[rustc.git] / library / core / src / cell.rs

//! Shareable mutable containers.
//!
//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
//! have one of the following:
//!
//! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
//! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
//!
//! This is enforced by the Rust compiler. However, there are situations where this rule is not
//! flexible enough. Sometimes it is required to have multiple references to an object and yet
//! mutate it.
//!
//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
//! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded
//! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
//! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to
//! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types.
//!
//! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
//! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
//! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
//! with typical Rust types that exhibit 'inherited mutability'.
//!
//! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
//! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
//! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
//! methods to retrieve and change the current interior value:
//!
//!  - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
//!    interior value.
//!  - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
//!    interior value with [`Default::default()`] and returns the replaced value.
//!  - For all types, the [`replace`](Cell::replace) method replaces the current interior value and
//!    returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the
//!    `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method
//!    replaces the interior value, dropping the replaced value.
//!
//! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
//! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
//! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
//! to borrow a value that is already mutably borrowed; when this happens it results in thread
//! panic.
//!
//! # When to choose interior mutability
//!
//! The more common inherited mutability, where one must have unique access to mutate a value, is
//! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
//! interior mutability is something of a last resort. Since cell types enable mutation where it
//! would otherwise be disallowed though, there are occasions when interior mutability might be
//! appropriate, or even *must* be used, e.g.
//!
//! * Introducing mutability 'inside' of something immutable
//! * Implementation details of logically-immutable methods.
//! * Mutating implementations of [`Clone`].
//!
//! ## Introducing mutability 'inside' of something immutable
//!
//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
//! be cloned and shared between multiple parties. Because the contained values may be
//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
//! impossible to mutate data inside of these smart pointers at all.
//!
//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
//! mutability:
//!
//! ```
//! use std::cell::{RefCell, RefMut};
//! use std::collections::HashMap;
//! use std::rc::Rc;
//!
//! fn main() {
//!     let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
//!     // Create a new block to limit the scope of the dynamic borrow
//!     {
//!         let mut map: RefMut<_> = shared_map.borrow_mut();
//!         map.insert("africa", 92388);
//!         map.insert("kyoto", 11837);
//!         map.insert("piccadilly", 11826);
//!         map.insert("marbles", 38);
//!     }
//!
//!     // Note that if we had not let the previous borrow of the cache fall out
//!     // of scope then the subsequent borrow would cause a dynamic thread panic.
//!     // This is the major hazard of using `RefCell`.
//!     let total: i32 = shared_map.borrow().values().sum();
//!     println!("{}", total);
//! }
//! ```
//!
//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
//! multi-threaded situation.
//!
//! ## Implementation details of logically-immutable methods
//!
//! Occasionally it may be desirable not to expose in an API that there is mutation happening
//! "under the hood". This may be because logically the operation is immutable, but e.g., caching
//! forces the implementation to perform mutation; or because you must employ mutation to implement
//! a trait method that was originally defined to take `&self`.
//!
//! ```
//! # #![allow(dead_code)]
//! use std::cell::RefCell;
//!
//! struct Graph {
//!     edges: Vec<(i32, i32)>,
//!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
//! }
//!
//! impl Graph {
//!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
//!         self.span_tree_cache.borrow_mut()
//!             .get_or_insert_with(|| self.calc_span_tree())
//!             .clone()
//!     }
//!
//!     fn calc_span_tree(&self) -> Vec<(i32, i32)> {
//!         // Expensive computation goes here
//!         vec![]
//!     }
//! }
//! ```
//!
//! ## Mutating implementations of `Clone`
//!
//! This is simply a special - but common - case of the previous: hiding mutability for operations
//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
//! reference counts within a `Cell<T>`.
//!
//! ```
//! use std::cell::Cell;
//! use std::ptr::NonNull;
//! use std::process::abort;
//! use std::marker::PhantomData;
//!
//! struct Rc<T: ?Sized> {
//!     ptr: NonNull<RcBox<T>>,
//!     phantom: PhantomData<RcBox<T>>,
//! }
//!
//! struct RcBox<T: ?Sized> {
//!     strong: Cell<usize>,
//!     refcount: Cell<usize>,
//!     value: T,
//! }
//!
//! impl<T: ?Sized> Clone for Rc<T> {
//!     fn clone(&self) -> Rc<T> {
//!         self.inc_strong();
//!         Rc {
//!             ptr: self.ptr,
//!             phantom: PhantomData,
//!         }
//!     }
//! }
//!
//! trait RcBoxPtr<T: ?Sized> {
//!
//!     fn inner(&self) -> &RcBox<T>;
//!
//!     fn strong(&self) -> usize {
//!         self.inner().strong.get()
//!     }
//!
//!     fn inc_strong(&self) {
//!         self.inner()
//!             .strong
//!             .set(self.strong()
//!                      .checked_add(1)
//!                      .unwrap_or_else(|| abort() ));
//!     }
//! }
//!
//! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
//!    fn inner(&self) -> &RcBox<T> {
//!        unsafe {
//!            self.ptr.as_ref()
//!        }
//!    }
//! }
//! ```
//!
//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
//! [`atomic`]: crate::sync::atomic

