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Diffstat (limited to 'library/core/src/cell.rs')
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diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs new file mode 100644 index 000000000..fb4454c94 --- /dev/null +++ b/library/core/src/cell.rs @@ -0,0 +1,2122 @@ +//! Shareable mutable containers. +//! +//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to +//! have one of the following: +//! +//! - Having several immutable references (`&T`) to the object (also known as **aliasing**). +//! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**). +//! +//! This is enforced by the Rust compiler. However, there are situations where this rule is not +//! flexible enough. Sometimes it is required to have multiple references to an object and yet +//! mutate it. +//! +//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the +//! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded +//! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement +//! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to +//! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types. +//! +//! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e. +//! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`) +//! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast +//! with typical Rust types that exhibit 'inherited mutability'. +//! +//! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior +//! mutability by moving values in and out of the `Cell<T>`. To use references instead of values, +//! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides +//! methods to retrieve and change the current interior value: +//! +//! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current +//! interior value. +//! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current +//! interior value with [`Default::default()`] and returns the replaced value. +//! - For all types, the [`replace`](Cell::replace) method replaces the current interior value and +//! returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the +//! `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method +//! replaces the interior value, dropping the replaced value. +//! +//! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can +//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are +//! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked +//! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt +//! to borrow a value that is already mutably borrowed; when this happens it results in thread +//! panic. +//! +//! # When to choose interior mutability +//! +//! The more common inherited mutability, where one must have unique access to mutate a value, is +//! one of the key language elements that enables Rust to reason strongly about pointer aliasing, +//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and +//! interior mutability is something of a last resort. Since cell types enable mutation where it +//! would otherwise be disallowed though, there are occasions when interior mutability might be +//! appropriate, or even *must* be used, e.g. +//! +//! * Introducing mutability 'inside' of something immutable +//! * Implementation details of logically-immutable methods. +//! * Mutating implementations of [`Clone`]. +//! +//! ## Introducing mutability 'inside' of something immutable +//! +//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can +//! be cloned and shared between multiple parties. Because the contained values may be +//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be +//! impossible to mutate data inside of these smart pointers at all. +//! +//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce +//! mutability: +//! +//! ``` +//! use std::cell::{RefCell, RefMut}; +//! use std::collections::HashMap; +//! use std::rc::Rc; +//! +//! fn main() { +//! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new())); +//! // Create a new block to limit the scope of the dynamic borrow +//! { +//! let mut map: RefMut<_> = shared_map.borrow_mut(); +//! map.insert("africa", 92388); +//! map.insert("kyoto", 11837); +//! map.insert("piccadilly", 11826); +//! map.insert("marbles", 38); +//! } +//! +//! // Note that if we had not let the previous borrow of the cache fall out +//! // of scope then the subsequent borrow would cause a dynamic thread panic. +//! // This is the major hazard of using `RefCell`. +//! let total: i32 = shared_map.borrow().values().sum(); +//! println!("{total}"); +//! } +//! ``` +//! +//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded +//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a +//! multi-threaded situation. +//! +//! ## Implementation details of logically-immutable methods +//! +//! Occasionally it may be desirable not to expose in an API that there is mutation happening +//! "under the hood". This may be because logically the operation is immutable, but e.g., caching +//! forces the implementation to perform mutation; or because you must employ mutation to implement +//! a trait method that was originally defined to take `&self`. +//! +//! ``` +//! # #![allow(dead_code)] +//! use std::cell::RefCell; +//! +//! struct Graph { +//! edges: Vec<(i32, i32)>, +//! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>> +//! } +//! +//! impl Graph { +//! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> { +//! self.span_tree_cache.borrow_mut() +//! .get_or_insert_with(|| self.calc_span_tree()) +//! .clone() +//! } +//! +//! fn calc_span_tree(&self) -> Vec<(i32, i32)> { +//! // Expensive computation goes here +//! vec![] +//! } +//! } +//! ``` +//! +//! ## Mutating implementations of `Clone` +//! +//! This is simply a special - but common - case of the previous: hiding mutability for operations +//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the +//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that +//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its +//! reference counts within a `Cell<T>`. +//! +//! ``` +//! use std::cell::Cell; +//! use std::ptr::NonNull; +//! use std::process::abort; +//! use std::marker::PhantomData; +//! +//! struct Rc<T: ?Sized> { +//! ptr: NonNull<RcBox<T>>, +//! phantom: PhantomData<RcBox<T>>, +//! } +//! +//! struct RcBox<T: ?Sized> { +//! strong: Cell<usize>, +//! refcount: Cell<usize>, +//! value: T, +//! } +//! +//! impl<T: ?Sized> Clone for Rc<T> { +//! fn clone(&self) -> Rc<T> { +//! self.inc_strong(); +//! Rc { +//! ptr: self.ptr, +//! phantom: PhantomData, +//! } +//! } +//! } +//! +//! trait RcBoxPtr<T: ?Sized> { +//! +//! fn inner(&self) -> &RcBox<T>; +//! +//! fn strong(&self) -> usize { +//! self.inner().strong.get() +//! } +//! +//! fn inc_strong(&self) { +//! self.inner() +//! .strong +//! .set(self.strong() +//! .checked_add(1) +//! .unwrap_or_else(|| abort() )); +//! } +//! } +//! +//! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> { +//! fn inner(&self) -> &RcBox<T> { +//! unsafe { +//! self.ptr.as_ref() +//! } +//! } +//! } +//! ``` +//! +//! [`Arc<T>`]: ../../std/sync/struct.Arc.html +//! [`Rc<T>`]: ../../std/rc/struct.Rc.html +//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html +//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html +//! [`atomic`]: crate::sync::atomic + +#![stable(feature = "rust1", since = "1.0.0")] + +use crate::cmp::Ordering; +use crate::fmt::{self, Debug, Display}; +use crate::marker::{PhantomData, Unsize}; +use crate::mem; +use crate::ops::{CoerceUnsized, Deref, DerefMut}; +use crate::ptr::{self, NonNull}; + +mod lazy; +mod once; + +#[unstable(feature = "once_cell", issue = "74465")] +pub use lazy::LazyCell; +#[unstable(feature = "once_cell", issue = "74465")] +pub use once::OnceCell; + +/// A mutable memory location. +/// +/// # Examples +/// +/// In this example, you can see that `Cell<T>` enables mutation inside an +/// immutable struct. In other words, it enables "interior mutability". +/// +/// ``` +/// use std::cell::Cell; +/// +/// struct SomeStruct { +/// regular_field: u8, +/// special_field: Cell<u8>, +/// } +/// +/// let my_struct = SomeStruct { +/// regular_field: 0, +/// special_field: Cell::new(1), +/// }; +/// +/// let new_value = 100; +/// +/// // ERROR: `my_struct` is immutable +/// // my_struct.regular_field = new_value; +/// +/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, +/// // which can always be mutated +/// my_struct.special_field.set(new_value); +/// assert_eq!(my_struct.special_field.get(), new_value); +/// ``` +/// +/// See the [module-level documentation](self) for more. +#[stable(feature = "rust1", since = "1.0.0")] +#[repr(transparent)] +pub struct Cell<T: ?Sized> { + value: UnsafeCell<T>, +} + +#[stable(feature = "rust1", since = "1.0.0")] +unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} + +// 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")] +impl<T: Default> Default for Cell<T> { + /// Creates a `Cell<T>`, with the `Default` value for T. + #[inline] + fn default() -> Cell<T> { + Cell::new(Default::default()) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: PartialEq + Copy> PartialEq for Cell<T> { + #[inline] + fn eq(&self, other: &Cell<T>) -> bool { + self.get() == other.get() + } +} + +#[stable(feature = "cell_eq", since = "1.2.0")] +impl<T: Eq + Copy> Eq for Cell<T> {} + +#[stable(feature = "cell_ord", since = "1.10.0")] +impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { + #[inline] + fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { + self.get().partial_cmp(&other.get()) + } + + #[inline] + fn lt(&self, other: &Cell<T>) -> bool { + self.get() < other.get() + } + + #[inline] + fn le(&self, other: &Cell<T>) -> bool { + self.get() <= other.get() + } + + #[inline] + fn gt(&self, other: &Cell<T>) -> bool { + self.get() > other.get() + } + + #[inline] + fn ge(&self, other: &Cell<T>) -> bool { + self.get() >= other.get() + } +} + +#[stable(feature = "cell_ord", since = "1.10.0")] +impl<T: Ord + Copy> Ord for Cell<T> { + #[inline] + fn cmp(&self, other: &Cell<T>) -> Ordering { + self.get().cmp(&other.get()) + } +} + +#[stable(feature = "cell_from", since = "1.12.0")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +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")] + pub fn set(&self, val: T) { + let old = self.replace(val); + drop(old); + } + + /// Swaps the values of two `Cell`s. + /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let c1 = Cell::new(5i32); + /// let c2 = Cell::new(10i32); + /// c1.swap(&c2); + /// assert_eq!(10, c1.get()); + /// assert_eq!(5, c2.get()); + /// ``` + #[inline] + #[stable(feature = "move_cell", since = "1.17.0")] + pub fn swap(&self, other: &Self) { + if ptr::eq(self, other) { + return; + } + // SAFETY: This can be risky if called from separate threads, but `Cell` + // is `!Sync` so this won't happen. This also won't invalidate any + // pointers since `Cell` makes sure nothing else will be pointing into + // either of these `Cell`s. + unsafe { + ptr::swap(self.value.get(), other.value.get()); + } + } + + /// Replaces the contained value with `val`, and returns the old contained value. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let cell = Cell::new(5); + /// assert_eq!(cell.get(), 5); + /// assert_eq!(cell.replace(10), 5); + /// assert_eq!(cell.get(), 10); + /// ``` + #[stable(feature = "move_cell", since = "1.17.0")] + pub fn replace(&self, val: T) -> T { + // SAFETY: This can cause data races if called from a separate thread, + // but `Cell` is `!Sync` so this won't happen. + mem::replace(unsafe { &mut *self.value.get() }, val) + } + + /// Unwraps the value. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let c = Cell::new(5); + /// let five = c.into_inner(); + /// + /// assert_eq!(five, 5); + /// ``` + #[stable(feature = "move_cell", since = "1.17.0")] + #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] + pub const fn into_inner(self) -> T { + self.value.into_inner() + } +} + +impl<T: Copy> Cell<T> { + /// Returns a copy of the contained value. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let c = Cell::new(5); + /// + /// let five = c.get(); + /// ``` + #[inline] + #[stable(feature = "rust1", since = "1.0.0")] + pub fn get(&self) -> T { + // SAFETY: This can cause data races if called from a separate thread, + // but `Cell` is `!Sync` so this won't happen. + unsafe { *self.value.get() } + } + + /// Updates the contained value using a function and returns the new value. + /// + /// # Examples + /// + /// ``` + /// #![feature(cell_update)] + /// + /// use std::cell::Cell; + /// + /// let c = Cell::new(5); + /// let new = c.update(|x| x + 1); + /// + /// assert_eq!(new, 6); + /// assert_eq!(c.get(), 6); + /// ``` + #[inline] + #[unstable(feature = "cell_update", issue = "50186")] + pub fn update<F>(&self, f: F) -> T + where + F: FnOnce(T) -> T, + { + let old = self.get(); + let new = f(old); + self.set(new); + new + } +} + +impl<T: ?Sized> Cell<T> { + /// Returns a raw pointer to the underlying data in this cell. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let c = Cell::new(5); + /// + /// let ptr = c.as_ptr(); + /// ``` + #[inline] + #[stable(feature = "cell_as_ptr", since = "1.12.0")] + #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")] + pub const fn as_ptr(&self) -> *mut T { + self.value.get() + } + + /// Returns a mutable reference to the underlying data. + /// + /// This call borrows `Cell` mutably (at compile-time) which guarantees + /// that we possess the only reference. + /// + /// However be cautious: this method expects `self` to be mutable, which is + /// generally not the case when using a `Cell`. If you require interior + /// mutability by reference, consider using `RefCell` which provides + /// run-time checked mutable borrows through its [`borrow_mut`] method. + /// + /// [`borrow_mut`]: RefCell::borrow_mut() + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let mut c = Cell::new(5); + /// *c.get_mut() += 1; + /// + /// assert_eq!(c.get(), 6); + /// ``` + #[inline] + #[stable(feature = "cell_get_mut", since = "1.11.0")] + pub fn get_mut(&mut self) -> &mut T { + self.value.get_mut() + } + + /// Returns a `&Cell<T>` from a `&mut T` + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let slice: &mut [i32] = &mut [1, 2, 3]; + /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); + /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); + /// + /// assert_eq!(slice_cell.len(), 3); + /// ``` + #[inline] + #[stable(feature = "as_cell", since = "1.37.0")] + pub fn from_mut(t: &mut T) -> &Cell<T> { + // SAFETY: `&mut` ensures unique access. + unsafe { &*(t as *mut T as *const Cell<T>) } + } +} + +impl<T: Default> Cell<T> { + /// Takes the value of the cell, leaving `Default::default()` in its place. + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let c = Cell::new(5); + /// let five = c.take(); + /// + /// assert_eq!(five, 5); + /// assert_eq!(c.into_inner(), 0); + /// ``` + #[stable(feature = "move_cell", since = "1.17.0")] + pub fn take(&self) -> T { + self.replace(Default::default()) + } +} + +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {} + +impl<T> Cell<[T]> { + /// Returns a `&[Cell<T>]` from a `&Cell<[T]>` + /// + /// # Examples + /// + /// ``` + /// use std::cell::Cell; + /// + /// let slice: &mut [i32] = &mut [1, 2, 3]; + /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); + /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); + /// + /// assert_eq!(slice_cell.len(), 3); + /// ``` + #[stable(feature = "as_cell", since = "1.37.0")] + pub fn as_slice_of_cells(&self) -> &[Cell<T>] { + // SAFETY: `Cell<T>` has the same memory layout as `T`. + unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) } + } +} + +impl<T, const N: usize> Cell<[T; N]> { + /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>` + /// + /// # Examples + /// + /// ``` + /// #![feature(as_array_of_cells)] + /// 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(); + /// ``` + #[unstable(feature = "as_array_of_cells", issue = "88248")] + pub 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]) } + } +} + +/// A mutable memory location with dynamically checked borrow rules +/// +/// See the [module-level documentation](self) for more. +#[stable(feature = "rust1", since = "1.0.0")] +pub struct RefCell<T: ?Sized> { + borrow: Cell<BorrowFlag>, + // Stores the location of the earliest currently active borrow. + // This gets updated 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] +pub struct BorrowError { + #[cfg(feature = "debug_refcell")] + location: &'static crate::panic::Location<'static>, +} + +#[stable(feature = "try_borrow", since = "1.13.0")] +impl Debug for BorrowError { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + let mut builder = f.debug_struct("BorrowError"); + + #[cfg(feature = "debug_refcell")] + builder.field("location", self.location); + + builder.finish() + } +} + +#[stable(feature = "try_borrow", since = "1.13.0")] +impl Display for BorrowError { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + Display::fmt("already mutably borrowed", f) + } +} + +/// An error returned by [`RefCell::try_borrow_mut`]. +#[stable(feature = "try_borrow", since = "1.13.0")] +#[non_exhaustive] +pub struct BorrowMutError { + #[cfg(feature = "debug_refcell")] + location: &'static crate::panic::Location<'static>, +} + +#[stable(feature = "try_borrow", since = "1.13.0")] +impl Debug for BorrowMutError { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + let mut builder = f.debug_struct("BorrowMutError"); + + #[cfg(feature = "debug_refcell")] + builder.field("location", self.location); + + builder.finish() + } +} + +#[stable(feature = "try_borrow", since = "1.13.0")] +impl Display for BorrowMutError { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + Display::fmt("already borrowed", f) + } +} + +// Positive values represent the number of `Ref` active. Negative values +// represent the number of `RefMut` active. Multiple `RefMut`s can only be +// active at a time if they refer to distinct, nonoverlapping components of a +// `RefCell` (e.g., different ranges of a slice). +// +// `Ref` and `RefMut` are both two words in size, and so there will likely never +// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` +// range. Thus, a `BorrowFlag` will probably never overflow or underflow. +// However, this is not a guarantee, as a pathological program could repeatedly +// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must +// explicitly check for overflow and underflow in order to avoid unsafety, or at +// least behave correctly in the event that overflow or underflow happens (e.g., +// see BorrowRef::new). +type BorrowFlag = isize; +const UNUSED: BorrowFlag = 0; + +#[inline(always)] +fn is_writing(x: BorrowFlag) -> bool { + x < UNUSED +} + +#[inline(always)] +fn is_reading(x: BorrowFlag) -> bool { + x > UNUSED +} + +impl<T> RefCell<T> { + /// Creates a new `RefCell` containing `value`. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")] + #[inline] + pub const fn new(value: T) -> RefCell<T> { + RefCell { + value: UnsafeCell::new(value), + borrow: Cell::new(UNUSED), + #[cfg(feature = "debug_refcell")] + borrowed_at: Cell::new(None), + } + } + + /// Consumes the `RefCell`, returning the wrapped value. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// let five = c.into_inner(); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] + #[inline] + pub const fn into_inner(self) -> T { + // Since this function takes `self` (the `RefCell`) by value, the + // compiler statically verifies that it is not currently borrowed. + self.value.into_inner() + } + + /// Replaces the wrapped value with a new one, returning the old value, + /// without deinitializing either one. + /// + /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). + /// + /// # Panics + /// + /// Panics if the value is currently borrowed. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// let cell = RefCell::new(5); + /// let old_value = cell.replace(6); + /// assert_eq!(old_value, 5); + /// assert_eq!(cell, RefCell::new(6)); + /// ``` + #[inline] + #[stable(feature = "refcell_replace", since = "1.24.0")] + #[track_caller] + pub fn replace(&self, t: T) -> T { + mem::replace(&mut *self.borrow_mut(), t) + } + + /// Replaces the wrapped value with a new one computed from `f`, returning + /// the old value, without deinitializing either one. + /// + /// # Panics + /// + /// Panics if the value is currently borrowed. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// let cell = RefCell::new(5); + /// let old_value = cell.replace_with(|&mut old| old + 1); + /// assert_eq!(old_value, 5); + /// assert_eq!(cell, RefCell::new(6)); + /// ``` + #[inline] + #[stable(feature = "refcell_replace_swap", since = "1.35.0")] + #[track_caller] + pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { + let mut_borrow = &mut *self.borrow_mut(); + let replacement = f(mut_borrow); + mem::replace(mut_borrow, replacement) + } + + /// Swaps the wrapped value of `self` with the wrapped value of `other`, + /// without deinitializing either one. + /// + /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). + /// + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// let c = RefCell::new(5); + /// let d = RefCell::new(6); + /// c.swap(&d); + /// assert_eq!(c, RefCell::new(6)); + /// assert_eq!(d, RefCell::new(5)); + /// ``` + #[inline] + #[stable(feature = "refcell_swap", since = "1.24.0")] + pub fn swap(&self, other: &Self) { + mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) + } +} + +impl<T: ?Sized> RefCell<T> { + /// Immutably borrows the wrapped value. + /// + /// The borrow lasts until the returned `Ref` exits scope. Multiple + /// immutable borrows can be taken out at the same time. + /// + /// # Panics + /// + /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use + /// [`try_borrow`](#method.try_borrow). + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// let borrowed_five = c.borrow(); + /// let borrowed_five2 = c.borrow(); + /// ``` + /// + /// An example of panic: + /// + /// ```should_panic + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// let m = c.borrow_mut(); + /// let b = c.borrow(); // this causes a panic + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + #[inline] + #[track_caller] + pub fn borrow(&self) -> Ref<'_, T> { + self.try_borrow().expect("already mutably borrowed") + } + + /// Immutably borrows the wrapped value, returning an error if the value is currently mutably + /// borrowed. + /// + /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be + /// taken out at the same time. + /// + /// This is the non-panicking variant of [`borrow`](#method.borrow). + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// { + /// let m = c.borrow_mut(); + /// assert!(c.try_borrow().is_err()); + /// } + /// + /// { + /// let m = c.borrow(); + /// assert!