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nest-open-source / manifest_repos / toolchain / 6143c57c644e0da7c4a10d1b4af322f92e74e6f7 / . / arm / usr / lib / rustlib / src / rust / library / alloc / src / rc.rs
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//! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
//! Counted'.
//!
//! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
//! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
//! pointer to the same allocation in the heap. When the last [`Rc`] pointer to a
//! given allocation is destroyed, the value stored in that allocation (often
//! referred to as "inner value") is also dropped.
//!
//! Shared references in Rust disallow mutation by default, and [`Rc`]
//! is no exception: you cannot generally obtain a mutable reference to
//! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
//! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
//! inside an `Rc`][mutability].
//!
//! [`Rc`] uses non-atomic reference counting. This means that overhead is very
//! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
//! does not implement [`Send`][send]. As a result, the Rust compiler
//! will check *at compile time* that you are not sending [`Rc`]s between
//! threads. If you need multi-threaded, atomic reference counting, use
//! [`sync::Arc`][arc].
//!
//! The [`downgrade`][downgrade] method can be used to create a non-owning
//! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
//! to an [`Rc`], but this will return [`None`] if the value stored in the allocation has
//! already been dropped. In other words, `Weak` pointers do not keep the value
//! inside the allocation alive; however, they *do* keep the allocation
//! (the backing store for the inner value) alive.
//!
//! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
//! [`Weak`] is used to break cycles. For example, a tree could have strong
//! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
//! children back to their parents.
//!
//! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
//! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
//! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are associated
//! functions, called using [fully qualified syntax]:
//!
//! ```
//! use std::rc::Rc;
//!
//! let my_rc = Rc::new(());
//! let my_weak = Rc::downgrade(&my_rc);
//! ```
//!
//! `Rc<T>`'s implementations of traits like `Clone` may also be called using
//! fully qualified syntax. Some people prefer to use fully qualified syntax,
//! while others prefer using method-call syntax.
//!
//! ```
//! use std::rc::Rc;
//!
//! let rc = Rc::new(());
//! // Method-call syntax
//! let rc2 = rc.clone();
//! // Fully qualified syntax
//! let rc3 = Rc::clone(&rc);
//! ```
//!
//! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the inner value may have
//! already been dropped.
//!
//! # Cloning references
//!
//! Creating a new reference to the same allocation as an existing reference counted pointer
//! is done using the `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
//!
//! ```
//! use std::rc::Rc;
//!
//! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
//! // The two syntaxes below are equivalent.
//! let a = foo.clone();
//! let b = Rc::clone(&foo);
//! // a and b both point to the same memory location as foo.
//! ```
//!
//! The `Rc::clone(&from)` syntax is the most idiomatic because it conveys more explicitly
//! the meaning of the code. In the example above, this syntax makes it easier to see that
//! this code is creating a new reference rather than copying the whole content of foo.
//!
//! # Examples
//!
//! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
//! We want to have our `Gadget`s point to their `Owner`. We can't do this with
//! unique ownership, because more than one gadget may belong to the same
//! `Owner`. [`Rc`] allows us to share an `Owner` between multiple `Gadget`s,
//! and have the `Owner` remain allocated as long as any `Gadget` points at it.
//!
//! ```
//! use std::rc::Rc;
//!
//! struct Owner {
//! name: String,
//! // ...other fields
//! }
//!
//! struct Gadget {
//! id: i32,
//! owner: Rc<Owner>,
//! // ...other fields
//! }
//!
//! fn main() {
//! // Create a reference-counted `Owner`.
//! let gadget_owner: Rc<Owner> = Rc::new(
//! Owner {
//! name: "Gadget Man".to_string(),
//! }
//! );
//!
//! // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
//! // gives us a new pointer to the same `Owner` allocation, incrementing
//! // the reference count in the process.
//! let gadget1 = Gadget {
//! id: 1,
//! owner: Rc::clone(&gadget_owner),
//! };
//! let gadget2 = Gadget {
//! id: 2,
//! owner: Rc::clone(&gadget_owner),
//! };
//!
//! // Dispose of our local variable `gadget_owner`.
//! drop(gadget_owner);
//!
//! // Despite dropping `gadget_owner`, we're still able to print out the name
//! // of the `Owner` of the `Gadget`s. This is because we've only dropped a
//! // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
//! // other `Rc<Owner>` pointing at the same `Owner` allocation, it will remain
//! // live. The field projection `gadget1.owner.name` works because
//! // `Rc<Owner>` automatically dereferences to `Owner`.
//! println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
//! println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
//!
//! // At the end of the function, `gadget1` and `gadget2` are destroyed, and
//! // with them the last counted references to our `Owner`. Gadget Man now
//! // gets destroyed as well.
//! }
//! ```
//!
//! If our requirements change, and we also need to be able to traverse from
//! `Owner` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
//! to `Gadget` introduces a cycle. This means that their
//! reference counts can never reach 0, and the allocation will never be destroyed:
//! a memory leak. In order to get around this, we can use [`Weak`]
//! pointers.
//!
//! Rust actually makes it somewhat difficult to produce this loop in the first
//! place. In order to end up with two values that point at each other, one of
//! them needs to be mutable. This is difficult because [`Rc`] enforces
//! memory safety by only giving out shared references to the value it wraps,
//! and these don't allow direct mutation. We need to wrap the part of the
//! value we wish to mutate in a [`RefCell`], which provides *interior
//! mutability*: a method to achieve mutability through a shared reference.
//! [`RefCell`] enforces Rust's borrowing rules at runtime.
//!
//! ```
//! use std::rc::Rc;
//! use std::rc::Weak;
//! use std::cell::RefCell;
//!
//! struct Owner {
//! name: String,
//! gadgets: RefCell<Vec<Weak<Gadget>>>,
//! // ...other fields
//! }
//!
//! struct Gadget {
//! id: i32,
//! owner: Rc<Owner>,
//! // ...other fields
//! }
//!
//! fn main() {
//! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
//! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
//! // a shared reference.
//! let gadget_owner: Rc<Owner> = Rc::new(
//! Owner {
//! name: "Gadget Man".to_string(),
//! gadgets: RefCell::new(vec![]),
//! }
//! );
//!
//! // Create `Gadget`s belonging to `gadget_owner`, as before.
//! let gadget1 = Rc::new(
//! Gadget {
//! id: 1,
//! owner: Rc::clone(&gadget_owner),
//! }
//! );
//! let gadget2 = Rc::new(
//! Gadget {
//! id: 2,
//! owner: Rc::clone(&gadget_owner),
//! }
//! );
//!
//! // Add the `Gadget`s to their `Owner`.
//! {
//! let mut gadgets = gadget_owner.gadgets.borrow_mut();
//! gadgets.push(Rc::downgrade(&gadget1));
//! gadgets.push(Rc::downgrade(&gadget2));
//!
//! // `RefCell` dynamic borrow ends here.
//! }
//!
//! // Iterate over our `Gadget`s, printing their details out.
//! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
//!
//! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
//! // guarantee the allocation still exists, we need to call
//! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
//! //
//! // In this case we know the allocation still exists, so we simply
//! // `unwrap` the `Option`. In a more complicated program, you might
//! // need graceful error handling for a `None` result.
//!
//! let gadget = gadget_weak.upgrade().unwrap();
//! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
//! }
//!
//! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
//! // are destroyed. There are now no strong (`Rc`) pointers to the
//! // gadgets, so they are destroyed. This zeroes the reference count on
//! // Gadget Man, so he gets destroyed as well.
//! }
//! ```
//!