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

use crate::cmp::Ordering;
use crate::fmt::{self, Debug, Display};
use crate::marker::Unsize;
use crate::mem;
use crate::ops::{CoerceUnsized, Deref, DerefMut};
use crate::ptr;

/// A mutable memory location.
///
/// # Examples
///
/// In this example, you can see that `Cell<T>` enables mutation inside an
/// immutable struct. In other words, it enables "interior mutability".
///
/// ```
/// use std::cell::Cell;
///
/// struct SomeStruct {
///     regular_field: u8,
///     special_field: Cell<u8>,
/// }
///
/// let my_struct = SomeStruct {
///     regular_field: 0,
///     special_field: Cell::new(1),
/// };
///
/// let new_value = 100;
///
/// // ERROR: `my_struct` is immutable
/// // my_struct.regular_field = new_value;
///
/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
/// // which can always be mutated
/// my_struct.special_field.set(new_value);
/// assert_eq!(my_struct.special_field.get(), new_value);
/// ```
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
pub struct Cell<T: ?Sized> {
    value: UnsafeCell<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for Cell<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Copy> Clone for Cell<T> {
    #[inline]
    fn clone(&self) -> Cell<T> {
        Cell::new(self.get())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Cell<T> {
    /// Creates a `Cell<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> Cell<T> {
        Cell::new(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialEq + Copy> PartialEq for Cell<T> {
    #[inline]
    fn eq(&self, other: &Cell<T>) -> bool {
        self.get() == other.get()
    }
}

#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: Eq + Copy> Eq for Cell<T> {}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
    #[inline]
    fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
        self.get().partial_cmp(&other.get())
    }

    #[inline]
    fn lt(&self, other: &Cell<T>) -> bool {
        self.get() < other.get()
    }

    #[inline]
    fn le(&self, other: &Cell<T>) -> bool {
        self.get() <= other.get()
    }

    #[inline]
    fn gt(&self, other: &Cell<T>) -> bool {
        self.get() > other.get()
    }

    #[inline]
    fn ge(&self, other: &Cell<T>) -> bool {
        self.get() >= other.get()
    }
}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: Ord + Copy> Ord for Cell<T> {
    #[inline]
    fn cmp(&self, other: &Cell<T>) -> Ordering {
        self.get().cmp(&other.get())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for Cell<T> {
    fn from(t: T) -> Cell<T> {
        Cell::new(t)
    }
}

impl<T> Cell<T> {
    /// Creates a new `Cell` containing the given value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
    #[inline]
    pub const fn new(value: T) -> Cell<T> {
        Cell { value: UnsafeCell::new(value) }
    }

    /// Sets the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// c.set(10);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn set(&self, val: T) {
        let old = self.replace(val);
        drop(old);
    }

    /// Swaps the values of two Cells.
    /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c1 = Cell::new(5i32);
    /// let c2 = Cell::new(10i32);
    /// c1.swap(&c2);
    /// assert_eq!(10, c1.get());
    /// assert_eq!(5, c2.get());
    /// ```
    #[inline]
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn swap(&self, other: &Self) {
        if ptr::eq(self, other) {
            return;
        }
        // SAFETY: This can be risky if called from separate threads, but `Cell`
        // is `!Sync` so this won't happen. This also won't invalidate any
        // pointers since `Cell` makes sure nothing else will be pointing into
        // either of these `Cell`s.
        unsafe {
            ptr::swap(self.value.get(), other.value.get());
        }
    }

    /// Replaces the contained value with `val`, and returns the old contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let cell = Cell::new(5);
    /// assert_eq!(cell.get(), 5);
    /// assert_eq!(cell.replace(10), 5);
    /// assert_eq!(cell.get(), 10);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn replace(&self, val: T) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        mem::replace(unsafe { &mut *self.value.get() }, val)
    }

    /// Unwraps the value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let five = c.into_inner();
    ///
    /// assert_eq!(five, 5);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    pub const fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

impl<T: Copy> Cell<T> {
    /// Returns a copy of the contained value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// let five = c.get();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn get(&self) -> T {
        // SAFETY: This can cause data races if called from a separate thread,
        // but `Cell` is `!Sync` so this won't happen.
        unsafe { *self.value.get() }
    }

