core/

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:
/!
/! - Several immutable references (`&T`) to the object (also known as **aliasing**).
/! - 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. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
/! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
/! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
/! types are the correct data structures to do so).
/!
/! Values of the `Cell<T>`, `RefCell<T>`, and `OnceCell<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 these cell types provide 'interior mutability'
/! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
/! (mutable only via `&mut T`).
/!
/! Cell types come in four flavors: `Cell<T>`, `RefCell<T>`, `OnceCell<T>`, and `LazyCell<T>`.
/! Each provides a different way of providing safe interior mutability.
/!
/! ## `Cell<T>`
/!
/! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
/! `&mut T` to the inner value can never be obtained, and the value itself cannot be directly
/! obtained without replacing it with something else. Both of these rules ensure that there is
/! never more than one reference pointing to the inner value. This type provides the following
/! methods:
/!
/!  - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
/!    interior value by duplicating it.
/!  - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
/!    interior value with [`Default::default()`] and returns the replaced value.
/!  - All types have:
/!    - [`replace`](Cell::replace): replaces the current interior value and returns the replaced
/!      value.
/!    - [`into_inner`](Cell::into_inner): this method consumes the `Cell<T>` and returns the
/!      interior value.
/!    - [`set`](Cell::set): this method replaces the interior value, dropping the replaced value.
/!
/! `Cell<T>` is typically used for more simple types where copying or moving values isn't too
/! resource intensive (e.g. numbers), and should usually be preferred over other cell types when
/! possible. For larger and non-copy types, `RefCell` provides some advantages.
/!
/! ## `RefCell<T>`
/!
/! [`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.
/!
/! An immutable reference to a `RefCell`'s inner value (`&T`) can be obtained with
/! [`borrow`](`RefCell::borrow`), and a mutable borrow (`&mut T`) can be obtained with
/! [`borrow_mut`](`RefCell::borrow_mut`). When these functions are called, they first verify that
/! Rust's borrow rules will be satisfied: any number of immutable borrows are allowed or a
/! single mutable borrow is allowed, but never both. If a borrow is attempted that would violate
/! these rules, the thread will panic.
/!
/! The corresponding [`Sync`] version of `RefCell<T>` is [`RwLock<T>`].
/!
/! ## `OnceCell<T>`
/!
/! [`OnceCell<T>`] is somewhat of a hybrid of `Cell` and `RefCell` that works for values that
/! typically only need to be set once. This means that a reference `&T` can be obtained without
/! moving or copying the inner value (unlike `Cell`) but also without runtime checks (unlike
/! `RefCell`). However, its value can also not be updated once set unless you have a mutable
/! reference to the `OnceCell`.
/!
/! `OnceCell` provides the following methods:
/!
/! - [`get`](OnceCell::get): obtain a reference to the inner value
/! - [`set`](OnceCell::set): set the inner value if it is unset (returns a `Result`)
/! - [`get_or_init`](OnceCell::get_or_init): return the inner value, initializing it if needed
/! - [`get_mut`](OnceCell::get_mut): provide a mutable reference to the inner value, only available
/!   if you have a mutable reference to the cell itself.
/!
/! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
/!
/! ## `LazyCell<T, F>`
/!
/! A common pattern with OnceCell is, for a given OnceCell, to use the same function on every
/! call to [`OnceCell::get_or_init`] with that cell. This is what is offered by [`LazyCell`],
/! which pairs cells of `T` with functions of `F`, and always calls `F` before it yields `&T`.
/! This happens implicitly by simply attempting to dereference the LazyCell to get its contents,
/! so its use is much more transparent with a place which has been initialized by a constant.
/!
/! More complicated patterns that don't fit this description can be built on `OnceCell<T>` instead.
/!
/! `LazyCell` works by providing an implementation of `impl Deref` that calls the function,
/! so you can just use it by dereference (e.g. `*lazy_cell` or `lazy_cell.deref()`).
/!
/! The corresponding [`Sync`] version of `LazyCell<T, F>` is [`LazyLock<T, F>`].
/!
/! # 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::OnceCell;
/!
/! struct Graph {
/!     edges: Vec<(i32, i32)>,
/!     span_tree_cache: OnceCell<Vec<(i32, i32)>>
/! }
/!
/! impl Graph {
/!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
/!         self.span_tree_cache
/!             .get_or_init(|| 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<RcInner<T>>,
/!     phantom: PhantomData<RcInner<T>>,
/! }
/!
/! struct RcInner<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 RcInnerPtr<T: ?Sized> {
/!
/!     fn inner(&self) -> &RcInner<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> RcInnerPtr<T> for Rc<T> {
/!    fn inner(&self) -> &RcInner<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
/! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
/! [`LazyLock<T, F>`]: ../../std/sync/struct.LazyLock.html
/! [`Sync`]: ../../std/marker/trait.Sync.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::{Destruct, PhantomData, Unsize};
use crate::mem::{self, ManuallyDrop};
use crate::ops::{self, CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
use crate::panic::const_panic;
use crate::pin::PinCoerceUnsized;
use crate::ptr::{self, NonNull};
use crate::range;

mod lazy;
mod once;