//! [clone]: Clone::clone
//! [`Cell`]: core::cell::Cell
//! [`RefCell`]: core::cell::RefCell
//! [send]: core::marker::Send
//! [arc]: crate::sync::Arc
//! [`Deref`]: core::ops::Deref
//! [downgrade]: Rc::downgrade
//! [upgrade]: Weak::upgrade
//! [mutability]: core::cell#introducing-mutability-inside-of-something-immutable
//! [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
#![stable(feature = "rust1", since = "1.0.0")]
#[cfg(not(test))]
use crate::boxed::Box;
#[cfg(test)]
use std::boxed::Box;
use core::any::Any;
use core::borrow;
use core::cell::Cell;
use core::cmp::Ordering;
use core::convert::{From, TryFrom};
use core::fmt;
use core::hash::{Hash, Hasher};
use core::intrinsics::abort;
#[cfg(not(no_global_oom_handling))]
use core::iter;
use core::marker::{self, PhantomData, Unpin, Unsize};
#[cfg(not(no_global_oom_handling))]
use core::mem::size_of_val;
use core::mem::{self, align_of_val_raw, forget};
use core::ops::{CoerceUnsized, Deref, DispatchFromDyn, Receiver};
use core::panic::{RefUnwindSafe, UnwindSafe};
#[cfg(not(no_global_oom_handling))]
use core::pin::Pin;
use core::ptr::{self, NonNull};
#[cfg(not(no_global_oom_handling))]
use core::slice::from_raw_parts_mut;
#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
#[cfg(not(no_global_oom_handling))]
use crate::alloc::{box_free, WriteCloneIntoRaw};
use crate::alloc::{AllocError, Allocator, Global, Layout};
use crate::borrow::{Cow, ToOwned};
#[cfg(not(no_global_oom_handling))]
use crate::string::String;
#[cfg(not(no_global_oom_handling))]
use crate::vec::Vec;
#[cfg(test)]
mod tests;
// This is repr(C) to future-proof against possible field-reordering, which
// would interfere with otherwise safe [into|from]_raw() of transmutable
// inner types.
#[repr(C)]
struct RcBox<T: ?Sized> {
strong: Cell<usize>,
weak: Cell<usize>,
value: T,
}
/// Calculate layout for `RcBox<T>` using the inner value's layout
fn rcbox_layout_for_value_layout(layout: Layout) -> Layout {
// Calculate layout using the given value layout.
// Previously, layout was calculated on the expression
// `&*(ptr as *const RcBox<T>)`, but this created a misaligned
// reference (see #54908).
Layout::new::<RcBox<()>>().extend(layout).unwrap().0.pad_to_align()
}
/// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
/// Counted'.
///
/// See the [module-level documentation](./index.html) for more details.
///
/// The inherent methods of `Rc` are all associated functions, which means
/// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
/// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
///
/// [get_mut]: Rc::get_mut
#[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_insignificant_dtor]
pub struct Rc<T: ?Sized> {
ptr: NonNull<RcBox<T>>,
phantom: PhantomData<RcBox<T>>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !marker::Send for Rc<T> {}
// Note that this negative impl isn't strictly necessary for correctness,
// as `Rc` transitively contains a `Cell`, which is itself `!Sync`.
// However, given how important `Rc`'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> !marker::Sync for Rc<T> {}
#[stable(feature = "catch_unwind", since = "1.9.0")]
impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Rc<T> {}
#[stable(feature = "rc_ref_unwind_safe", since = "1.58.0")]
impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for Rc<T> {}
#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
impl<T: ?Sized> Rc<T> {
#[inline(always)]
fn inner(&self) -> &RcBox<T> {
// This unsafety is ok because while this Rc is alive we're guaranteed
// that the inner pointer is valid.
unsafe { self.ptr.as_ref() }
}
unsafe fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
Self { ptr, phantom: PhantomData }
}
unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
}
}
impl<T> Rc<T> {
/// Constructs a new `Rc<T>`.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
/// ```
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn new(value: T) -> Rc<T> {
// There is an implicit weak pointer owned by all the strong
// pointers, which ensures that the weak destructor never frees
// the allocation while the strong destructor is running, even
// if the weak pointer is stored inside the strong one.
unsafe {
Self::from_inner(
Box::leak(Box::new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value }))
.into(),
)
}
}
/// Constructs a new `Rc<T>` while giving you a `Weak<T>` to the allocation,
/// to allow you to construct a `T` which holds a weak pointer to itself.
///
/// Generally, a structure circularly referencing itself, either directly or
/// indirectly, should not hold a strong reference to itself to prevent a memory leak.
/// Using this function, you get access to the weak pointer during the
/// initialization of `T`, before the `Rc<T>` is created, such that you can
/// clone and store it inside the `T`.
///
/// `new_cyclic` first allocates the managed allocation for the `Rc<T>`,
/// then calls your closure, giving it a `Weak<T>` to this allocation,
/// and only afterwards completes the construction of the `Rc<T>` by placing
/// the `T` returned from your closure into the allocation.
///
/// Since the new `Rc<T>` is not fully-constructed until `Rc<T>::new_cyclic`
/// returns, calling [`upgrade`] on the weak reference inside your closure will
/// fail and result in a `None` value.
///
/// # Panics
///
/// If `data_fn` panics, the panic is propagated to the caller, and the
/// temporary [`Weak<T>`] is dropped normally.
///
/// # Examples
///
/// ```
/// # #![allow(dead_code)]
/// use std::rc::{Rc, Weak};
///
/// struct Gadget {
/// me: Weak<Gadget>,
/// }
///
/// impl Gadget {
/// /// Construct a reference counted Gadget.
/// fn new() -> Rc<Self> {
/// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
/// // `Rc` we're constructing.
/// Rc::new_cyclic(|me| {
/// // Create the actual struct here.
/// Gadget { me: me.clone() }
/// })
/// }
///
/// /// Return a reference counted pointer to Self.
/// fn me(&self) -> Rc<Self> {
/// self.me.upgrade().unwrap()
/// }
/// }
/// ```
/// [`upgrade`]: Weak::upgrade
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "arc_new_cyclic", since = "1.60.0")]
pub fn new_cyclic<F>(data_fn: F) -> Rc<T>
where
F: FnOnce(&Weak<T>) -> T,
{
// Construct the inner in the "uninitialized" state with a single
// weak reference.
let uninit_ptr: NonNull<_> = Box::leak(Box::new(RcBox {
strong: Cell::new(0),
weak: Cell::new(1),
value: mem::MaybeUninit::<T>::uninit(),
}))
.into();
let init_ptr: NonNull<RcBox<T>> = uninit_ptr.cast();
let weak = Weak { ptr: init_ptr };
// It's important we don't give up ownership of the weak pointer, or
// else the memory might be freed by the time `data_fn` returns. If
// we really wanted to pass ownership, we could create an additional
// weak pointer for ourselves, but this would result in additional
// updates to the weak reference count which might not be necessary
// otherwise.
let data = data_fn(&weak);
let strong = unsafe {
let inner = init_ptr.as_ptr();
ptr::write(ptr::addr_of_mut!((*inner).value), data);
let prev_value = (*inner).strong.get();
debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
(*inner).strong.set(1);
Rc::from_inner(init_ptr)
};
// Strong references should collectively own a shared weak reference,
// so don't run the destructor for our old weak reference.
mem::forget(weak);
strong
}
/// Constructs a new `Rc` with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut five = Rc::<u32>::new_uninit();
///
/// // Deferred initialization:
/// Rc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
|mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
))
}
}
/// Constructs a new `Rc` with uninitialized contents, with the memory
/// being filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
/// incorrect usage of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// use std::rc::Rc;
///
/// let zero = Rc::<u32>::new_zeroed();
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0)
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed() -> Rc<mem::MaybeUninit<T>> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
|mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
))
}
}
/// Constructs a new `Rc<T>`, returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api)]
/// use std::rc::Rc;
///
/// let five = Rc::try_new(5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
// There is an implicit weak pointer owned by all the strong
// pointers, which ensures that the weak destructor never frees
// the allocation while the strong destructor is running, even
// if the weak pointer is stored inside the strong one.
unsafe {
Ok(Self::from_inner(
Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
.into(),
))
}
}
/// Constructs a new `Rc` with uninitialized contents, returning an error if the allocation fails
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut five = Rc::<u32>::try_new_uninit()?;
///
/// // Deferred initialization:
/// Rc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
#[unstable(feature = "allocator_api", issue = "32838")]
// #[unstable(feature = "new_uninit", issue = "63291")]
pub fn try_new_uninit() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(layout),
|mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
)?))
}
}
/// Constructs a new `Rc` with uninitialized contents, with the memory
/// being filled with `0` bytes, returning an error if the allocation fails
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
/// incorrect usage of this method.
///
/// # Examples
///
/// ```
/// #![feature(allocator_api, new_uninit)]
///
/// use std::rc::Rc;
///
/// let zero = Rc::<u32>::try_new_zeroed()?;
/// let zero = unsafe { zero.assume_init() };
///
/// assert_eq!(*zero, 0);
/// # Ok::<(), std::alloc::AllocError>(())
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[unstable(feature = "allocator_api", issue = "32838")]
//#[unstable(feature = "new_uninit", issue = "63291")]
pub fn try_new_zeroed() -> Result<Rc<mem::MaybeUninit<T>>, AllocError> {
unsafe {
Ok(Rc::from_ptr(Rc::try_allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate_zeroed(layout),
|mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
)?))