    /// Updates the contained value using a function and returns the new value.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_update)]
    ///
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let new = c.update(|x| x + 1);
    ///
    /// assert_eq!(new, 6);
    /// assert_eq!(c.get(), 6);
    /// ```
    #[inline]
    #[unstable(feature = "cell_update", issue = "50186")]
    pub fn update<F>(&self, f: F) -> T
    where
        F: FnOnce(T) -> T,
    {
        let old = self.get();
        let new = f(old);
        self.set(new);
        new
    }
}

impl<T: ?Sized> Cell<T> {
    /// Returns a raw pointer to the underlying data in this cell.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    ///
    /// let ptr = c.as_ptr();
    /// ```
    #[inline]
    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
    #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
    pub const fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows `Cell` mutably (at compile-time) which guarantees
    /// that we possess the only reference.
    ///
    /// However be cautious: this method expects `self` to be mutable, which is
    /// generally not the case when using a `Cell`. If you require interior
    /// mutability by reference, consider using `RefCell` which provides
    /// run-time checked mutable borrows through its [`borrow_mut`] method.
    ///
    /// [`borrow_mut`]: RefCell::borrow_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let mut c = Cell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(c.get(), 6);
    /// ```
    #[inline]
    #[stable(feature = "cell_get_mut", since = "1.11.0")]
    pub fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    /// Returns a `&Cell<T>` from a `&mut T`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let slice: &mut [i32] = &mut [1, 2, 3];
    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
    ///
    /// assert_eq!(slice_cell.len(), 3);
    /// ```
    #[inline]
    #[stable(feature = "as_cell", since = "1.37.0")]
    pub fn from_mut(t: &mut T) -> &Cell<T> {
        // SAFETY: `&mut` ensures unique access.
        unsafe { &*(t as *mut T as *const Cell<T>) }
    }
}

impl<T: Default> Cell<T> {
    /// Takes the value of the cell, leaving `Default::default()` in its place.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let c = Cell::new(5);
    /// let five = c.take();
    ///
    /// assert_eq!(five, 5);
    /// assert_eq!(c.into_inner(), 0);
    /// ```
    #[stable(feature = "move_cell", since = "1.17.0")]
    pub fn take(&self) -> T {
        self.replace(Default::default())
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}

impl<T> Cell<[T]> {
    /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// let slice: &mut [i32] = &mut [1, 2, 3];
    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
    ///
    /// assert_eq!(slice_cell.len(), 3);
    /// ```
    #[stable(feature = "as_cell", since = "1.37.0")]
    pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
        // SAFETY: `Cell<T>` has the same memory layout as `T`.
        unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
    }
}

/// A mutable memory location with dynamically checked borrow rules
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowFlag>,
    // Stores the location of the earliest currently active borrow.
    // This gets updated whenver we go from having zero borrows
    // to having a single borrow. When a borrow occurs, this gets included
    // in the generated `BorrowError/`BorrowMutError`
    #[cfg(feature = "debug_refcell")]
    borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
    value: UnsafeCell<T>,
}

/// An error returned by [`RefCell::try_borrow`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowError {
    #[cfg(feature = "debug_refcell")]
    location: &'static crate::panic::Location<'static>,
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut builder = f.debug_struct("BorrowError");

        #[cfg(feature = "debug_refcell")]
        builder.field("location", self.location);

        builder.finish()
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Display::fmt("already mutably borrowed", f)
    }
}

/// An error returned by [`RefCell::try_borrow_mut`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowMutError {
    #[cfg(feature = "debug_refcell")]
    location: &'static crate::panic::Location<'static>,
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut builder = f.debug_struct("BorrowMutError");

        #[cfg(feature = "debug_refcell")]
        builder.field("location", self.location);

        builder.finish()
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Display::fmt("already borrowed", f)
    }
}

// Positive values represent the number of `Ref` active. Negative values
// represent the number of `RefMut` active. Multiple `RefMut`s can only be
// active at a time if they refer to distinct, nonoverlapping components of a
// `RefCell` (e.g., different ranges of a slice).
//
// `Ref` and `RefMut` are both two words in size, and so there will likely never
// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
// range. Thus, a `BorrowFlag` will probably never overflow or underflow.
// However, this is not a guarantee, as a pathological program could repeatedly
// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
// explicitly check for overflow and underflow in order to avoid unsafety, or at
// least behave correctly in the event that overflow or underflow happens (e.g.,
// see BorrowRef::new).
type BorrowFlag = isize;
const UNUSED: BorrowFlag = 0;

#[inline(always)]
fn is_writing(x: BorrowFlag) -> bool {
    x < UNUSED
}

#[inline(always)]
fn is_reading(x: BorrowFlag) -> bool {
    x > UNUSED
}

impl<T> RefCell<T> {
    /// Creates a new `RefCell` containing `value`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
    #[inline]
    pub const fn new(value: T) -> RefCell<T> {
        RefCell {
            value: UnsafeCell::new(value),
            borrow: Cell::new(UNUSED),
            #[cfg(feature = "debug_refcell")]
            borrowed_at: Cell::new(None),
        }
    }

    /// Consumes the `RefCell`, returning the wrapped value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let five = c.into_inner();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    #[inline]
    pub const fn into_inner(self) -> T {
        // Since this function takes `self` (the `RefCell`) by value, the
        // compiler statically verifies that it is not currently borrowed.
        self.value.into_inner()
    }

