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//! 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) that is accessible by safe code (for example, because you returned it),
/// then you must not access the data in any way that contradicts that reference for the
/// remainder of `'a`. For example, this means that if you take the `*mut T` from an
/// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
/// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
/// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
/// safe code, then you must not access the data within the `UnsafeCell` until that
/// reference expires.
///
/// - At all times, you must avoid data races. If multiple threads have access to
/// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
/// accesses (or use atomics).
///
/// To assist with proper design, the following scenarios are explicitly declared legal
/// for single-threaded code:
///
/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
/// references, but not with a `&mut T`
///
/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
/// co-exist with it. A `&mut T` must always be unique.
///
/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
/// `&UnsafeCell<T>` references alias the cell) is
/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
/// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
/// may be aliased for the duration of that `&mut` borrow.
/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
/// a `&mut T`.
///
/// [`.get_mut()`]: `UnsafeCell::get_mut`
///
/// # Examples
///
/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
/// there being multiple references aliasing the cell:
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let x: UnsafeCell<i32> = 42.into();
/// // Get multiple / concurrent / shared references to the same `x`.
/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
///
/// unsafe {
/// // SAFETY: within this scope there are no other references to `x`'s contents,
/// // so ours is effectively unique.
/// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
/// *p1_exclusive += 27; // |
/// } // <---------- cannot go beyond this point -------------------+
///
/// unsafe {
/// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
/// // so we can have multiple shared accesses concurrently.
/// let p2_shared: &i32 = &*p2.get();
/// assert_eq!(*p2_shared, 42 + 27);
/// let p1_shared: &i32 = &*p1.get();
/// assert_eq!(*p1_shared, *p2_shared);
/// }
/// ```
///
/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
/// implies exclusive access to its `T`:
///
/// ```rust
/// #![forbid(unsafe_code)] // with exclusive accesses,
/// // `UnsafeCell` is a transparent no-op wrapper,
/// // so no need for `unsafe` here.
/// use std::cell::UnsafeCell;
///
/// let mut x: UnsafeCell<i32> = 42.into();
///
/// // Get a compile-time-checked unique reference to `x`.
/// let p_unique: &mut UnsafeCell<i32> = &mut x;
/// // With an exclusive reference, we can mutate the contents for free.
/// *p_unique.get_mut() = 0;
/// // Or, equivalently:
/// x = UnsafeCell::new(0);
///
/// // When we own the value, we can extract the contents for free.
/// let contents: i32 = x.into_inner();
/// assert_eq!(contents, 0);
/// ```
#[lang = "unsafe_cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
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;
}