}
}
/// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
/// `value` will be pinned in memory and unable to be moved.
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "pin", since = "1.33.0")]
#[must_use]
pub fn pin(value: T) -> Pin<Rc<T>> {
unsafe { Pin::new_unchecked(Rc::new(value)) }
}
/// Returns the inner value, if the `Rc` has exactly one strong reference.
///
/// Otherwise, an [`Err`] is returned with the same `Rc` that was
/// passed in.
///
/// This will succeed even if there are outstanding weak references.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let x = Rc::new(3);
/// assert_eq!(Rc::try_unwrap(x), Ok(3));
///
/// let x = Rc::new(4);
/// let _y = Rc::clone(&x);
/// assert_eq!(*Rc::try_unwrap(x).unwrap_err(), 4);
/// ```
#[inline]
#[stable(feature = "rc_unique", since = "1.4.0")]
pub fn try_unwrap(this: Self) -> Result<T, Self> {
if Rc::strong_count(&this) == 1 {
unsafe {
let val = ptr::read(&*this); // copy the contained object
// Indicate to Weaks that they can't be promoted by decrementing
// the strong count, and then remove the implicit "strong weak"
// pointer while also handling drop logic by just crafting a
// fake Weak.
this.inner().dec_strong();
let _weak = Weak { ptr: this.ptr };
forget(this);
Ok(val)
}
} else {
Err(this)
}
}
}
impl<T> Rc<[T]> {
/// Constructs a new reference-counted slice with uninitialized contents.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut values = Rc::<[u32]>::new_uninit_slice(3);
///
/// // Deferred initialization:
/// let data = Rc::get_mut(&mut values).unwrap();
/// data[0].write(1);
/// data[1].write(2);
/// data[2].write(3);
///
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
unsafe { Rc::from_ptr(Rc::allocate_for_slice(len)) }
}
/// Constructs a new reference-counted slice with uninitialized contents, with the memory being
/// filled with `0` bytes.
///
/// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
/// incorrect usage of this method.
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
///
/// use std::rc::Rc;
///
/// let values = Rc::<[u32]>::new_zeroed_slice(3);
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [0, 0, 0])
/// ```
///
/// [zeroed]: mem::MaybeUninit::zeroed
#[cfg(not(no_global_oom_handling))]
#[unstable(feature = "new_uninit", issue = "63291")]
#[must_use]
pub fn new_zeroed_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::array::<T>(len).unwrap(),
|layout| Global.allocate_zeroed(layout),
|mem| {
ptr::slice_from_raw_parts_mut(mem as *mut T, len)
as *mut RcBox<[mem::MaybeUninit<T>]>
},
))
}
}
}
impl<T> Rc<mem::MaybeUninit<T>> {
/// Converts to `Rc<T>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the inner value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut five = Rc::<u32>::new_uninit();
///
/// // Deferred initialization:
/// Rc::get_mut(&mut five).unwrap().write(5);
///
/// let five = unsafe { five.assume_init() };
///
/// assert_eq!(*five, 5)
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[inline]
pub unsafe fn assume_init(self) -> Rc<T> {
unsafe { Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast()) }
}
}
impl<T> Rc<[mem::MaybeUninit<T>]> {
/// Converts to `Rc<[T]>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the inner value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// #![feature(new_uninit)]
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut values = Rc::<[u32]>::new_uninit_slice(3);
///
/// // Deferred initialization:
/// let data = Rc::get_mut(&mut values).unwrap();
/// data[0].write(1);
/// data[1].write(2);
/// data[2].write(3);
///
/// let values = unsafe { values.assume_init() };
///
/// assert_eq!(*values, [1, 2, 3])
/// ```
#[unstable(feature = "new_uninit", issue = "63291")]
#[inline]
pub unsafe fn assume_init(self) -> Rc<[T]> {
unsafe { Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
}
}
impl<T: ?Sized> Rc<T> {
/// Consumes the `Rc`, returning the wrapped pointer.
///
/// To avoid a memory leak the pointer must be converted back to an `Rc` using
/// [`Rc::from_raw`].
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let x = Rc::new("hello".to_owned());
/// let x_ptr = Rc::into_raw(x);
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// ```
#[stable(feature = "rc_raw", since = "1.17.0")]
pub fn into_raw(this: Self) -> *const T {
let ptr = Self::as_ptr(&this);
mem::forget(this);
ptr
}
/// Provides a raw pointer to the data.
///
/// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
/// for as long there are strong counts in the `Rc`.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let x = Rc::new("hello".to_owned());
/// let y = Rc::clone(&x);
/// let x_ptr = Rc::as_ptr(&x);
/// assert_eq!(x_ptr, Rc::as_ptr(&y));
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// ```
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub fn as_ptr(this: &Self) -> *const T {
let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
// SAFETY: This cannot go through Deref::deref or Rc::inner because
// this is required to retain raw/mut provenance such that e.g. `get_mut` can
// write through the pointer after the Rc is recovered through `from_raw`.
unsafe { ptr::addr_of_mut!((*ptr).value) }
}
/// Constructs an `Rc<T>` from a raw pointer.
///
/// The raw pointer must have been previously returned by a call to
/// [`Rc<U>::into_raw`][into_raw] where `U` must have the same size
/// and alignment as `T`. This is trivially true if `U` is `T`.
/// Note that if `U` is not `T` but has the same size and alignment, this is
/// basically like transmuting references of different types. See
/// [`mem::transmute`] for more information on what
/// restrictions apply in this case.
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Rc<T>` is never accessed.
///
/// [into_raw]: Rc::into_raw
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let x = Rc::new("hello".to_owned());
/// let x_ptr = Rc::into_raw(x);
///
/// unsafe {
/// // Convert back to an `Rc` to prevent leak.
/// let x = Rc::from_raw(x_ptr);
/// assert_eq!(&*x, "hello");
///
/// // Further calls to `Rc::from_raw(x_ptr)` would be memory-unsafe.
/// }
///
/// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
/// ```
#[stable(feature = "rc_raw", since = "1.17.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
let offset = unsafe { data_offset(ptr) };
// Reverse the offset to find the original RcBox.
let rc_ptr = unsafe { ptr.byte_sub(offset) as *mut RcBox<T> };
unsafe { Self::from_ptr(rc_ptr) }
}
/// Creates a new [`Weak`] pointer to this allocation.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// let weak_five = Rc::downgrade(&five);
/// ```
#[must_use = "this returns a new `Weak` pointer, \
without modifying the original `Rc`"]
#[stable(feature = "rc_weak", since = "1.4.0")]
pub fn downgrade(this: &Self) -> Weak<T> {
this.inner().inc_weak();
// Make sure we do not create a dangling Weak
debug_assert!(!is_dangling(this.ptr.as_ptr()));
Weak { ptr: this.ptr }
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
/// let _weak_five = Rc::downgrade(&five);
///
/// assert_eq!(1, Rc::weak_count(&five));
/// ```
#[inline]
#[stable(feature = "rc_counts", since = "1.15.0")]
pub fn weak_count(this: &Self) -> usize {
this.inner().weak() - 1
}
/// Gets the number of strong (`Rc`) pointers to this allocation.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
/// let _also_five = Rc::clone(&five);
///
/// assert_eq!(2, Rc::strong_count(&five));
/// ```
#[inline]
#[stable(feature = "rc_counts", since = "1.15.0")]
pub fn strong_count(this: &Self) -> usize {
this.inner().strong()
}
/// Increments the strong reference count on the `Rc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Rc::into_raw`, and the
/// associated `Rc` instance must be valid (i.e. the strong count must be at
/// least 1) for the duration of this method.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// unsafe {
/// let ptr = Rc::into_raw(five);
/// Rc::increment_strong_count(ptr);
///
/// let five = Rc::from_raw(ptr);
/// assert_eq!(2, Rc::strong_count(&five));
/// }
/// ```
#[inline]
#[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
pub unsafe fn increment_strong_count(ptr: *const T) {
// Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
let rc = unsafe { mem::ManuallyDrop::new(Rc::<T>::from_raw(ptr)) };
// Now increase refcount, but don't drop new refcount either
let _rc_clone: mem::ManuallyDrop<_> = rc.clone();
}
/// Decrements the strong reference count on the `Rc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Rc::into_raw`, and the
/// associated `Rc` instance must be valid (i.e. the strong count must be at
/// least 1) when invoking this method. This method can be used to release
/// the final `Rc` and backing storage, but **should not** be called after
/// the final `Rc` has been released.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// unsafe {
/// let ptr = Rc::into_raw(five);
/// Rc::increment_strong_count(ptr);
///
/// let five = Rc::from_raw(ptr);
/// assert_eq!(2, Rc::strong_count(&five));
/// Rc::decrement_strong_count(ptr);
/// assert_eq!(1, Rc::strong_count(&five));
/// }
/// ```
#[inline]
#[stable(feature = "rc_mutate_strong_count", since = "1.53.0")]
pub unsafe fn decrement_strong_count(ptr: *const T) {
unsafe { mem::drop(Rc::from_raw(ptr)) };
}
/// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
/// this allocation.