#[stable(feature = "lazy_cell", since = "1.80.0")]
pub use lazy::LazyCell;
#[stable(feature = "once_cell", since = "1.70.0")]
pub use once::OnceCell;

/ A mutable memory location.
/
/ # Memory layout
/
/ `Cell<T>` has the same [memory layout and caveats as
/ `UnsafeCell<T>`](UnsafeCell#memory-layout). In particular, this means that
/ `Cell<T>` has the same in-memory representation as its inner type `T`.
/
/ # 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.
#[rustc_diagnostic_item = "Cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[rustc_pub_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 {}

/ Note that this negative impl isn't strictly necessary for correctness,
/ as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
/ However, given how important `Cell`'s `!Sync`-ness is,
/ having an explicit negative impl is nice for documentation purposes
/ and results in nicer error messages.
#[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")]
#[rustc_const_unstable(feature = "const_default", issue = "143894")]
impl<T: [const] Default> const 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")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<T> for Cell<T> {
    / Creates a new `Cell<T>` containing the given value.
    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")]
    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
    pub const fn set(&self, val: T)
    where
        T: [const] Destruct,
    {
        self.replace(val);
    }

    / Swaps the values of two `Cell`s.
    /
    / The difference with `std::mem::swap` is that this function doesn't
    / require a `&mut` reference.
    /
    / # Panics
    /
    / This function will panic if `self` and `other` are different `Cell`s that partially overlap.
    / (Using just standard library methods, it is impossible to create such partially overlapping `Cell`s.
    / However, unsafe code is allowed to e.g. create two `&Cell<[i32; 2]>` that partially overlap.)
    /
    / # 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) {
        / This function documents that it *will* panic, and intrinsics::is_nonoverlapping doesn't
        / do the check in const, so trying to use it here would be inviting unnecessary fragility.
        fn is_nonoverlapping<T>(src: *const T, dst: *const T) -> bool {
            let src_usize = src.addr();
            let dst_usize = dst.addr();
            let diff = src_usize.abs_diff(dst_usize);
            diff >= size_of::<T>()
        }

        if ptr::eq(self, other) {
            / Swapping wouldn't change anything.
            return;
        }
        if !is_nonoverlapping(self, other) {
            / See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
            panic!("`Cell::swap` on overlapping non-identical `Cell`s");
        }
        / 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. We also excluded shenanigans like partially overlapping `Cell`s,
        / so `swap` will just properly copy two full values of type `T` back and forth.
        unsafe {
            mem::swap(&mut *self.value.get(), &mut *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);
    / ```
    #[inline]
    #[stable(feature = "move_cell", since = "1.17.0")]
    #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
    #[rustc_confusables("swap")]
    pub const 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, consuming the cell.
    /
    / # 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_stable(feature = "const_cell_into_inner", since = "1.83.0")]
    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
    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")]
    #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
    pub const 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.
    /
    / # Examples
    /
    / ```
    / use std::cell::Cell;
    /
    / let c = Cell::new(5);
    / c.update(|x| x + 1);
    / assert_eq!(c.get(), 6);
    / ```
    #[inline]
    #[stable(feature = "cell_update", since = "1.88.0")]
    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
    pub const fn update(&self, f: impl [const] FnOnce(T) -> T)
    where
        / FIXME(const-hack): `Copy` should imply `const Destruct`
        T: [const] Destruct,
    {
        let old = self.get();
        self.set(f(old));
    }
}

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")]
    #[rustc_as_ptr]
    #[rustc_never_returns_null_ptr]
    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")]
    #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
    pub const 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")]
    #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
    pub const 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")]
    #[rustc_const_unstable(feature = "const_cell_traits", issue = "147787")]
    pub const fn take(&self) -> T
    where
        T: [const] Default,
    {
        self.replace(Default::default())
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}