(c.try_borrow().is_ok()); + /// } + /// ``` + #[stable(feature = "try_borrow", since = "1.13.0")] + #[inline] + #[cfg_attr(feature = "debug_refcell", track_caller)] + pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { + match BorrowRef::new(&self.borrow) { + Some(b) => { + #[cfg(feature = "debug_refcell")] + { + // `borrowed_at` is always the *first* active borrow + if b.borrow.get() == 1 { + self.borrowed_at.set(Some(crate::panic::Location::caller())); + } + } + + // SAFETY: `BorrowRef` ensures that there is only immutable access + // to the value while borrowed. + 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] + pub fn borrow_mut(&self) -> RefMut<'_, T> { + self.try_borrow_mut().expect("already borrowed") + } + + /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. + /// + /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived + /// from it exit scope. The value cannot be borrowed while this borrow is + /// active. + /// + /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// { + /// let m = c.borrow(); + /// assert!(c.try_borrow_mut().is_err()); + /// } + /// + /// assert!(c.try_borrow_mut().is_ok()); + /// ``` + #[stable(feature = "try_borrow", since = "1.13.0")] + #[inline] + #[cfg_attr(feature = "debug_refcell", track_caller)] + pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { + match BorrowRefMut::new(&self.borrow) { + Some(b) => { + #[cfg(feature = "debug_refcell")] + { + self.borrowed_at.set(Some(crate::panic::Location::caller())); + } + + // SAFETY: `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")] + pub fn as_ptr(&self) -> *mut T { + self.value.get() + } + + /// Returns a mutable reference to the underlying data. + /// + /// This call borrows `RefCell` mutably (at compile-time) so there is no + /// need for dynamic checks. + /// + /// However be cautious: this method expects `self` to be mutable, which is + /// generally not the case when using a `RefCell`. Take a look at the + /// [`borrow_mut`] method instead if `self` isn't mutable. + /// + /// Also, please be aware that this method is only for special circumstances and is usually + /// not what you want. In case of doubt, use [`borrow_mut`] instead. + /// + /// [`borrow_mut`]: RefCell::borrow_mut() + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let mut c = RefCell::new(5); + /// *c.get_mut() += 1; + /// + /// assert_eq!(c, RefCell::new(6)); + /// ``` + #[inline] + #[stable(feature = "cell_get_mut", since = "1.11.0")] + pub fn get_mut(&mut self) -> &mut T { + self.value.get_mut() + } + + /// Undo the effect of leaked guards on the borrow state of the `RefCell`. + /// + /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to + /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant + /// if some `Ref` or `RefMut` borrows have been leaked. + /// + /// [`get_mut`]: RefCell::get_mut() + /// + /// # Examples + /// + /// ``` + /// #![feature(cell_leak)] + /// use std::cell::RefCell; + /// + /// let mut c = RefCell::new(0); + /// std::mem::forget(c.borrow_mut()); + /// + /// assert!(c.try_borrow().is_err()); + /// c.undo_leak(); + /// assert!(c.try_borrow().is_ok()); + /// ``` + #[unstable(feature = "cell_leak", issue = "69099")] + pub fn undo_leak(&mut self) -> &mut T { + *self.borrow.get_mut() = UNUSED; + self.get_mut() + } + + /// Immutably borrows the wrapped value, returning an error if the value is + /// currently mutably borrowed. + /// + /// # Safety + /// + /// Unlike `RefCell::borrow`, this method is unsafe because it does not + /// return a `Ref`, thus leaving the borrow flag untouched. Mutably + /// borrowing the `RefCell` while the reference returned by this method + /// is alive is undefined behaviour. + /// + /// # Examples + /// + /// ``` + /// use std::cell::RefCell; + /// + /// let c = RefCell::new(5); + /// + /// { + /// let m = c.borrow_mut(); + /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); + /// } + /// + /// { + /// let m = c.borrow(); + /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); + /// } + /// ``` + #[stable(feature = "borrow_state", since = "1.37.0")] + #[inline] + pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { + if !is_writing(self.borrow.get()) { + // SAFETY: We check that nobody is actively writing now, but it is + // the caller's responsibility to ensure that nobody writes until + // the returned reference is no longer in use. + // Also, `self.value.get()` refers to the value owned by `self` + // and is thus guaranteed to be valid for the lifetime of `self`. + Ok(unsafe { &*self.value.get() }) + } else { + Err(BorrowError { + // If a borrow 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 `other` is currently mutably borrowed. + #[inline] + #[track_caller] + fn clone_from(&mut self, other: &Self) { + self.get_mut().clone_from(&other.borrow()) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: Default> Default for RefCell<T> { + /// Creates a `RefCell<T>`, with the `Default` value for T. + #[inline] + fn default() -> RefCell<T> { + RefCell::new(Default::default()) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn eq(&self, other: &RefCell<T>) -> bool { + *self.borrow() == *other.borrow() + } +} + +#[stable(feature = "cell_eq", since = "1.2.0")] +impl<T: ?Sized + Eq> Eq for RefCell<T> {} + +#[stable(feature = "cell_ord", since = "1.10.0")] +impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { + self.borrow().partial_cmp(&*other.borrow()) + } + + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn lt(&self, other: &RefCell<T>) -> bool { + *self.borrow() < *other.borrow() + } + + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn le(&self, other: &RefCell<T>) -> bool { + *self.borrow() <= *other.borrow() + } + + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn gt(&self, other: &RefCell<T>) -> bool { + *self.borrow() > *other.borrow() + } + + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn ge(&self, other: &RefCell<T>) -> bool { + *self.borrow() >= *other.borrow() + } +} + +#[stable(feature = "cell_ord", since = "1.10.0")] +impl<T: ?Sized + Ord> Ord for RefCell<T> { + /// # Panics + /// + /// Panics if the value in either `RefCell` is currently borrowed. + #[inline] + fn cmp(&self, other: &RefCell<T>) -> Ordering { + self.borrow().cmp(&*other.borrow()) + } +} + +#[stable(feature = "cell_from", since = "1.12.0")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +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 = "27732")] +impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} + +struct BorrowRef<'b> { + borrow: &'b Cell<BorrowFlag>, +} + +impl<'b> BorrowRef<'b> { + #[inline] + fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { + let b = borrow.get().wrapping_add(1); + if !is_reading(b) { + // Incrementing borrow can result in a non-reading value (<= 0) in these cases: + // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow + // due to Rust's reference aliasing rules + // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed + // into isize::MIN (the max amount of writing borrows) so we can't allow + // an additional read borrow because isize can't represent so many read borrows + // (this can only happen if you mem::forget more than a small constant amount of + // `Ref`s, which is not good practice) + None + } else { + // Incrementing borrow can result in a reading value (> 0) in these cases: + // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow + // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize + // is large enough to represent having one more read borrow + borrow.set(b); + Some(BorrowRef { borrow }) + } + } +} + +impl Drop for BorrowRef<'_> { + #[inline] + fn drop(&mut self) { + let borrow = self.borrow.get(); + debug_assert!(is_reading(borrow)); + self.borrow.set(borrow - 1); + } +} + +impl Clone for BorrowRef<'_> { + #[inline] + fn clone(&self) -> Self { + // Since this Ref exists, we know the borrow flag + // is a reading borrow. + let borrow = self.borrow.get(); + debug_assert!(is_reading(borrow)); + // Prevent the borrow counter from overflowing into + // a writing borrow. + assert!(borrow != isize::MAX); + self.borrow.set(borrow + 1); + BorrowRef { borrow: self.borrow } + } +} + +/// Wraps a borrowed reference to a value in a `RefCell` box. +/// A wrapper type for an immutably borrowed value from a `RefCell<T>`. +/// +/// See the [module-level documentation](self) for more. +#[stable(feature = "rust1", since = "1.0.0")] +#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"] +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")] +impl<T: ?Sized> 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() } + } +} + +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] + pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { + Ref { value: orig.value, borrow: orig.borrow.clone() } + } + + /// Makes a new `Ref` for a component of the borrowed data. + /// + /// The `RefCell` is already immutably borrowed, so this cannot fail. + /// + /// This is an associated function that needs to be used as `Ref::map(...)`. + /// A method would interfere with methods of the same name on the contents + /// of a `RefCell` used through `Deref`. + /// + /// # Examples + /// + /// ``` + /// use std::cell::{RefCell, Ref}; + /// + /// let c = RefCell::new((5, 'b')); + /// let b1: Ref<(u32, char)> = c.borrow(); + /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); + /// assert_eq!(*b2, 5) + /// ``` + #[stable(feature = "cell_map", since = "1.8.0")] + #[inline] + pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> + where + F: FnOnce(&T) -> &U, + { + Ref { value: 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), + } + } + + /// 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 }, + ) + } + + /// Convert into a reference to the underlying data. + /// + /// The underlying `RefCell` can never be mutably borrowed from again and will always appear + /// already immutably borrowed. It is not a good idea to leak more than a constant number of + /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks + /// have occurred in total. + /// + /// This is an associated function that needs to be used as + /// `Ref::leak(...)`. A method would interfere with methods of the + /// same name on the contents of a `RefCell` used through `Deref`. + /// + /// # Examples + /// + /// ``` + /// #![feature(cell_leak)] + /// use std::cell::{RefCell, Ref}; + /// let cell = RefCell::new(0); + /// + /// let value = Ref::leak(cell.borrow()); + /// assert_eq!(*value, 0); + /// + /// assert!(cell.try_borrow().is_ok()); + /// assert!(cell.try_borrow_mut().is_err()); + /// ``` + #[unstable(feature = "cell_leak", issue = "69099")] + pub fn leak(orig: Ref<'b, T>) -> &'b T { + // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to + // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a + // unique reference to the borrowed RefCell. No further mutable references can be created + // from the original cell. + mem::forget(orig.borrow); + // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`. + unsafe { orig.value.as_ref() } + } +} + +#[unstable(feature = "coerce_unsized", issue = "27732")] +impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} + +#[stable(feature = "std_guard_impls", since = "1.20.0")] +impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + (**self).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), + } + } + + /// 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 }, + ) + } + + /// Convert into a mutable reference to the underlying data. + /// + /// The underlying `RefCell` can not be borrowed from again and will always appear already + /// mutably borrowed, making the returned reference the only to the interior. + /// + /// This is an associated function that needs to be used as + /// `RefMut::leak(...)