#[inline]
fn is_unique(this: &Self) -> bool {
Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
}
/// Returns a mutable reference into the given `Rc`, if there are
/// no other `Rc` or [`Weak`] pointers to the same allocation.
///
/// Returns [`None`] otherwise, because it is not safe to
/// mutate a shared value.
///
/// See also [`make_mut`][make_mut], which will [`clone`][clone]
/// the inner value when there are other `Rc` pointers.
///
/// [make_mut]: Rc::make_mut
/// [clone]: Clone::clone
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let mut x = Rc::new(3);
/// *Rc::get_mut(&mut x).unwrap() = 4;
/// assert_eq!(*x, 4);
///
/// let _y = Rc::clone(&x);
/// assert!(Rc::get_mut(&mut x).is_none());
/// ```
#[inline]
#[stable(feature = "rc_unique", since = "1.4.0")]
pub fn get_mut(this: &mut Self) -> Option<&mut T> {
if Rc::is_unique(this) { unsafe { Some(Rc::get_mut_unchecked(this)) } } else { None }
}
/// Returns a mutable reference into the given `Rc`,
/// without any check.
///
/// See also [`get_mut`], which is safe and does appropriate checks.
///
/// [`get_mut`]: Rc::get_mut
///
/// # Safety
///
/// If any other `Rc` or [`Weak`] pointers to the same allocation exist, then
/// they must be must not be dereferenced or have active borrows for the duration
/// of the returned borrow, and their inner type must be exactly the same as the
/// inner type of this Rc (including lifetimes). This is trivially the case if no
/// such pointers exist, for example immediately after `Rc::new`.
///
/// # Examples
///
/// ```
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let mut x = Rc::new(String::new());
/// unsafe {
/// Rc::get_mut_unchecked(&mut x).push_str("foo")
/// }
/// assert_eq!(*x, "foo");
/// ```
/// Other `Rc` pointers to the same allocation must be to the same type.
/// ```no_run
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let x: Rc<str> = Rc::from("Hello, world!");
/// let mut y: Rc<[u8]> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type is str, not [u8]
/// Rc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
/// }
/// println!("{}", &*x); // Invalid UTF-8 in a str
/// ```
/// Other `Rc` pointers to the same allocation must be to the exact same type, including lifetimes.
/// ```no_run
/// #![feature(get_mut_unchecked)]
///
/// use std::rc::Rc;
///
/// let x: Rc<&str> = Rc::new("Hello, world!");
/// {
/// let s = String::from("Oh, no!");
/// let mut y: Rc<&str> = x.clone().into();
/// unsafe {
/// // this is Undefined Behavior, because x's inner type
/// // is &'long str, not &'short str
/// *Rc::get_mut_unchecked(&mut y) = &s;
/// }
/// }
/// println!("{}", &*x); // Use-after-free
/// ```
#[inline]
#[unstable(feature = "get_mut_unchecked", issue = "63292")]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
// We are careful to *not* create a reference covering the "count" fields, as
// this would conflict with accesses to the reference counts (e.g. by `Weak`).
unsafe { &mut (*this.ptr.as_ptr()).value }
}
#[inline]
#[stable(feature = "ptr_eq", since = "1.17.0")]
/// Returns `true` if the two `Rc`s point to the same allocation in a vein similar to
/// [`ptr::eq`]. See [that function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
/// let same_five = Rc::clone(&five);
/// let other_five = Rc::new(5);
///
/// assert!(Rc::ptr_eq(&five, &same_five));
/// assert!(!Rc::ptr_eq(&five, &other_five));
/// ```
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
this.ptr.as_ptr() == other.ptr.as_ptr()
}
}
impl<T: Clone> Rc<T> {
/// Makes a mutable reference into the given `Rc`.
///
/// If there are other `Rc` pointers to the same allocation, then `make_mut` will
/// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
/// referred to as clone-on-write.
///
/// However, if there are no other `Rc` pointers to this allocation, but some [`Weak`]
/// pointers, then the [`Weak`] pointers will be disassociated and the inner value will not
/// be cloned.
///
/// See also [`get_mut`], which will fail rather than cloning the inner value
/// or disassociating [`Weak`] pointers.
///
/// [`clone`]: Clone::clone
/// [`get_mut`]: Rc::get_mut
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let mut data = Rc::new(5);
///
/// *Rc::make_mut(&mut data) += 1; // Won't clone anything
/// let mut other_data = Rc::clone(&data); // Won't clone inner data
/// *Rc::make_mut(&mut data) += 1; // Clones inner data
/// *Rc::make_mut(&mut data) += 1; // Won't clone anything
/// *Rc::make_mut(&mut other_data) *= 2; // Won't clone anything
///
/// // Now `data` and `other_data` point to different allocations.
/// assert_eq!(*data, 8);
/// assert_eq!(*other_data, 12);
/// ```
///
/// [`Weak`] pointers will be disassociated:
///
/// ```
/// use std::rc::Rc;
///
/// let mut data = Rc::new(75);
/// let weak = Rc::downgrade(&data);
///
/// assert!(75 == *data);
/// assert!(75 == *weak.upgrade().unwrap());
///
/// *Rc::make_mut(&mut data) += 1;
///
/// assert!(76 == *data);
/// assert!(weak.upgrade().is_none());
/// ```
#[cfg(not(no_global_oom_handling))]
#[inline]
#[stable(feature = "rc_unique", since = "1.4.0")]
pub fn make_mut(this: &mut Self) -> &mut T {
if Rc::strong_count(this) != 1 {
// Gotta clone the data, there are other Rcs.
// Pre-allocate memory to allow writing the cloned value directly.
let mut rc = Self::new_uninit();
unsafe {
let data = Rc::get_mut_unchecked(&mut rc);
(**this).write_clone_into_raw(data.as_mut_ptr());
*this = rc.assume_init();
}
} else if Rc::weak_count(this) != 0 {
// Can just steal the data, all that's left is Weaks
let mut rc = Self::new_uninit();
unsafe {
let data = Rc::get_mut_unchecked(&mut rc);
data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
this.inner().dec_strong();
// Remove implicit strong-weak ref (no need to craft a fake
// Weak here -- we know other Weaks can clean up for us)
this.inner().dec_weak();
ptr::write(this, rc.assume_init());
}
}
// This unsafety is ok because we're guaranteed that the pointer
// returned is the *only* pointer that will ever be returned to T. Our
// reference count is guaranteed to be 1 at this point, and we required
// the `Rc<T>` itself to be `mut`, so we're returning the only possible
// reference to the allocation.
unsafe { &mut this.ptr.as_mut().value }
}
/// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
/// clone.
///
/// Assuming `rc_t` is of type `Rc<T>`, this function is functionally equivalent to
/// `(*rc_t).clone()`, but will avoid cloning the inner value where possible.
///
/// # Examples
///
/// ```
/// #![feature(arc_unwrap_or_clone)]
/// # use std::{ptr, rc::Rc};
/// let inner = String::from("test");
/// let ptr = inner.as_ptr();
///
/// let rc = Rc::new(inner);
/// let inner = Rc::unwrap_or_clone(rc);
/// // The inner value was not cloned
/// assert!(ptr::eq(ptr, inner.as_ptr()));
///
/// let rc = Rc::new(inner);
/// let rc2 = rc.clone();
/// let inner = Rc::unwrap_or_clone(rc);
/// // Because there were 2 references, we had to clone the inner value.