/ Allow types that wrap `Cell` to also implement `DispatchFromDyn`
/ and become dyn-compatible method receivers.
/ Note that currently `Cell` itself cannot be a method receiver
/ because it does not implement Deref.
/ In other words:
/ `self: Cell<&Self>` won't work
/ `self: CellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<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")]
    #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
    pub const 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>]) }
    }
}

impl<T, const N: usize> Cell<[T; N]> {
    / Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
    /
    / # Examples
    /
    / ```
    / use std::cell::Cell;
    /
    / let mut array: [i32; 3] = [1, 2, 3];
    / let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
    / let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
    / ```
    #[stable(feature = "as_array_of_cells", since = "1.91.0")]
    #[rustc_const_stable(feature = "as_array_of_cells", since = "1.91.0")]
    pub const fn as_array_of_cells(&self) -> &[Cell<T>; N] {
        / SAFETY: `Cell<T>` has the same memory layout as `T`.
        unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
    }
}

/ Types for which cloning `Cell<Self>` is sound.
/
/ # Safety
/
/ Implementing this trait for a type is sound if and only if the following code is sound for T =
/ that type.
/
/ ```
/ #![feature(cell_get_cloned)]
/ # use std::cell::{CloneFromCell, Cell};
/ fn clone_from_cell<T: CloneFromCell>(cell: &Cell<T>) -> T {
/     unsafe { T::clone(&*cell.as_ptr()) }
/ }
/ ```
/
/ Importantly, you can't just implement `CloneFromCell` for any arbitrary `Copy` type, e.g. the
/ following is unsound:
/
/ ```rust
/ #![feature(cell_get_cloned)]
/ # use std::cell::Cell;
/
/ #[derive(Copy, Debug)]
/ pub struct Bad<'a>(Option<&'a Cell<Bad<'a>>>, u8);
/
/ impl Clone for Bad<'_> {
/     fn clone(&self) -> Self {
/         let a: &u8 = &self.1;
/         / when self.0 points to self, we write to self.1 while we have a live `&u8` pointing to
/         / it -- this is UB
/         self.0.unwrap().set(Self(None, 1));
/         dbg!((a, self));
/         Self(None, 0)
/     }
/ }
/
/ / this is not sound
/ / unsafe impl CloneFromCell for Bad<'_> {}
/ ```
#[unstable(feature = "cell_get_cloned", issue = "145329")]
/ Allow potential overlapping implementations in user code
#[marker]
pub unsafe trait CloneFromCell: Clone {}

/ `CloneFromCell` can be implemented for types that don't have indirection and which don't access
/ `Cell`s in their `Clone` implementation. A commonly-used subset is covered here.
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell, const N: usize> CloneFromCell for [T; N] {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell> CloneFromCell for Option<T> {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell, E: CloneFromCell> CloneFromCell for Result<T, E> {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: ?Sized> CloneFromCell for PhantomData<T> {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell> CloneFromCell for ManuallyDrop<T> {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell> CloneFromCell for ops::Range<T> {}
#[unstable(feature = "cell_get_cloned", issue = "145329")]
unsafe impl<T: CloneFromCell> CloneFromCell for range::Range<T> {}

#[unstable(feature = "cell_get_cloned", issue = "145329")]
impl<T: CloneFromCell> Cell<T> {
    / Get a clone of the `Cell` that contains a copy of the original value.
    /
    / This allows a cheaply `Clone`-able type like an `Rc` to be stored in a `Cell`, exposing the
    / cheaper `clone()` method.
    /
    / # Examples
    /
    / ```
    / #![feature(cell_get_cloned)]
    /
    / use core::cell::Cell;
    / use std::rc::Rc;
    /
    / let rc = Rc::new(1usize);
    / let c1 = Cell::new(rc);
    / let c2 = c1.get_cloned();
    / assert_eq!(*c2.into_inner(), 1);
    / ```
    pub fn get_cloned(&self) -> Self {
        / SAFETY: T is CloneFromCell, which guarantees that this is sound.
        Cell::new(T::clone(unsafe { &*self.as_ptr() }))
    }
}

/ A mutable memory location with dynamically checked borrow rules
/
/ See the [module-level documentation](self) for more.
#[rustc_diagnostic_item = "RefCell"]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RefCell<T: ?Sized> {
    borrow: Cell<BorrowCounter>,
    / Stores the location of the earliest currently active borrow.
    / This gets updated whenever 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]
#[derive(Debug)]
pub struct BorrowError {
    #[cfg(feature = "debug_refcell")]
    location: &'static crate::panic::Location<'static>,
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        #[cfg(feature = "debug_refcell")]
        let res = write!(
            f,
            "RefCell already mutably borrowed; a previous borrow was at {}",
            self.location
        );

        #[cfg(not(feature = "debug_refcell"))]
        let res = Display::fmt("RefCell already mutably borrowed", f);

        res
    }
}

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

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowMutError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        #[cfg(feature = "debug_refcell")]
        let res = write!(f, "RefCell already borrowed; a previous borrow was at {}", self.location);

