::force(self)
    }
}

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