/// assert!(!ptr::eq(ptr, inner.as_ptr()));
/// // `rc2` is the last reference, so when we unwrap it we get back
/// // the original `String`.
/// let inner = Rc::unwrap_or_clone(rc2);
/// assert!(ptr::eq(ptr, inner.as_ptr()));
/// ```
#[inline]
#[unstable(feature = "arc_unwrap_or_clone", issue = "93610")]
pub fn unwrap_or_clone(this: Self) -> T {
Rc::try_unwrap(this).unwrap_or_else(|rc| (*rc).clone())
}
}
impl Rc<dyn Any> {
/// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
///
/// # Examples
///
/// ```
/// use std::any::Any;
/// use std::rc::Rc;
///
/// fn print_if_string(value: Rc<dyn Any>) {
/// if let Ok(string) = value.downcast::<String>() {
/// println!("String ({}): {}", string.len(), string);
/// }
/// }
///
/// let my_string = "Hello World".to_string();
/// print_if_string(Rc::new(my_string));
/// print_if_string(Rc::new(0i8));
/// ```
#[inline]
#[stable(feature = "rc_downcast", since = "1.29.0")]
pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
if (*self).is::<T>() {
unsafe {
let ptr = self.ptr.cast::<RcBox<T>>();
forget(self);
Ok(Rc::from_inner(ptr))
}
} else {
Err(self)
}
}
/// Downcasts the `Rc<dyn Any>` to a concrete type.
///
/// For a safe alternative see [`downcast`].
///
/// # Examples
///
/// ```
/// #![feature(downcast_unchecked)]
///
/// use std::any::Any;
/// use std::rc::Rc;
///
/// let x: Rc<dyn Any> = Rc::new(1_usize);
///
/// unsafe {
/// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
/// }
/// ```
///
/// # Safety
///
/// The contained value must be of type `T`. Calling this method
/// with the incorrect type is *undefined behavior*.
///
///
/// [`downcast`]: Self::downcast
#[inline]
#[unstable(feature = "downcast_unchecked", issue = "90850")]
pub unsafe fn downcast_unchecked<T: Any>(self) -> Rc<T> {
unsafe {
let ptr = self.ptr.cast::<RcBox<T>>();
mem::forget(self);
Rc::from_inner(ptr)
}
}
}
impl<T: ?Sized> Rc<T> {
/// Allocates an `RcBox<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided.
///
/// The function `mem_to_rcbox` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
) -> *mut RcBox<T> {
let layout = rcbox_layout_for_value_layout(value_layout);
unsafe {
Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rcbox)
.unwrap_or_else(|_| handle_alloc_error(layout))
}
}
/// Allocates an `RcBox<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided,
/// returning an error if allocation fails.
///
/// The function `mem_to_rcbox` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
#[inline]
unsafe fn try_allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
) -> Result<*mut RcBox<T>, AllocError> {
let layout = rcbox_layout_for_value_layout(value_layout);
// Allocate for the layout.
let ptr = allocate(layout)?;
// Initialize the RcBox
let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
unsafe {
debug_assert_eq!(Layout::for_value(&*inner), layout);
ptr::write(&mut (*inner).strong, Cell::new(1));
ptr::write(&mut (*inner).weak, Cell::new(1));
}
Ok(inner)
}
/// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
// Allocate for the `RcBox<T>` using the given value.
unsafe {
Self::allocate_for_layout(
Layout::for_value(&*ptr),
|layout| Global.allocate(layout),
|mem| mem.with_metadata_of(ptr as *const RcBox<T>),
)
}
}
#[cfg(not(no_global_oom_handling))]
fn from_box(v: Box<T>) -> Rc<T> {
unsafe {
let (box_unique, alloc) = Box::into_unique(v);
let bptr = box_unique.as_ptr();
let value_size = size_of_val(&*bptr);
let ptr = Self::allocate_for_ptr(bptr);
// Copy value as bytes
ptr::copy_nonoverlapping(
bptr as *const T as *const u8,
&mut (*ptr).value as *mut _ as *mut u8,
value_size,
);
// Free the allocation without dropping its contents
box_free(box_unique, alloc);
Self::from_ptr(ptr)
}
}
}
impl<T> Rc<[T]> {
/// Allocates an `RcBox<[T]>` with the given length.
#[cfg(not(no_global_oom_handling))]
unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
unsafe {
Self::allocate_for_layout(
Layout::array::<T>(len).unwrap(),
|layout| Global.allocate(layout),
|mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
)
}
}
/// Copy elements from slice into newly allocated `Rc<[T]>`
///
/// Unsafe because the caller must either take ownership or bind `T: Copy`
#[cfg(not(no_global_oom_handling))]
unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
unsafe {
let ptr = Self::allocate_for_slice(v.len());
ptr::copy_nonoverlapping(v.as_ptr(), &mut (*ptr).value as *mut [T] as *mut T, v.len());
Self::from_ptr(ptr)
}
}
/// Constructs an `Rc<[T]>` from an iterator known to be of a certain size.
///
/// Behavior is undefined should the size be wrong.
#[cfg(not(no_global_oom_handling))]
unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Rc<[T]> {
// Panic guard while cloning T elements.
// In the event of a panic, elements that have been written
// into the new RcBox will be dropped, then the memory freed.
struct Guard<T> {
mem: NonNull<u8>,
elems: *mut T,
layout: Layout,
n_elems: usize,
}
impl<T> Drop for Guard<T> {
fn drop(&mut self) {
unsafe {
let slice = from_raw_parts_mut(self.elems, self.n_elems);
ptr::drop_in_place(slice);
Global.deallocate(self.mem, self.layout);
}
}
}
unsafe {
let ptr = Self::allocate_for_slice(len);
let mem = ptr as *mut _ as *mut u8;
let layout = Layout::for_value(&*ptr);
// Pointer to first element
let elems = &mut (*ptr).value as *mut [T] as *mut T;
let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
for (i, item) in iter.enumerate() {
ptr::write(elems.add(i), item);
guard.n_elems += 1;
}
// All clear. Forget the guard so it doesn't free the new RcBox.
forget(guard);
Self::from_ptr(ptr)
}
}
}
/// Specialization trait used for `From<&[T]>`.
trait RcFromSlice<T> {
fn from_slice(slice: &[T]) -> Self;
}
#[cfg(not(no_global_oom_handling))]
impl<T: Clone> RcFromSlice<T> for Rc<[T]> {
#[inline]
default fn from_slice(v: &[T]) -> Self {
unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
}
}
#[cfg(not(no_global_oom_handling))]
impl<T: Copy> RcFromSlice<T> for Rc<[T]> {
#[inline]
fn from_slice(v: &[T]) -> Self {
unsafe { Rc::copy_from_slice(v) }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Rc<T> {
type Target = T;
#[inline(always)]
fn deref(&self) -> &T {
&self.inner().value
}
}
#[unstable(feature = "receiver_trait", issue = "none")]
impl<T: ?Sized> Receiver for Rc<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
/// Drops the `Rc`.
///
/// This will decrement the strong reference count. If the strong reference
/// count reaches zero then the only other references (if any) are
/// [`Weak`], so we `drop` the inner value.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// struct Foo;
///
/// impl Drop for Foo {
/// fn drop(&mut self) {
/// println!("dropped!");
/// }
/// }
///
/// let foo = Rc::new(Foo);
/// let foo2 = Rc::clone(&foo);
///
/// drop(foo); // Doesn't print anything
/// drop(foo2); // Prints "dropped!"
/// ```
fn drop(&mut self) {
unsafe {
self.inner().dec_strong();
if self.inner().strong() == 0 {
// destroy the contained object
ptr::drop_in_place(Self::get_mut_unchecked(self));
// remove the implicit "strong weak" pointer now that we've
// destroyed the contents.
self.inner().dec_weak();
if self.inner().weak() == 0 {
Global.deallocate(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
}
}
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Clone for Rc<T> {
/// Makes a clone of the `Rc` pointer.