    /// Replaces the wrapped value with a new one, returning the old value,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace(6);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "refcell_replace", since = "1.24.0")]
    #[track_caller]
    pub fn replace(&self, t: T) -> T {
        mem::replace(&mut *self.borrow_mut(), t)
    }

    /// Replaces the wrapped value with a new one computed from `f`, returning
    /// the old value, without deinitializing either one.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let cell = RefCell::new(5);
    /// let old_value = cell.replace_with(|&mut old| old + 1);
    /// assert_eq!(old_value, 5);
    /// assert_eq!(cell, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
    #[track_caller]
    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
        let mut_borrow = &mut *self.borrow_mut();
        let replacement = f(mut_borrow);
        mem::replace(mut_borrow, replacement)
    }

    /// Swaps the wrapped value of `self` with the wrapped value of `other`,
    /// without deinitializing either one.
    ///
    /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
    ///
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    /// let c = RefCell::new(5);
    /// let d = RefCell::new(6);
    /// c.swap(&d);
    /// assert_eq!(c, RefCell::new(6));
    /// assert_eq!(d, RefCell::new(5));
    /// ```
    #[inline]
    #[stable(feature = "refcell_swap", since = "1.24.0")]
    pub fn swap(&self, other: &Self) {
        mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
    }
}

impl<T: ?Sized> RefCell<T> {
    /// Immutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple
    /// immutable borrows can be taken out at the same time.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
    /// [`try_borrow`](#method.try_borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let borrowed_five = c.borrow();
    /// let borrowed_five2 = c.borrow();
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```should_panic
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let m = c.borrow_mut();
    /// let b = c.borrow(); // this causes a panic
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    #[track_caller]
    pub fn borrow(&self) -> Ref<'_, T> {
        self.try_borrow().expect("already mutably borrowed")
    }

    /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
    /// borrowed.
    ///
    /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
    /// taken out at the same time.
    ///
    /// This is the non-panicking variant of [`borrow`](#method.borrow).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow_mut();
    ///     assert!(c.try_borrow().is_err());
    /// }
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow().is_ok());
    /// }
    /// ```
    #[stable(feature = "try_borrow", since = "1.13.0")]
    #[inline]
    #[cfg_attr(feature = "debug_refcell", track_caller)]
    pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
        match BorrowRef::new(&self.borrow) {
            Some(b) => {
                #[cfg(feature = "debug_refcell")]
                {
                    // `borrowed_at` is always the *first* active borrow
                    if b.borrow.get() == 1 {
                        self.borrowed_at.set(Some(crate::panic::Location::caller()));
                    }
                }

                // SAFETY: `BorrowRef` ensures that there is only immutable access
                // to the value while borrowed.
                Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
            }
            None => Err(BorrowError {
                // If a borrow occured, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            }),
        }
    }

    /// Mutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed. For a non-panicking variant, use
    /// [`try_borrow_mut`](#method.try_borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new("hello".to_owned());
    ///
    /// *c.borrow_mut() = "bonjour".to_owned();
    ///
    /// assert_eq!(&*c.borrow(), "bonjour");
    /// ```
    ///
    /// An example of panic:
    ///
    /// ```should_panic
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// let m = c.borrow();
    ///
    /// let b = c.borrow_mut(); // this causes a panic
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[inline]
    #[track_caller]
    pub fn borrow_mut(&self) -> RefMut<'_, T> {
        self.try_borrow_mut().expect("already borrowed")
    }

    /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
    ///
    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
    /// from it exit scope. The value cannot be borrowed while this borrow is
    /// active.
    ///
    /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(c.try_borrow_mut().is_err());
    /// }
    ///
    /// assert!(c.try_borrow_mut().is_ok());
    /// ```
    #[stable(feature = "try_borrow", since = "1.13.0")]
    #[inline]
    #[cfg_attr(feature = "debug_refcell", track_caller)]
    pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
        match BorrowRefMut::new(&self.borrow) {
            Some(b) => {
                #[cfg(feature = "debug_refcell")]
                {
                    self.borrowed_at.set(Some(crate::panic::Location::caller()));
                }

                // SAFETY: `BorrowRef` guarantees unique access.
                Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
            }
            None => Err(BorrowMutError {
                // If a borrow occured, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            }),
        }
    }

    /// Returns a raw pointer to the underlying data in this cell.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// let ptr = c.as_ptr();
    /// ```
    #[inline]
    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
    pub fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows `RefCell` mutably (at compile-time) so there is no
    /// need for dynamic checks.
    ///
    /// However be cautious: this method expects `self` to be mutable, which is
    /// generally not the case when using a `RefCell`. Take a look at the
    /// [`borrow_mut`] method instead if `self` isn't mutable.
    ///
    /// Also, please be aware that this method is only for special circumstances and is usually
    /// not what you want. In case of doubt, use [`borrow_mut`] instead.
    ///
    /// [`borrow_mut`]: RefCell::borrow_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let mut c = RefCell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(c, RefCell::new(6));
    /// ```
    #[inline]
    #[stable(feature = "cell_get_mut", since = "1.11.0")]
    pub fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
    ///
    /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
    /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
    /// if some `Ref` or `RefMut` borrows have been leaked.
    ///
    /// [`get_mut`]: RefCell::get_mut()
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::RefCell;
    ///
    /// let mut c = RefCell::new(0);
    /// std::mem::forget(c.borrow_mut());
    ///
    /// assert!(c.try_borrow().is_err());
    /// c.undo_leak();
    /// assert!(c.try_borrow().is_ok());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn undo_leak(&mut self) -> &mut T {
        *self.borrow.get_mut() = UNUSED;
        self.get_mut()
    }