        #[cfg(not(feature = "debug_refcell"))]
        let res = Display::fmt("RefCell already borrowed", f);

        res
    }
}

/ This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
#[cfg_attr(not(panic = "immediate-abort"), inline(never))]
#[track_caller]
#[cold]
const fn panic_already_borrowed(err: BorrowMutError) -> ! {
    const_panic!(
        "RefCell already borrowed",
        "{err}",
        err: BorrowMutError = err,
    )
}

/ This ensures the panicking code is outlined from `borrow` for `RefCell`.
#[cfg_attr(not(panic = "immediate-abort"), inline(never))]
#[track_caller]
#[cold]
const fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
    const_panic!(
        "RefCell already mutably borrowed",
        "{err}",
        err: BorrowError = err,
    )
}

/ 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 `BorrowCounter` 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 BorrowCounter = isize;
const UNUSED: BorrowCounter = 0;

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

#[inline(always)]
const fn is_reading(x: BorrowCounter) -> 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_stable(feature = "const_cell_into_inner", since = "1.83.0")]
    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
    #[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]
    #[rustc_confusables("swap")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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, or
    / if `self` and `other` point to the same `RefCell`.
    /
    / # 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")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const fn borrow(&self) -> Ref<'_, T> {
        match self.try_borrow() {
            Ok(b) => b,
            Err(err) => panic_already_mutably_borrowed(err),
        }
    }

    / 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)]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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.replace(Some(crate::panic::Location::caller()));
                    }
                }

                / SAFETY: `BorrowRef` ensures that there is only immutable access
                / to the value while borrowed.
                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
                Ok(Ref { value, borrow: b })
            }
            None => Err(BorrowError {
                / If a borrow occurred, 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]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const fn borrow_mut(&self) -> RefMut<'_, T> {
        match self.try_borrow_mut() {
            Ok(b) => b,
            Err(err) => panic_already_borrowed(err),
        }
    }

    / 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)]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
        match BorrowRefMut::new(&self.borrow) {
            Some(b) => {
                #[cfg(feature = "debug_refcell")]
                {
                    self.borrowed_at.replace(Some(crate::panic::Location::caller()));
                }

                / SAFETY: `BorrowRefMut` guarantees unique access.
                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
                Ok(RefMut { value, borrow: b, marker: PhantomData })
            }
            None => Err(BorrowMutError {
                / If a borrow occurred, 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")]
    #[rustc_as_ptr]
    #[rustc_never_returns_null_ptr]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const fn as_ptr(&self) -> *mut T {
        self.value.get()
    }

    / Returns a mutable reference to the underlying data.
    /
    / Since this method borrows `RefCell` mutably, it is statically guaranteed
    / that no borrows to the underlying data exist. The dynamic checks inherent
    / in [`borrow_mut`] and most other methods of `RefCell` are therefore
    / unnecessary. Note that this method does not reset the borrowing state if borrows were previously leaked
    / (e.g., via [`forget()`] on a [`Ref`] or [`RefMut`]). For that purpose,
    / consider using the unstable [`undo_leak`] method.
    /
    / This method can only be called if `RefCell` can be mutably borrowed,
    / which in general is only the case directly after the `RefCell` has
    / been created. In these situations, skipping the aforementioned dynamic
    / borrowing checks may yield better ergonomics and runtime-performance.
    /
    / In most situations where `RefCell` is used, it can't be borrowed mutably.
    / Use [`borrow_mut`] to get mutable access to the underlying data then.
    /
    / [`borrow_mut`]: RefCell::borrow_mut()
    / [`forget()`]: mem::forget
    / [`undo_leak`]: RefCell::undo_leak()
    /
    / # 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")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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 behavior.
    /
    / # 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]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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 occurred, 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 `source` is currently mutably borrowed.
    #[inline]
    #[track_caller]
    fn clone_from(&mut self, source: &Self) {
        self.get_mut().clone_from(&source.borrow())
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_default", issue = "143894")]
impl<T: [const] Default> const 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 mutably 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 mutably 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 mutably borrowed.
    #[inline]
    fn lt(&self, other: &RefCell<T>) -> bool {
        *self.borrow() < *other.borrow()
    }

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

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

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

#[stable(feature = "cell_from", since = "1.12.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<T> for RefCell<T> {
    / Creates a new `RefCell<T>` containing the given value.
    fn from(t: T) -> RefCell<T> {
        RefCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}