///
/// This creates another pointer to the same allocation, increasing the
/// strong reference count.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// let _ = Rc::clone(&five);
/// ```
#[inline]
fn clone(&self) -> Rc<T> {
unsafe {
self.inner().inc_strong();
Self::from_inner(self.ptr)
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Rc<T> {
/// Creates a new `Rc<T>`, with the `Default` value for `T`.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let x: Rc<i32> = Default::default();
/// assert_eq!(*x, 0);
/// ```
#[inline]
fn default() -> Rc<T> {
Rc::new(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
trait RcEqIdent<T: ?Sized + PartialEq> {
fn eq(&self, other: &Rc<T>) -> bool;
fn ne(&self, other: &Rc<T>) -> bool;
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> RcEqIdent<T> for Rc<T> {
#[inline]
default fn eq(&self, other: &Rc<T>) -> bool {
**self == **other
}
#[inline]
default fn ne(&self, other: &Rc<T>) -> bool {
**self != **other
}
}
// Hack to allow specializing on `Eq` even though `Eq` has a method.
#[rustc_unsafe_specialization_marker]
pub(crate) trait MarkerEq: PartialEq<Self> {}
impl<T: Eq> MarkerEq for T {}
/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
/// would otherwise add a cost to all equality checks on refs. We assume that `Rc`s are used to
/// store large values, that are slow to clone, but also heavy to check for equality, causing this
/// cost to pay off more easily. It's also more likely to have two `Rc` clones, that point to
/// the same value, than two `&T`s.
///
/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + MarkerEq> RcEqIdent<T> for Rc<T> {
#[inline]
fn eq(&self, other: &Rc<T>) -> bool {
Rc::ptr_eq(self, other) || **self == **other
}
#[inline]
fn ne(&self, other: &Rc<T>) -> bool {
!Rc::ptr_eq(self, other) && **self != **other
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
/// Equality for two `Rc`s.
///
/// Two `Rc`s are equal if their inner values are equal, even if they are
/// stored in different allocation.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Rc`s that point to the same allocation are
/// always equal.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five == Rc::new(5));
/// ```
#[inline]
fn eq(&self, other: &Rc<T>) -> bool {
RcEqIdent::eq(self, other)
}
/// Inequality for two `Rc`s.
///
/// Two `Rc`s are unequal if their inner values are unequal.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Rc`s that point to the same allocation are
/// never unequal.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five != Rc::new(6));
/// ```
#[inline]
fn ne(&self, other: &Rc<T>) -> bool {
RcEqIdent::ne(self, other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Eq> Eq for Rc<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
/// Partial comparison for two `Rc`s.
///
/// The two are compared by calling `partial_cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
/// use std::cmp::Ordering;
///
/// let five = Rc::new(5);
///
/// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
/// ```
#[inline(always)]
fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
(**self).partial_cmp(&**other)
}
/// Less-than comparison for two `Rc`s.
///
/// The two are compared by calling `<` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five < Rc::new(6));
/// ```
#[inline(always)]
fn lt(&self, other: &Rc<T>) -> bool {
**self < **other
}
/// 'Less than or equal to' comparison for two `Rc`s.
///
/// The two are compared by calling `<=` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five <= Rc::new(5));
/// ```
#[inline(always)]
fn le(&self, other: &Rc<T>) -> bool {
**self <= **other
}
/// Greater-than comparison for two `Rc`s.
///
/// The two are compared by calling `>` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five > Rc::new(4));
/// ```
#[inline(always)]
fn gt(&self, other: &Rc<T>) -> bool {
**self > **other
}
/// 'Greater than or equal to' comparison for two `Rc`s.
///
/// The two are compared by calling `>=` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five >= Rc::new(5));
/// ```
#[inline(always)]
fn ge(&self, other: &Rc<T>) -> bool {
**self >= **other
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Ord> Ord for Rc<T> {
/// Comparison for two `Rc`s.
///
/// The two are compared by calling `cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
/// use std::cmp::Ordering;
///
/// let five = Rc::new(5);
///
/// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
/// ```
#[inline]
fn cmp(&self, other: &Rc<T>) -> Ordering {
(**self).cmp(&**other)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + Hash> Hash for Rc<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state);
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> fmt::Pointer for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&(&**self as *const T), f)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "from_for_ptrs", since = "1.6.0")]
impl<T> From<T> for Rc<T> {
/// Converts a generic type `T` into an `Rc<T>`
///
/// The conversion allocates on the heap and moves `t`
/// from the stack into it.
///
/// # Example
/// ```rust
/// # use std::rc::Rc;
/// let x = 5;
/// let rc = Rc::new(5);
///
/// assert_eq!(Rc::from(x), rc);
/// ```
fn from(t: T) -> Self {
Rc::new(t)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: Clone> From<&[T]> for Rc<[T]> {
/// Allocate a reference-counted slice and fill it by cloning `v`'s items.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let original: &[i32] = &[1, 2, 3];
/// let shared: Rc<[i32]> = Rc::from(original);
/// assert_eq!(&[1, 2, 3], &shared[..]);
/// ```
#[inline]
fn from(v: &[T]) -> Rc<[T]> {
<Self as RcFromSlice<T>>::from_slice(v)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<&str> for Rc<str> {
/// Allocate a reference-counted string slice and copy `v` into it.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let shared: Rc<str> = Rc::from("statue");
/// assert_eq!("statue", &shared[..]);
/// ```
#[inline]
fn from(v: &str) -> Rc<str> {
let rc = Rc::<[u8]>::from(v.as_bytes());
unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) }
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl From<String> for Rc<str> {
/// Allocate a reference-counted string slice and copy `v` into it.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let original: String = "statue".to_owned();
/// let shared: Rc<str> = Rc::from(original);
/// assert_eq!("statue", &shared[..]);
/// ```
#[inline]
fn from(v: String) -> Rc<str> {
Rc::from(&v[..])
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T: ?Sized> From<Box<T>> for Rc<T> {
/// Move a boxed object to a new, reference counted, allocation.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let original: Box<i32> = Box::new(1);
/// let shared: Rc<i32> = Rc::from(original);
/// assert_eq!(1, *shared);
/// ```
#[inline]
fn from(v: Box<T>) -> Rc<T> {
Rc::from_box(v)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_slice", since = "1.21.0")]
impl<T> From<Vec<T>> for Rc<[T]> {
/// Allocate a reference-counted slice and move `v`'s items into it.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let original: Box<Vec<i32>> = Box::new(vec![1, 2, 3]);
/// let shared: Rc<Vec<i32>> = Rc::from(original);
/// assert_eq!(vec![1, 2, 3], *shared);
/// ```
#[inline]
fn from(mut v: Vec<T>) -> Rc<[T]> {
unsafe {
let rc = Rc::copy_from_slice(&v);
// Allow the Vec to free its memory, but not destroy its contents
v.set_len(0);
rc
}
}
}
#[stable(feature = "shared_from_cow", since = "1.45.0")]
impl<'a, B> From<Cow<'a, B>> for Rc<B>
where
B: ToOwned + ?Sized,
Rc<B>: From<&'a B> + From<B::Owned>,
{
/// Create a reference-counted pointer from
/// a clone-on-write pointer by copying its content.
///
/// # Example
///
/// ```rust
/// # use std::rc::Rc;
/// # use std::borrow::Cow;
/// let cow: Cow<str> = Cow::Borrowed("eggplant");
/// let shared: Rc<str> = Rc::from(cow);
/// assert_eq!("eggplant", &shared[..]);
/// ```
#[inline]
fn from(cow: Cow<'a, B>) -> Rc<B> {
match cow {
Cow::Borrowed(s) => Rc::from(s),
Cow::Owned(s) => Rc::from(s),
}
}
}
#[stable(feature = "shared_from_str", since = "1.62.0")]
impl From<Rc<str>> for Rc<[u8]> {
/// Converts a reference-counted string slice into a byte slice.
///
/// # Example
///
/// ```
/// # use std::rc::Rc;
/// let string: Rc<str> = Rc::from("eggplant");
/// let bytes: Rc<[u8]> = Rc::from(string);
/// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
/// ```
#[inline]
fn from(rc: Rc<str>) -> Self {
// SAFETY: `str` has the same layout as `[u8]`.
unsafe { Rc::from_raw(Rc::into_raw(rc) as *const [u8]) }
}
}
#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]> {
type Error = Rc<[T]>;
fn try_from(boxed_slice: Rc<[T]>) -> Result<Self, Self::Error> {
if boxed_slice.len() == N {
Ok(unsafe { Rc::from_raw(Rc::into_raw(boxed_slice) as *mut [T; N]) })
} else {
Err(boxed_slice)
}
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "shared_from_iter", since = "1.37.0")]
impl<T> iter::FromIterator<T> for Rc<[T]> {
/// Takes each element in the `Iterator` and collects it into an `Rc<[T]>`.