    /// Immutably borrows the wrapped value, returning an error if the value is
    /// currently mutably borrowed.
    ///
    /// # Safety
    ///
    /// Unlike `RefCell::borrow`, this method is unsafe because it does not
    /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
    /// borrowing the `RefCell` while the reference returned by this method
    /// is alive is undefined behaviour.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    ///
    /// {
    ///     let m = c.borrow_mut();
    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_err());
    /// }
    ///
    /// {
    ///     let m = c.borrow();
    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
    /// }
    /// ```
    #[stable(feature = "borrow_state", since = "1.37.0")]
    #[inline]
    pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
        if !is_writing(self.borrow.get()) {
            // SAFETY: We check that nobody is actively writing now, but it is
            // the caller's responsibility to ensure that nobody writes until
            // the returned reference is no longer in use.
            // Also, `self.value.get()` refers to the value owned by `self`
            // and is thus guaranteed to be valid for the lifetime of `self`.
            Ok(unsafe { &*self.value.get() })
        } else {
            Err(BorrowError {
                // If a borrow occured, then we must already have an outstanding borrow,
                // so `borrowed_at` will be `Some`
                #[cfg(feature = "debug_refcell")]
                location: self.borrowed_at.get().unwrap(),
            })
        }
    }
}

impl<T: Default> RefCell<T> {
    /// Takes the wrapped value, leaving `Default::default()` in its place.
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let c = RefCell::new(5);
    /// let five = c.take();
    ///
    /// assert_eq!(five, 5);
    /// assert_eq!(c.into_inner(), 0);
    /// ```
    #[stable(feature = "refcell_take", since = "1.50.0")]
    pub fn take(&self) -> T {
        self.replace(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for RefCell<T> {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> Clone for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value is currently mutably borrowed.
    #[inline]
    #[track_caller]
    fn clone(&self) -> RefCell<T> {
        RefCell::new(self.borrow().clone())
    }

    /// # Panics
    ///
    /// Panics if `other` is currently mutably borrowed.
    #[inline]
    #[track_caller]
    fn clone_from(&mut self, other: &Self) {
        self.get_mut().clone_from(&other.borrow())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for RefCell<T> {
    /// Creates a `RefCell<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> RefCell<T> {
        RefCell::new(Default::default())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn eq(&self, other: &RefCell<T>) -> bool {
        *self.borrow() == *other.borrow()
    }
}

#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: ?Sized + Eq> Eq for RefCell<T> {}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
        self.borrow().partial_cmp(&*other.borrow())
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn lt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() < *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn le(&self, other: &RefCell<T>) -> bool {
        *self.borrow() <= *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn gt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() > *other.borrow()
    }

    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn ge(&self, other: &RefCell<T>) -> bool {
        *self.borrow() >= *other.borrow()
    }
}

#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + Ord> Ord for RefCell<T> {
    /// # Panics
    ///
    /// Panics if the value in either `RefCell` is currently borrowed.
    #[inline]
    fn cmp(&self, other: &RefCell<T>) -> Ordering {
        self.borrow().cmp(&*other.borrow())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for RefCell<T> {
    fn from(t: T) -> RefCell<T> {
        RefCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}

struct BorrowRef<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl<'b> BorrowRef<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
        let b = borrow.get().wrapping_add(1);
        if !is_reading(b) {
            // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
            // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
            //    due to Rust's reference aliasing rules
            // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
            //    into isize::MIN (the max amount of writing borrows) so we can't allow
            //    an additional read borrow because isize can't represent so many read borrows
            //    (this can only happen if you mem::forget more than a small constant amount of
            //    `Ref`s, which is not good practice)
            None
        } else {
            // Incrementing borrow can result in a reading value (> 0) in these cases:
            // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
            // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
            //    is large enough to represent having one more read borrow
            borrow.set(b);
            Some(BorrowRef { borrow })
        }
    }
}

impl Drop for BorrowRef<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        self.borrow.set(borrow - 1);
    }
}

impl Clone for BorrowRef<'_> {
    #[inline]
    fn clone(&self) -> Self {
        // Since this Ref exists, we know the borrow flag
        // is a reading borrow.
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        // Prevent the borrow counter from overflowing into
        // a writing borrow.
        assert!(borrow != isize::MAX);
        self.borrow.set(borrow + 1);
        BorrowRef { borrow: self.borrow }
    }
}