`. A method would interfere with methods of the + /// same name on the contents of a `RefCell` used through `Deref`. + /// + /// # Examples + /// + /// ``` + /// #![feature(cell_leak)] + /// use std::cell::{RefCell, RefMut}; + /// let cell = RefCell::new(0); + /// + /// let value = RefMut::leak(cell.borrow_mut()); + /// assert_eq!(*value, 0); + /// *value = 1; + /// + /// assert!(cell.try_borrow_mut().is_err()); + /// ``` + #[unstable(feature = "cell_leak", issue = "69099")] + pub fn leak(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<BorrowFlag>, +} + +impl Drop for BorrowRefMut<'_> { + #[inline] + fn drop(&mut self) { + let borrow = self.borrow.get(); + debug_assert!(is_writing(borrow)); + self.borrow.set(borrow + 1); + } +} + +impl<'b> BorrowRefMut<'b> { + #[inline] + fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { + // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial + // mutable reference, and so there must currently be no existing + // references. Thus, while clone increments the mutable refcount, here + // we explicitly only allow going from UNUSED to UNUSED - 1. + match borrow.get() { + UNUSED => { + borrow.set(UNUSED - 1); + Some(BorrowRefMut { borrow }) + } + _ => None, + } + } + + // Clones a `BorrowRefMut`. + // + // This is only valid if each `BorrowRefMut` is used to track a mutable + // reference to a distinct, nonoverlapping range of the original object. + // This isn't in a Clone impl so that code doesn't call this implicitly. + #[inline] + fn clone(&self) -> BorrowRefMut<'b> { + let borrow = self.borrow.get(); + debug_assert!(is_writing(borrow)); + // Prevent the borrow counter from underflowing. + assert!(borrow != isize::MIN); + self.borrow.set(borrow - 1); + BorrowRefMut { borrow: self.borrow } + } +} + +/// A wrapper type for a mutably borrowed value from a `RefCell<T>`. +/// +/// See the [module-level documentation](self) for more. +#[stable(feature = "rust1", since = "1.0.0")] +#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"] +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")] +impl<T: ?Sized> 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")] +impl<T: ?Sized> 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 = "coerce_unsized", issue = "27732")] +impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} + +#[stable(feature = "std_guard_impls", since = "1.20.0")] +impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + (**self).fmt(f) + } +} + +/// The core primitive for interior mutability in Rust. +/// +/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on +/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an +/// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior. +/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference +/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability". +/// +/// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally +/// use `UnsafeCell` to wrap their data. +/// +/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The +/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain +/// aliasing `&mut`, not even with `UnsafeCell<T>`. +/// +/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer +/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer +/// correctly. +/// +/// [`.get()`]: `UnsafeCell::get` +/// +/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: +/// +/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), 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 deallocted 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. +/// +/// - At all times, you must avoid data races. If multiple threads have access to +/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other +/// accesses (or use atomics). +/// +/// To assist with proper design, the following scenarios are explicitly declared legal +/// for single-threaded code: +/// +/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` +/// references, but not with a `&mut T` +/// +/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` +/// co-exist with it. A `&mut T` must always be unique. +/// +/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other +/// `&UnsafeCell<T>` references alias the cell) is +/// ok (provided you enforce the above invariants some other way), it is still undefined behavior +/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper +/// designed to have a special interaction with _shared_ accesses (_i.e._, through an +/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_ +/// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value +/// may be aliased for the duration of that `&mut` borrow. +/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields +/// a `&mut T`. +/// +/// [`.get_mut()`]: `UnsafeCell::get_mut` +/// +/// # Examples +/// +/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite +/// there being multiple references aliasing the cell: +/// +/// ``` +/// use std::cell::UnsafeCell; +/// +/// let x: UnsafeCell<i32> = 42.into(); +/// // Get multiple / concurrent / shared references to the same `x`. +/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x); +/// +/// unsafe { +/// // SAFETY: within this scope there are no other references to `x`'s contents, +/// // so ours is effectively unique. +/// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+ +/// *p1_exclusive += 27; // | +/// } // <---------- cannot go beyond this point -------------------+ +/// +/// unsafe { +/// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents, +/// // so we can have multiple shared accesses concurrently. +/// let p2_shared: &i32 = &*p2.get(); +/// assert_eq!