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

impl<'b> BorrowRef<'b> {
    #[inline]
    const fn new(borrow: &'b Cell<BorrowCounter>) -> 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.replace(b);
            Some(BorrowRef { borrow })
        }
    }
}

#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
impl const Drop for BorrowRef<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_reading(borrow));
        self.borrow.replace(borrow - 1);
    }
}

#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
impl const 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 != BorrowCounter::MAX);
        self.borrow.replace(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")]
#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
#[rustc_diagnostic_item = "RefCellRef"]
pub struct Ref<'b, T: ?Sized + 'b> {
    / NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
    / `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
    / `NonNull` is also covariant over `T`, just like we would have with `&T`.
    value: NonNull<T>,
    borrow: BorrowRef<'b>,
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: ?Sized> const Deref for Ref<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        / SAFETY: the value is accessible as long as we hold our borrow.
        unsafe { self.value.as_ref() }
    }
}

#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}

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")]
    #[must_use]
    #[inline]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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: NonNull::from(f(&*orig)), 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
    /
    / ```
    / 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);
    / ```
    #[stable(feature = "cell_filter_map", since = "1.63.0")]
    #[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) {
            Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
            None => Err(orig),
        }
    }

    / Tries to makes a new `Ref` for a component of the borrowed data.
    / On failure, the original guard is returned alongside with the error
    / returned by the closure.
    /
    / The `RefCell` is already immutably borrowed, so this cannot fail.
    /
    / This is an associated function that needs to be used as
    / `Ref::try_map(...)`. A method would interfere with methods of the same
    / name on the contents of a `RefCell` used through `Deref`.
    /
    / # Examples
    /
    / ```
    / #![feature(refcell_try_map)]
    / use std::cell::{RefCell, Ref};
    / use std::str::{from_utf8, Utf8Error};
    /
    / let c = RefCell::new(vec![0xF0, 0x9F, 0xA6 ,0x80]);
    / let b1: Ref<'_, Vec<u8>> = c.borrow();
    / let b2: Result<Ref<'_, str>, _> = Ref::try_map(b1, |v| from_utf8(v));
    / assert_eq!(&*b2.unwrap(), "🦀");
    /
    / let c = RefCell::new(vec![0xF0, 0x9F, 0xA6]);
    / let b1: Ref<'_, Vec<u8>> = c.borrow();
    / let b2: Result<_, (Ref<'_, Vec<u8>>, Utf8Error)> = Ref::try_map(b1, |v| from_utf8(v));
    / let (b3, e) = b2.unwrap_err();
    / assert_eq!(*b3, vec![0xF0, 0x9F, 0xA6]);
    / assert_eq!(e.valid_up_to(), 0);
    / ```
    #[unstable(feature = "refcell_try_map", issue = "143801")]
    #[inline]
    pub fn try_map<U: ?Sized, E>(
        orig: Ref<'b, T>,
        f: impl FnOnce(&T) -> Result<&U, E>,
    ) -> Result<Ref<'b, U>, (Self, E)> {
        match f(&*orig) {
            Ok(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
            Err(e) => Err((orig, e)),
        }
    }

    / 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);
        let borrow = orig.borrow.clone();
        (
            Ref { value: NonNull::from(a), borrow },
            Ref { value: NonNull::from(b), borrow: orig.borrow },
        )
    }

    / Converts 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")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const 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);
        / SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
        unsafe { orig.value.as_ref() }
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
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).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>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
    where
        F: FnOnce(&mut T) -> &mut U,
    {
        let value = NonNull::from(f(&mut *orig));
        RefMut { value, borrow: orig.borrow, marker: PhantomData }
    }

    / 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
    /
    / ```
    / 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]);
    / ```
    #[stable(feature = "cell_filter_map", since = "1.63.0")]
    #[inline]
    pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
    where
        F: FnOnce(&mut T) -> Option<&mut U>,
    {
        / 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(&mut *orig) {
            Some(value) => {
                Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
            }
            None => Err(orig),
        }
    }