/// Wraps a borrowed reference to a value in a `RefCell` box.
/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Ref<'b, T: ?Sized + 'b> {
    value: &'b T,
    borrow: BorrowRef<'b>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Ref<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        self.value
    }
}

impl<'b, T: ?Sized> Ref<'b, T> {
    /// Copies a `Ref`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
    /// with the widespread use of `r.borrow().clone()` to clone the contents of
    /// a `RefCell`.
    #[stable(feature = "cell_extras", since = "1.15.0")]
    #[inline]
    pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
        Ref { value: orig.value, borrow: orig.borrow.clone() }
    }

    /// Makes a new `Ref` for a component of the borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as `Ref::map(...)`.
    /// A method would interfere with methods of the same name on the contents
    /// of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, Ref};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// let b1: Ref<(u32, char)> = c.borrow();
    /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
    /// assert_eq!(*b2, 5)
    /// ```
    #[stable(feature = "cell_map", since = "1.8.0")]
    #[inline]
    pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
    where
        F: FnOnce(&T) -> &U,
    {
        Ref { value: f(orig.value), borrow: orig.borrow }
    }

    /// Makes a new `Ref` for an optional component of the borrowed data. The
    /// original guard is returned as an `Err(..)` if the closure returns
    /// `None`.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::filter_map(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_filter_map)]
    ///
    /// use std::cell::{RefCell, Ref};
    ///
    /// let c = RefCell::new(vec![1, 2, 3]);
    /// let b1: Ref<Vec<u32>> = c.borrow();
    /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
    /// assert_eq!(*b2.unwrap(), 2);
    /// ```
    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
    #[inline]
    pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
    where
        F: FnOnce(&T) -> Option<&U>,
    {
        match f(orig.value) {
            Some(value) => Ok(Ref { value, borrow: orig.borrow }),
            None => Err(orig),
        }
    }

    /// Splits a `Ref` into multiple `Ref`s for different components of the
    /// borrowed data.
    ///
    /// The `RefCell` is already immutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::map_split(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{Ref, RefCell};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow();
    /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// ```
    #[stable(feature = "refcell_map_split", since = "1.35.0")]
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
    where
        F: FnOnce(&T) -> (&U, &V),
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
    }

    /// Convert into a reference to the underlying data.
    ///
    /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
    /// already immutably borrowed. It is not a good idea to leak more than a constant number of
    /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
    /// have occurred in total.
    ///
    /// This is an associated function that needs to be used as
    /// `Ref::leak(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::{RefCell, Ref};
    /// let cell = RefCell::new(0);
    ///
    /// let value = Ref::leak(cell.borrow());
    /// assert_eq!(*value, 0);
    ///
    /// assert!(cell.try_borrow().is_ok());
    /// assert!(cell.try_borrow_mut().is_err());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn leak(orig: Ref<'b, T>) -> &'b T {
        // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
        // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
        // unique reference to the borrowed RefCell. No further mutable references can be created
        // from the original cell.
        mem::forget(orig.borrow);
        orig.value
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.value.fmt(f)
    }
}

impl<'b, T: ?Sized> RefMut<'b, T> {
    /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
    /// variant.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map(...)`. A method would interfere with methods of the same
    /// name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let c = RefCell::new((5, 'b'));
    /// {
    ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
    ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
    ///     assert_eq!(*b2, 5);
    ///     *b2 = 42;
    /// }
    /// assert_eq!(*c.borrow(), (42, 'b'));
    /// ```
    #[stable(feature = "cell_map", since = "1.8.0")]
    #[inline]
    pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
    where
        F: FnOnce(&mut T) -> &mut U,
    {
        // FIXME(nll-rfc#40): fix borrow-check
        let RefMut { value, borrow } = orig;
        RefMut { value: f(value), borrow }
    }

    /// Makes a new `RefMut` for an optional component of the borrowed data. The
    /// original guard is returned as an `Err(..)` if the closure returns
    /// `None`.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::filter_map(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_filter_map)]
    ///
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let c = RefCell::new(vec![1, 2, 3]);
    ///
    /// {
    ///     let b1: RefMut<Vec<u32>> = c.borrow_mut();
    ///     let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
    ///
    ///     if let Ok(mut b2) = b2 {
    ///         *b2 += 2;
    ///     }
    /// }
    ///
    /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
    /// ```
    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
    #[inline]
    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
    {
        // FIXME(nll-rfc#40): fix borrow-check
        let RefMut { value, borrow } = orig;
        let value = value as *mut T;
        // SAFETY: function holds onto an exclusive reference for the duration
        // of its call through `orig`, and the pointer is only de-referenced
        // inside of the function call never allowing the exclusive reference to
        // escape.
        match f(unsafe { &mut *value }) {
            Some(value) => Ok(RefMut { value, borrow }),
            None => {
                // SAFETY: same as above.
                Err(RefMut { value: unsafe { &mut *value }, borrow })
            }
        }
    }