(*p2_shared, 42 + 27); +/// let p1_shared: &i32 = &*p1.get(); +/// assert_eq!(*p1_shared, *p2_shared); +/// } +/// ``` +/// +/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>` +/// implies exclusive access to its `T`: +/// +/// ```rust +/// #![forbid(unsafe_code)] // with exclusive accesses, +/// // `UnsafeCell` is a transparent no-op wrapper, +/// // so no need for `unsafe` here. +/// use std::cell::UnsafeCell; +/// +/// let mut x: UnsafeCell<i32> = 42.into(); +/// +/// // Get a compile-time-checked unique reference to `x`. +/// let p_unique: &mut UnsafeCell<i32> = &mut x; +/// // With an exclusive reference, we can mutate the contents for free. +/// *p_unique.get_mut() = 0; +/// // Or, equivalently: +/// x = UnsafeCell::new(0); +/// +/// // When we own the value, we can extract the contents for free. +/// let contents: i32 = x.into_inner(); +/// assert_eq!(contents, 0); +/// ``` +#[lang = "unsafe_cell"] +#[stable(feature = "rust1", since = "1.0.0")] +#[repr(transparent)] +pub struct UnsafeCell<T: ?Sized> { + value: T, +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized> !Sync for UnsafeCell<T> {} + +impl<T> UnsafeCell<T> { + /// Constructs a new instance of `UnsafeCell` which will wrap the specified + /// value. + /// + /// All access to the inner value through methods is `unsafe`. + /// + /// # Examples + /// + /// ``` + /// use std::cell::UnsafeCell; + /// + /// let uc = UnsafeCell::new(5); + /// ``` + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")] + #[inline(always)] + pub const fn new(value: T) -> UnsafeCell<T> { + UnsafeCell { value } + } + + /// Unwraps the value. + /// + /// # Examples + /// + /// ``` + /// use std::cell::UnsafeCell; + /// + /// let uc = UnsafeCell::new(5); + /// + /// let five = uc.into_inner(); + /// ``` + #[inline(always)] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] + pub const fn into_inner(self) -> T { + self.value + } +} + +impl<T: ?Sized> UnsafeCell<T> { + /// Gets a mutable pointer to the wrapped value. + /// + /// This can be cast to a pointer of any kind. + /// Ensure that the access is unique (no active references, mutable or not) + /// when casting to `&mut T`, and ensure that there are no mutations + /// or mutable aliases going on when casting to `&T` + /// + /// # Examples + /// + /// ``` + /// use std::cell::UnsafeCell; + /// + /// let uc = UnsafeCell::new(5); + /// + /// let five = uc.get(); + /// ``` + #[inline(always)] + #[stable(feature = "rust1", since = "1.0.0")] + #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")] + pub const fn get(&self) -> *mut T { + // We can just cast the pointer from `UnsafeCell<T>` to `T` because of + // #[repr(transparent)]. This exploits libstd's special status, there is + // no guarantee for user code that this will work in future versions of the compiler! + self as *const UnsafeCell<T> as *const T as *mut T + } + + /// Returns a mutable reference to the underlying data. + /// + /// This call borrows the `UnsafeCell` mutably (at compile-time) which + /// guarantees that we possess the only reference. + /// + /// # Examples + /// + /// ``` + /// use std::cell::UnsafeCell; + /// + /// let mut c = UnsafeCell::new(5); + /// *c.get_mut() += 1; + /// + /// assert_eq!(*c.get_mut(), 6); + /// ``` + #[inline(always)] + #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")] + #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")] + 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. + /// + /// The result can be cast to a pointer of any kind. + /// Ensure that the access is unique (no active references, mutable or not) + /// when casting to `&mut T`, and ensure that there are no mutations + /// or mutable aliases going on when casting to `&T`. + /// + /// [`get`]: UnsafeCell::get() + /// + /// # Examples + /// + /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as + /// calling `get` would require creating a reference to uninitialized data: + /// + /// ``` + /// use std::cell::UnsafeCell; + /// use std::mem::MaybeUninit; + /// + /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit(); + /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); } + /// let uc = unsafe { m.assume_init() }; + /// + /// assert_eq!(uc.into_inner(), 5); + /// ``` + #[inline(always)] + #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")] + #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")] + pub const fn raw_get(this: *const Self) -> *mut T { + // We can just cast the pointer from `UnsafeCell<T>` to `T` because of + // #[repr(transparent)]. This exploits libstd's special status, there is + // no guarantee for user code that this will work in future versions of the compiler! + this as *const T as *mut T + } +} + +#[stable(feature = "unsafe_cell_default", since = "1.10.0")] +impl<T: Default> Default for UnsafeCell<T> { + /// Creates an `UnsafeCell`, with the `Default` value for T. + fn default() -> UnsafeCell<T> { + UnsafeCell::new(Default::default()) + } +} + +#[stable(feature = "cell_from", since = "1.12.0")] +#[rustc_const_unstable(feature = "const_convert", issue = "88674")] +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 = "27732")] +impl<T: CoerceUnsized<U>, U> CoerceUnsized<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)] +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. + #[inline] + 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] + 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")] +impl<T: Default> 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 = "88674")] +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 = "27732")] +//#[unstable(feature = "sync_unsafe_cell", issue = "95439")] +impl<T: CoerceUnsized<U>, U> CoerceUnsized<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; +} |