    / Tries to makes a new `RefMut` for a component of the borrowed data.
    / On failure, the original guard is returned alongside with the error
    / returned by the closure.
    /
    / The `RefCell` is already mutably borrowed, so this cannot fail.
    /
    / This is an associated function that needs to be used as
    / `RefMut::try_map(...)`. A method would interfere with methods of the same
    / name on the contents of a `RefCell` used through `Deref`.
    /
    / # Examples
    /
    / ```
    / #![feature(refcell_try_map)]
    / use std::cell::{RefCell, RefMut};
    / use std::str::{from_utf8_mut, Utf8Error};
    /
    / let c = RefCell::new(vec![0x68, 0x65, 0x6C, 0x6C, 0x6F]);
    / {
    /     let b1: RefMut<'_, Vec<u8>> = c.borrow_mut();
    /     let b2: Result<RefMut<'_, str>, _> = RefMut::try_map(b1, |v| from_utf8_mut(v));
    /     let mut b2 = b2.unwrap();
    /     assert_eq!(&*b2, "hello");
    /     b2.make_ascii_uppercase();
    / }
    / assert_eq!(*c.borrow(), "HELLO".as_bytes());
    /
    / let c = RefCell::new(vec![0xFF]);
    / let b1: RefMut<'_, Vec<u8>> = c.borrow_mut();
    / let b2: Result<_, (RefMut<'_, Vec<u8>>, Utf8Error)> = RefMut::try_map(b1, |v| from_utf8_mut(v));
    / let (b3, e) = b2.unwrap_err();
    / assert_eq!(*b3, vec![0xFF]);
    / assert_eq!(e.valid_up_to(), 0);
    / ```
    #[unstable(feature = "refcell_try_map", issue = "143801")]
    #[inline]
    pub fn try_map<U: ?Sized, E>(
        mut orig: RefMut<'b, T>,
        f: impl FnOnce(&mut T) -> Result<&mut U, E>,
    ) -> Result<RefMut<'b, U>, (Self, E)> {
        / 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(&mut *orig) {
            Ok(value) => {
                Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
            }
            Err(e) => Err((orig, e)),
        }
    }

    / 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>(
        mut orig: RefMut<'b, T>,
        f: F,
    ) -> (RefMut<'b, U>, RefMut<'b, V>)
    where
        F: FnOnce(&mut T) -> (&mut U, &mut V),
    {
        let borrow = orig.borrow.clone();
        let (a, b) = f(&mut *orig);
        (
            RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
            RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
        )
    }

    / Converts 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")]
    #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
    pub const fn leak(mut 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);
        / SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
        unsafe { orig.value.as_mut() }
    }
}

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

#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
impl const Drop for BorrowRefMut<'_> {
    #[inline]
    fn drop(&mut self) {
        let borrow = self.borrow.get();
        debug_assert!(is_writing(borrow));
        self.borrow.replace(borrow + 1);
    }
}

impl<'b> BorrowRefMut<'b> {
    #[inline]
    const fn new(borrow: &'b Cell<BorrowCounter>) -> 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.replace(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 != BorrowCounter::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")]
#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
#[rustc_diagnostic_item = "RefCellRefMut"]
pub struct RefMut<'b, T: ?Sized + 'b> {
    / NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
    / `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
    value: NonNull<T>,
    borrow: BorrowRefMut<'b>,
    / `NonNull` is covariant over `T`, so we need to reintroduce invariance.
    marker: PhantomData<&'b mut T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: ?Sized> const Deref for RefMut<'_, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        / SAFETY: the value is accessible as long as we hold our borrow.
        unsafe { self.value.as_ref() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T: ?Sized> const DerefMut for RefMut<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        / SAFETY: the value is accessible as long as we hold our borrow.
        unsafe { self.value.as_mut() }
    }
}

#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}

#[unstable(feature = "coerce_unsized", issue = "18598")]
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).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 a `&T` into a `&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>`.
/
/ `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
/ threads have access to the same `UnsafeCell`, they must follow the usual rules of the
/ [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
/ [`core::sync::atomic`].
/
/ 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`
/ [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
/
/ # Aliasing rules
/
/ 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), 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.
/
/ - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
/   until the reference expires. As a special exception, given an `&T`, any part of it that is
/   inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
/   last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
/   of what a reference points to, this means the memory an `&T` points to can be deallocated only if
/   *every part of it* (including padding) is inside an `UnsafeCell`.
/
/ However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
/ live memory and the compiler is allowed to insert spurious reads if it can prove that this
/ memory has not yet been deallocated.
/
/ 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 a `&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`
/
/ # Memory layout
/
/ `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
/ of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
/ Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
/ to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
/ optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
/ 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
/ Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
/ having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
/ order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
/ thus this can cause distortions in the type size in these cases.
/
/ Note that the only valid way to obtain a `*mut T` pointer to the contents of a
/ _shared_ `UnsafeCell<T>` is through [`.get()`]  or [`.raw_get()`]. A `&mut T` reference
/ can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
/ on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
/ same memory layout, the following is not allowed and undefined behavior:
/
/ ```rust,compile_fail
/ # use std::cell::UnsafeCell;
/ unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
/   let t = ptr as *const UnsafeCell<T> as *mut T;
/   / This is undefined behavior, because the `*mut T` pointer
/   / was not obtained through `.get()` nor `.raw_get()`:
/   unsafe { &mut *t }
/ }
/ ```
/
/ Instead, do this:
/
/ ```rust
/ # use std::cell::UnsafeCell;
/ / Safety: the caller must ensure that there are no references that
/ / point to the *contents* of the `UnsafeCell`.
/ unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
/   unsafe { &mut *ptr.get() }
/ }
/ ```
/
/ Converting in the other direction from a `&mut T`
/ to an `&UnsafeCell<T>` is allowed:
/
/ ```rust
/ # use std::cell::UnsafeCell;
/ fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
/   let t = ptr as *mut T as *const UnsafeCell<T>;
/   / SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
/   unsafe { &*t }
/ }
/ ```
/
/ [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
/ [`.raw_get()`]: `UnsafeCell::raw_get`
/
/ # 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)]
#[rustc_pub_transparent]
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 `&UnsafeCell<T>` requires `unsafe` code.
    /
    / # 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, consuming the cell.
    /
    / # 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_stable(feature = "const_cell_into_inner", since = "1.83.0")]
    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
    pub const fn into_inner(self) -> T {
        self.value
    }