    /// Splits a `RefMut` into multiple `RefMut`s for different components of the
    /// borrowed data.
    ///
    /// The underlying `RefCell` will remain mutably borrowed until both
    /// returned `RefMut`s go out of scope.
    ///
    /// The `RefCell` is already mutably borrowed, so this cannot fail.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::map_split(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::{RefCell, RefMut};
    ///
    /// let cell = RefCell::new([1, 2, 3, 4]);
    /// let borrow = cell.borrow_mut();
    /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
    /// assert_eq!(*begin, [1, 2]);
    /// assert_eq!(*end, [3, 4]);
    /// begin.copy_from_slice(&[4, 3]);
    /// end.copy_from_slice(&[2, 1]);
    /// ```
    #[stable(feature = "refcell_map_split", since = "1.35.0")]
    #[inline]
    pub fn map_split<U: ?Sized, V: ?Sized, F>(
        orig: RefMut<'b, T>,
        f: F,
    ) -> (RefMut<'b, U>, RefMut<'b, V>)
    where
        F: FnOnce(&mut T) -> (&mut U, &mut V),
    {
        let (a, b) = f(orig.value);
        let borrow = orig.borrow.clone();
        (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
    }

    /// Convert into a mutable reference to the underlying data.
    ///
    /// The underlying `RefCell` can not be borrowed from again and will always appear already
    /// mutably borrowed, making the returned reference the only to the interior.
    ///
    /// This is an associated function that needs to be used as
    /// `RefMut::leak(...)`. A method would interfere with methods of the
    /// same name on the contents of a `RefCell` used through `Deref`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(cell_leak)]
    /// use std::cell::{RefCell, RefMut};
    /// let cell = RefCell::new(0);
    ///
    /// let value = RefMut::leak(cell.borrow_mut());
    /// assert_eq!(*value, 0);
    /// *value = 1;
    ///
    /// assert!(cell.try_borrow_mut().is_err());
    /// ```
    #[unstable(feature = "cell_leak", issue = "69099")]
    pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
        // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
        // require a unique reference to the borrowed RefCell. No further references can be created
        // from the original cell within that lifetime, making the current borrow the only
        // reference for the remaining lifetime.
        mem::forget(orig.borrow);
        orig.value
    }
}

struct BorrowRefMut<'b> {
    borrow: &'b Cell<BorrowFlag>,
}

impl Drop for BorrowRefMut<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        self.borrow.set(borrow + 1);
    }
}

impl<'b> BorrowRefMut<'b> {
    #[inline]
    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
        // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
        // mutable reference, and so there must currently be no existing
        // references. Thus, while clone increments the mutable refcount, here
        // we explicitly only allow going from UNUSED to UNUSED - 1.
        match borrow.get() {
            UNUSED => {
                borrow.set(UNUSED - 1);
                Some(BorrowRefMut { borrow })
            }
            _ => None,
        }
    }

    // Clones a `BorrowRefMut`.
    //
    // This is only valid if each `BorrowRefMut` is used to track a mutable
    // reference to a distinct, nonoverlapping range of the original object.
    // This isn't in a Clone impl so that code doesn't call this implicitly.
    #[inline]
    fn clone(&self) -> BorrowRefMut<'b> {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        // Prevent the borrow counter from underflowing.
        assert!(borrow != isize::MIN);
        self.borrow.set(borrow - 1);
        BorrowRefMut { borrow: self.borrow }
    }
}

/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RefMut<'b, T: ?Sized + 'b> {
    value: &'b mut T,
    borrow: BorrowRefMut<'b>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RefMut<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        self.value
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RefMut<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        self.value
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.value.fmt(f)
    }
}