    / Replace the value in this `UnsafeCell` and return the old value.
    /
    / # Safety
    /
    / The caller must take care to avoid aliasing and data races.
    /
    / - It is Undefined Behavior to allow calls to race with
    /   any other access to the wrapped value.
    / - It is Undefined Behavior to call this while any other
    /   reference(s) to the wrapped value are alive.
    /
    / # Examples
    /
    / ```
    / #![feature(unsafe_cell_access)]
    / use std::cell::UnsafeCell;
    /
    / let uc = UnsafeCell::new(5);
    /
    / let old = unsafe { uc.replace(10) };
    / assert_eq!(old, 5);
    / ```
    #[inline]
    #[unstable(feature = "unsafe_cell_access", issue = "136327")]
    pub const unsafe fn replace(&self, value: T) -> T {
        / SAFETY: pointer comes from `&self` so naturally satisfies invariants.
        unsafe { ptr::replace(self.get(), value) }
    }
}

impl<T: ?Sized> UnsafeCell<T> {
    / Converts from `&mut T` to `&mut UnsafeCell<T>`.
    /
    / # Examples
    /
    / ```
    / use std::cell::UnsafeCell;
    /
    / let mut val = 42;
    / let uc = UnsafeCell::from_mut(&mut val);
    /
    / *uc.get_mut() -= 1;
    / assert_eq!(*uc.get_mut(), 41);
    / ```
    #[inline(always)]
    #[stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
    #[rustc_const_stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
    pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
        / SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
        unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }
    }

    / Gets a mutable pointer to the wrapped value.
    /
    / This can be cast to a pointer of any kind. When creating references, you must uphold the
    / aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
    / caveats.
    /
    / # 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")]
    #[rustc_as_ptr]
    #[rustc_never_returns_null_ptr]
    pub const fn get(&self) -> *mut T {
        / We can just cast the pointer from `UnsafeCell<T>` to `T` because of
        / #[repr(transparent)]. This exploits std'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")]
    #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "1.83.0")]
    pub const fn get_mut(&mut self) -> &mut T {
        &mut self.value
    }

    / Gets a mutable pointer to the wrapped value.
    / The difference from [`get`] is that this function accepts a raw pointer,
    / which is useful to avoid the creation of temporary references.
    /
    / This can be cast to a pointer of any kind. When creating references, you must uphold the
    / aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
    / caveats.
    /
    / [`get`]: UnsafeCell::get()
    /
    / # Examples
    /
    / Gradual initialization of an `UnsafeCell` requires `raw_get`, as
    / calling `get` would require creating a reference to uninitialized data:
    /
    / ```
    / use std::cell::UnsafeCell;
    / use std::mem::MaybeUninit;
    /
    / let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
    / unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
    / / avoid below which references to uninitialized data
    / / unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
    / let uc = unsafe { m.assume_init() };
    /
    / assert_eq!(uc.into_inner(), 5);
    / ```
    #[inline(always)]
    #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
    #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
    #[rustc_diagnostic_item = "unsafe_cell_raw_get"]
    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 std'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
    }

    / Get a shared reference to the value within the `UnsafeCell`.
    /
    / # Safety
    /
    / - It is Undefined Behavior to call this while any mutable
    /   reference to the wrapped value is alive.
    / - Mutating the wrapped value while the returned
    /   reference is alive is Undefined Behavior.
    /
    / # Examples
    /
    / ```
    / #![feature(unsafe_cell_access)]
    / use std::cell::UnsafeCell;
    /
    / let uc = UnsafeCell::new(5);
    /
    / let val = unsafe { uc.as_ref_unchecked() };
    / assert_eq!(val, &5);
    / ```
    #[inline]
    #[unstable(feature = "unsafe_cell_access", issue = "136327")]
    pub const unsafe fn as_ref_unchecked(&self) -> &T {
        / SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
        unsafe { self.get().as_ref_unchecked() }
    }