/// The core primitive for interior mutability in Rust.
///
/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
/// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
///
/// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
/// use `UnsafeCell` to wrap their data.
///
/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
/// aliasing `&mut`, not even with `UnsafeCell<T>`.
///
/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
/// correctly.
///
/// [`.get()`]: `UnsafeCell::get`
///
/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
///
/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
/// reference) that is accessible by safe code (for example, because you returned it),
/// then you must not access the data in any way that contradicts that reference for the
/// remainder of `'a`. For example, this means that if you take the `*mut T` from an
/// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
/// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
/// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
/// safe code, then you must not access the data within the `UnsafeCell` until that
/// reference expires.
///
/// - At all times, you must avoid data races. If multiple threads have access to
/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
/// accesses (or use atomics).
///
/// To assist with proper design, the following scenarios are explicitly declared legal
/// for single-threaded code:
///
/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
/// references, but not with a `&mut T`
///
/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
/// co-exist with it. A `&mut T` must always be unique.
///
/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
/// `&UnsafeCell<T>` references alias the cell) is
/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
/// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
/// may be aliased for the duration of that `&mut` borrow.
/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
/// a `&mut T`.
///
/// [`.get_mut()`]: `UnsafeCell::get_mut`
///
/// # Examples
///
/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
/// there being multiple references aliasing the cell:
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let x: UnsafeCell<i32> = 42.into();
/// // Get multiple / concurrent / shared references to the same `x`.
/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
///
/// unsafe {
///     // SAFETY: within this scope there are no other references to `x`'s contents,
///     // so ours is effectively unique.
///     let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
///     *p1_exclusive += 27; //                                     |
/// } // <---------- cannot go beyond this point -------------------+
///
/// unsafe {
///     // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
///     // so we can have multiple shared accesses concurrently.
///     let p2_shared: &i32 = &*p2.get();
///     assert_eq!(*p2_shared, 42 + 27);
///     let p1_shared: &i32 = &*p1.get();
///     assert_eq!(*p1_shared, *p2_shared);
/// }
/// ```
///
/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
/// implies exclusive access to its `T`:
///
/// ```rust
/// #![forbid(unsafe_code)] // with exclusive accesses,
///                         // `UnsafeCell` is a transparent no-op wrapper,
///                         // so no need for `unsafe` here.
/// use std::cell::UnsafeCell;
///
/// let mut x: UnsafeCell<i32> = 42.into();
///
/// // Get a compile-time-checked unique reference to `x`.
/// let p_unique: &mut UnsafeCell<i32> = &mut x;
/// // With an exclusive reference, we can mutate the contents for free.
/// *p_unique.get_mut() = 0;
/// // Or, equivalently:
/// x = UnsafeCell::new(0);
///
/// // When we own the value, we can extract the contents for free.
/// let contents: i32 = x.into_inner();
/// assert_eq!(contents, 0);
/// ```
#[lang = "unsafe_cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[repr(no_niche)] // rust-lang/rust#68303.
pub struct UnsafeCell<T: ?Sized> {
    value: T,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for UnsafeCell<T> {}

impl<T> UnsafeCell<T> {
    /// Constructs a new instance of `UnsafeCell` which will wrap the specified
    /// value.
    ///
    /// All access to the inner value through methods is `unsafe`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
    #[inline(always)]
    pub const fn new(value: T) -> UnsafeCell<T> {
        UnsafeCell { value }
    }

    /// Unwraps the value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    ///
    /// let five = uc.into_inner();
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
    pub const fn into_inner(self) -> T {
        self.value
    }
}

impl<T: ?Sized> UnsafeCell<T> {
    /// Gets a mutable pointer to the wrapped value.
    ///
    /// This can be cast to a pointer of any kind.
    /// Ensure that the access is unique (no active references, mutable or not)
    /// when casting to `&mut T`, and ensure that there are no mutations
    /// or mutable aliases going on when casting to `&T`
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let uc = UnsafeCell::new(5);
    ///
    /// let five = uc.get();
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
    pub const fn get(&self) -> *mut T {
        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
        // #[repr(transparent)]. This exploits libstd's special status, there is
        // no guarantee for user code that this will work in future versions of the compiler!
        self as *const UnsafeCell<T> as *const T as *mut T
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// This call borrows the `UnsafeCell` mutably (at compile-time) which
    /// guarantees that we possess the only reference.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    ///
    /// let mut c = UnsafeCell::new(5);
    /// *c.get_mut() += 1;
    ///
    /// assert_eq!(*c.get_mut(), 6);
    /// ```
    #[inline(always)]
    #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.value
    }

    /// Gets a mutable pointer to the wrapped value.
    /// The difference to [`get`] is that this function accepts a raw pointer,
    /// which is useful to avoid the creation of temporary references.
    ///
    /// The result can be cast to a pointer of any kind.
    /// Ensure that the access is unique (no active references, mutable or not)
    /// when casting to `&mut T`, and ensure that there are no mutations
    /// or mutable aliases going on when casting to `&T`.
    ///
    /// [`get`]: UnsafeCell::get()
    ///
    /// # Examples
    ///
    /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
    /// calling `get` would require creating a reference to uninitialized data:
    ///
    /// ```
    /// #![feature(unsafe_cell_raw_get)]
    /// use std::cell::UnsafeCell;
    /// use std::mem::MaybeUninit;
    ///
    /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
    /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
    /// let uc = unsafe { m.assume_init() };
    ///
    /// assert_eq!(uc.into_inner(), 5);
    /// ```
    #[inline(always)]
    #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
    pub const fn raw_get(this: *const Self) -> *mut T {
        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
        // #[repr(transparent)]. This exploits libstd's special status, there is
        // no guarantee for user code that this will work in future versions of the compiler!
        this as *const T as *mut T
    }
}

#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
impl<T: Default> Default for UnsafeCell<T> {
    /// Creates an `UnsafeCell`, with the `Default` value for T.
    fn default() -> UnsafeCell<T> {
        UnsafeCell::new(Default::default())
    }
}

#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for UnsafeCell<T> {
    fn from(t: T) -> UnsafeCell<T> {
        UnsafeCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}

#[allow(unused)]
fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
    let _: UnsafeCell<&dyn Send> = a;
    let _: Cell<&dyn Send> = b;
    let _: RefCell<&dyn Send> = c;
}