    / Get an exclusive reference to the value within the `UnsafeCell`.
    /
    / # Safety
    /
    / - It is Undefined Behavior to call this while any other
    /   reference(s) to the wrapped value are alive.
    / - Mutating the wrapped value through other means while the
    /   returned reference is alive is Undefined Behavior.
    /
    / # Examples
    /
    / ```
    / #![feature(unsafe_cell_access)]
    / use std::cell::UnsafeCell;
    /
    / let uc = UnsafeCell::new(5);
    /
    / unsafe { *uc.as_mut_unchecked() += 1; }
    / assert_eq!(uc.into_inner(), 6);
    / ```
    #[inline]
    #[unstable(feature = "unsafe_cell_access", issue = "136327")]
    #[allow(clippy::mut_from_ref)]
    pub const unsafe fn as_mut_unchecked(&self) -> &mut T {
        / SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
        unsafe { self.get().as_mut_unchecked() }
    }
}

#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
#[rustc_const_unstable(feature = "const_default", issue = "143894")]
impl<T: [const] Default> const 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")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<T> for UnsafeCell<T> {
    / Creates a new `UnsafeCell<T>` containing the given value.
    fn from(t: T) -> UnsafeCell<T> {
        UnsafeCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}

/ Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
/ and become dyn-compatible method receivers.
/ Note that currently `UnsafeCell` itself cannot be a method receiver
/ because it does not implement Deref.
/ In other words:
/ `self: UnsafeCell<&Self>` won't work
/ `self: UnsafeCellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}

/ [`UnsafeCell`], but [`Sync`].
/
/ This is just an `UnsafeCell`, except it implements `Sync`
/ if `T` implements `Sync`.
/
/ `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
/ You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
/ shared between threads, if that's intentional.
/ Providing proper synchronization is still the task of the user,
/ making this type just as unsafe to use.
/
/ See [`UnsafeCell`] for details.
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
#[repr(transparent)]
#[rustc_diagnostic_item = "SyncUnsafeCell"]
#[rustc_pub_transparent]
pub struct SyncUnsafeCell<T: ?Sized> {
    value: UnsafeCell<T>,
}

#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}

#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T> SyncUnsafeCell<T> {
    / Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
    #[inline]
    pub const fn new(value: T) -> Self {
        Self { value: UnsafeCell { value } }
    }

    / Unwraps the value, consuming the cell.
    #[inline]
    #[rustc_const_unstable(feature = "sync_unsafe_cell", issue = "95439")]
    pub const fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: ?Sized> SyncUnsafeCell<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`
    #[inline]
    #[rustc_as_ptr]
    #[rustc_never_returns_null_ptr]
    pub const fn get(&self) -> *mut T {
        self.value.get()
    }

    / Returns a mutable reference to the underlying data.
    /
    / This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
    / guarantees that we possess the only reference.
    #[inline]
    pub const fn get_mut(&mut self) -> &mut T {
        self.value.get_mut()
    }

    / Gets a mutable pointer to the wrapped value.
    /
    / See [`UnsafeCell::get`] for details.
    #[inline]
    pub const fn raw_get(this: *const Self) -> *mut T {
        / We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
        / of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
        / See UnsafeCell::raw_get.
        this as *const T as *mut T
    }
}

#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
#[rustc_const_unstable(feature = "const_default", issue = "143894")]
impl<T: [const] Default> const Default for SyncUnsafeCell<T> {
    / Creates an `SyncUnsafeCell`, with the `Default` value for T.
    fn default() -> SyncUnsafeCell<T> {
        SyncUnsafeCell::new(Default::default())
    }
}

#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
impl<T> const From<T> for SyncUnsafeCell<T> {
    / Creates a new `SyncUnsafeCell<T>` containing the given value.
    fn from(t: T) -> SyncUnsafeCell<T> {
        SyncUnsafeCell::new(t)
    }
}

#[unstable(feature = "coerce_unsized", issue = "18598")]
/#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}

/ Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
/ and become dyn-compatible method receivers.
/ Note that currently `SyncUnsafeCell` itself cannot be a method receiver
/ because it does not implement Deref.
/ In other words:
/ `self: SyncUnsafeCell<&Self>` won't work
/ `self: SyncUnsafeCellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
/#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}

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

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}

#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}