///
/// # Performance characteristics
///
/// ## The general case
///
/// In the general case, collecting into `Rc<[T]>` is done by first
/// collecting into a `Vec<T>`. That is, when writing the following:
///
/// ```rust
/// # use std::rc::Rc;
/// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
/// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
/// ```
///
/// this behaves as if we wrote:
///
/// ```rust
/// # use std::rc::Rc;
/// let evens: Rc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
/// .collect::<Vec<_>>() // The first set of allocations happens here.
/// .into(); // A second allocation for `Rc<[T]>` happens here.
/// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
/// ```
///
/// This will allocate as many times as needed for constructing the `Vec<T>`
/// and then it will allocate once for turning the `Vec<T>` into the `Rc<[T]>`.
///
/// ## Iterators of known length
///
/// When your `Iterator` implements `TrustedLen` and is of an exact size,
/// a single allocation will be made for the `Rc<[T]>`. For example:
///
/// ```rust
/// # use std::rc::Rc;
/// let evens: Rc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
/// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
/// ```
fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
ToRcSlice::to_rc_slice(iter.into_iter())
}
}
/// Specialization trait used for collecting into `Rc<[T]>`.
#[cfg(not(no_global_oom_handling))]
trait ToRcSlice<T>: Iterator<Item = T> + Sized {
fn to_rc_slice(self) -> Rc<[T]>;
}
#[cfg(not(no_global_oom_handling))]
impl<T, I: Iterator<Item = T>> ToRcSlice<T> for I {
default fn to_rc_slice(self) -> Rc<[T]> {
self.collect::<Vec<T>>().into()
}
}
#[cfg(not(no_global_oom_handling))]
impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
fn to_rc_slice(self) -> Rc<[T]> {
// This is the case for a `TrustedLen` iterator.
let (low, high) = self.size_hint();
if let Some(high) = high {
debug_assert_eq!(
low,
high,
"TrustedLen iterator's size hint is not exact: {:?}",
(low, high)
);
unsafe {
// SAFETY: We need to ensure that the iterator has an exact length and we have.
Rc::from_iter_exact(self, low)
}
} else {
// TrustedLen contract guarantees that `upper_bound == `None` implies an iterator
// length exceeding `usize::MAX`.
// The default implementation would collect into a vec which would panic.
// Thus we panic here immediately without invoking `Vec` code.
panic!("capacity overflow");
}
}
}
/// `Weak` is a version of [`Rc`] that holds a non-owning reference to the
/// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
/// pointer, which returns an <code>[Option]<[Rc]\<T>></code>.
///
/// Since a `Weak` reference does not count towards ownership, it will not
/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
/// guarantees about the value still being present. Thus it may return [`None`]
/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
/// itself (the backing store) from being deallocated.
///
/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
/// managed by [`Rc`] without preventing its inner value from being dropped. It is also used to
/// prevent circular references between [`Rc`] pointers, since mutual owning references
/// would never allow either [`Rc`] to be dropped. For example, a tree could
/// have strong [`Rc`] pointers from parent nodes to children, and `Weak`
/// pointers from children back to their parents.
///
/// The typical way to obtain a `Weak` pointer is to call [`Rc::downgrade`].
///
/// [`upgrade`]: Weak::upgrade
#[stable(feature = "rc_weak", since = "1.4.0")]
pub struct Weak<T: ?Sized> {
// This is a `NonNull` to allow optimizing the size of this type in enums,
// but it is not necessarily a valid pointer.
// `Weak::new` sets this to `usize::MAX` so that it doesn’t need
// to allocate space on the heap. That's not a value a real pointer
// will ever have because RcBox has alignment at least 2.
// This is only possible when `T: Sized`; unsized `T` never dangle.
ptr: NonNull<RcBox<T>>,
}
#[stable(feature = "rc_weak", since = "1.4.0")]
impl<T: ?Sized> !marker::Send for Weak<T> {}
#[stable(feature = "rc_weak", since = "1.4.0")]
impl<T: ?Sized> !marker::Sync for Weak<T> {}
#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
impl<T> Weak<T> {
/// Constructs a new `Weak<T>`, without allocating any memory.
/// Calling [`upgrade`] on the return value always gives [`None`].
///
/// [`upgrade`]: Weak::upgrade
///
/// # Examples
///
/// ```
/// use std::rc::Weak;
///
/// let empty: Weak<i64> = Weak::new();
/// assert!(empty.upgrade().is_none());
/// ```
#[stable(feature = "downgraded_weak", since = "1.10.0")]
#[rustc_const_unstable(feature = "const_weak_new", issue = "95091", reason = "recently added")]
#[must_use]
pub const fn new() -> Weak<T> {
Weak { ptr: unsafe { NonNull::new_unchecked(ptr::invalid_mut::<RcBox<T>>(usize::MAX)) } }
}
}
pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
(ptr as *mut ()).addr() == usize::MAX
}
/// Helper type to allow accessing the reference counts without
/// making any assertions about the data field.
struct WeakInner<'a> {
weak: &'a Cell<usize>,
strong: &'a Cell<usize>,
}
impl<T: ?Sized> Weak<T> {
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. The pointer may be dangling,
/// unaligned or even [`null`] otherwise.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
/// use std::ptr;
///
/// let strong = Rc::new("hello".to_owned());
/// let weak = Rc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, weak.as_ptr()));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!("hello", unsafe { &*weak.as_ptr() });
///
/// drop(strong);
/// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
/// ```
///
/// [`null`]: ptr::null
#[must_use]
#[stable(feature = "rc_as_ptr", since = "1.45.0")]
pub fn as_ptr(&self) -> *const T {
let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
if is_dangling(ptr) {
// If the pointer is dangling, we return the sentinel directly. This cannot be
// a valid payload address, as the payload is at least as aligned as RcBox (usize).
ptr as *const T
} else {
// SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
// The payload may be dropped at this point, and we have to maintain provenance,
// so use raw pointer manipulation.
unsafe { ptr::addr_of_mut!((*ptr).value) }
}
}
/// Consumes the `Weak<T>` and turns it into a raw pointer.
///
/// This converts the weak pointer into a raw pointer, while still preserving the ownership of
/// one weak reference (the weak count is not modified by this operation). It can be turned
/// back into the `Weak<T>` with [`from_raw`].
///
/// The same restrictions of accessing the target of the pointer as with
/// [`as_ptr`] apply.
///
/// # Examples
///
/// ```
/// use std::rc::{Rc, Weak};
///
/// let strong = Rc::new("hello".to_owned());
/// let weak = Rc::downgrade(&strong);
/// let raw = weak.into_raw();
///
/// assert_eq!(1, Rc::weak_count(&strong));
/// assert_eq!("hello", unsafe { &*raw });
///
/// drop(unsafe { Weak::from_raw(raw) });
/// assert_eq!(0, Rc::weak_count(&strong));
/// ```
///
/// [`from_raw`]: Weak::from_raw
/// [`as_ptr`]: Weak::as_ptr
#[must_use = "`self` will be dropped if the result is not used"]
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub fn into_raw(self) -> *const T {
let result = self.as_ptr();
mem::forget(self);
result
}
/// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
///
/// This can be used to safely get a strong reference (by calling [`upgrade`]
/// later) or to deallocate the weak count by dropping the `Weak<T>`.
///
/// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
/// as these don't own anything; the method still works on them).
///
/// # Safety
///
/// The pointer must have originated from the [`into_raw`] and must still own its potential
/// weak reference.
///
/// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
/// takes ownership of one weak reference currently represented as a raw pointer (the weak
/// count is not modified by this operation) and therefore it must be paired with a previous
/// call to [`into_raw`].
///
/// # Examples
///
/// ```
/// use std::rc::{Rc, Weak};
///
/// let strong = Rc::new("hello".to_owned());
///
/// let raw_1 = Rc::downgrade(&strong).into_raw();
/// let raw_2 = Rc::downgrade(&strong).into_raw();
///
/// assert_eq!(2, Rc::weak_count(&strong));
///
/// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
/// assert_eq!(1, Rc::weak_count(&strong));
///
/// drop(strong);
///
/// // Decrement the last weak count.
/// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
/// ```
///
/// [`into_raw`]: Weak::into_raw
/// [`upgrade`]: Weak::upgrade
/// [`new`]: Weak::new
#[stable(feature = "weak_into_raw", since = "1.45.0")]
pub unsafe fn from_raw(ptr: *const T) -> Self {
// See Weak::as_ptr for context on how the input pointer is derived.
let ptr = if is_dangling(ptr as *mut T) {
// This is a dangling Weak.
ptr as *mut RcBox<T>
} else {
// Otherwise, we're guaranteed the pointer came from a nondangling Weak.
// SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
let offset = unsafe { data_offset(ptr) };
// Thus, we reverse the offset to get the whole RcBox.
// SAFETY: the pointer originated from a Weak, so this offset is safe.
unsafe { ptr.byte_sub(offset) as *mut RcBox<T> }
};
// SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
Weak { ptr: unsafe { NonNull::new_unchecked(ptr) } }
}
/// Attempts to upgrade the `Weak` pointer to an [`Rc`], delaying
/// dropping of the inner value if successful.
///
/// Returns [`None`] if the inner value has since been dropped.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let five = Rc::new(5);
///
/// let weak_five = Rc::downgrade(&five);
///
/// let strong_five: Option<Rc<_>> = weak_five.upgrade();
/// assert!(strong_five.is_some());
///
/// // Destroy all strong pointers.
/// drop(strong_five);
/// drop(five);
///
/// assert!(weak_five.upgrade().is_none());
/// ```
#[must_use = "this returns a new `Rc`, \
without modifying the original weak pointer"]
#[stable(feature = "rc_weak", since = "1.4.0")]
pub fn upgrade(&self) -> Option<Rc<T>> {
let inner = self.inner()?;
if inner.strong() == 0 {
None
} else {
unsafe {
inner.inc_strong();
Some(Rc::from_inner(self.ptr))
}
}
}
/// Gets the number of strong (`Rc`) pointers pointing to this allocation.
///
/// If `self` was created using [`Weak::new`], this will return 0.
#[must_use]
#[stable(feature = "weak_counts", since = "1.41.0")]
pub fn strong_count(&self) -> usize {
if let Some(inner) = self.inner() { inner.strong() } else { 0 }
}
/// Gets the number of `Weak` pointers pointing to this allocation.
///
/// If no strong pointers remain, this will return zero.
#[must_use]
#[stable(feature = "weak_counts", since = "1.41.0")]
pub fn weak_count(&self) -> usize {
self.inner()
.map(|inner| {
if inner.strong() > 0 {
inner.weak() - 1 // subtract the implicit weak ptr
} else {
0
}
})
.unwrap_or(0)
}
/// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
/// (i.e., when this `Weak` was created by `Weak::new`).
#[inline]
fn inner(&self) -> Option<WeakInner<'_>> {
if is_dangling(self.ptr.as_ptr()) {
None
} else {
// We are careful to *not* create a reference covering the "data" field, as
// the field may be mutated concurrently (for example, if the last `Rc`
// is dropped, the data field will be dropped in-place).
Some(unsafe {
let ptr = self.ptr.as_ptr();
WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak }
})
}
}
/// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
/// both don't point to any allocation (because they were created with `Weak::new()`). See [that
/// function][`ptr::eq`] for caveats when comparing `dyn Trait` pointers.
///
/// # Notes
///
/// Since this compares pointers it means that `Weak::new()` will equal each
/// other, even though they don't point to any allocation.
///
/// # Examples
///
/// ```
/// use std::rc::Rc;
///
/// let first_rc = Rc::new(5);
/// let first = Rc::downgrade(&first_rc);
/// let second = Rc::downgrade(&first_rc);
///
/// assert!(first.ptr_eq(&second));
///
/// let third_rc = Rc::new(5);
/// let third = Rc::downgrade(&third_rc);
///
/// assert!(!first.ptr_eq(&third));
/// ```
///
/// Comparing `Weak::new`.
///
/// ```
/// use std::rc::{Rc, Weak};
///
/// let first = Weak::new();
/// let second = Weak::new();
/// assert!(first.ptr_eq(&second));
///
/// let third_rc = Rc::new(());
/// let third = Rc::downgrade(&third_rc);
/// assert!(!first.ptr_eq(&third));
/// ```
#[inline]
#[must_use]
#[stable(feature = "weak_ptr_eq", since = "1.39.0")]
pub fn ptr_eq(&self, other: &Self) -> bool {
self.ptr.as_ptr() == other.ptr.as_ptr()
}
}
#[stable(feature = "rc_weak", since = "1.4.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Weak<T> {
/// Drops the `Weak` pointer.
///
/// # Examples
///
/// ```
/// use std::rc::{Rc, Weak};
///
/// struct Foo;
///
/// impl Drop for Foo {
/// fn drop(&mut self) {
/// println!("dropped!");
/// }
/// }
///
/// let foo = Rc::new(Foo);
/// let weak_foo = Rc::downgrade(&foo);
/// let other_weak_foo = Weak::clone(&weak_foo);
///
/// drop(weak_foo); // Doesn't print anything
/// drop(foo); // Prints "dropped!"
///
/// assert!(other_weak_foo.upgrade().is_none());
/// ```
fn drop(&mut self) {
let inner = if let Some(inner) = self.inner() { inner } else { return };
inner.dec_weak();
// the weak count starts at 1, and will only go to zero if all
// the strong pointers have disappeared.
if inner.weak() == 0 {
unsafe {
Global.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()));
}
}
}
}
#[stable(feature = "rc_weak", since = "1.4.0")]
impl<T: ?Sized> Clone for Weak<T> {
/// Makes a clone of the `Weak` pointer that points to the same allocation.
///
/// # Examples
///
/// ```
/// use std::rc::{Rc, Weak};
///
/// let weak_five = Rc::downgrade(&Rc::new(5));
///
/// let _ = Weak::clone(&weak_five);
/// ```
#[inline]
fn clone(&self) -> Weak<T> {
if let Some(inner) = self.inner() {
inner.inc_weak()
}
Weak { ptr: self.ptr }
}
}
#[stable(feature = "rc_weak", since = "1.4.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "(Weak)")
}
}
#[stable(feature = "downgraded_weak", since = "1.10.0")]
impl<T> Default for Weak<T> {
/// Constructs a new `Weak<T>`, without allocating any memory.
/// Calling [`upgrade`] on the return value always gives [`None`].
///
/// [`upgrade`]: Weak::upgrade
///
/// # Examples
///
/// ```
/// use std::rc::Weak;
///
/// let empty: Weak<i64> = Default::default();
/// assert!(empty.upgrade().is_none());
/// ```
fn default() -> Weak<T> {
Weak::new()
}
}
// NOTE: We checked_add here to deal with mem::forget safely. In particular
// if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
// you can free the allocation while outstanding Rcs (or Weaks) exist.
// We abort because this is such a degenerate scenario that we don't care about
// what happens -- no real program should ever experience this.
//
// This should have negligible overhead since you don't actually need to
// clone these much in Rust thanks to ownership and move-semantics.
#[doc(hidden)]
trait RcInnerPtr {
fn weak_ref(&self) -> &Cell<usize>;
fn strong_ref(&self) -> &Cell<usize>;
#[inline]
fn strong(&self) -> usize {
self.strong_ref().get()
}
#[inline]
fn inc_strong(&self) {
let strong = self.strong();
// We insert an `assume` here to hint LLVM at an otherwise
// missed optimization.
// SAFETY: The reference count will never be zero when this is
// called.
unsafe {
core::intrinsics::assume(strong != 0);
}
let strong = strong.wrapping_add(1);
self.strong_ref().set(strong);
// We want to abort on overflow instead of dropping the value.
// Checking for overflow after the store instead of before
// allows for slightly better code generation.
if core::intrinsics::unlikely(strong == 0) {
abort();
}
}
#[inline]
fn dec_strong(&self) {
self.strong_ref().set(self.strong() - 1);
}
#[inline]
fn weak(&self) -> usize {
self.weak_ref().get()
}
#[inline]
fn inc_weak(&self) {
let weak = self.weak();
// We insert an `assume` here to hint LLVM at an otherwise
// missed optimization.
// SAFETY: The reference count will never be zero when this is
// called.
unsafe {
core::intrinsics::assume(weak != 0);
}
let weak = weak.wrapping_add(1);
self.weak_ref().set(weak);
// We want to abort on overflow instead of dropping the value.
// Checking for overflow after the store instead of before
// allows for slightly better code generation.
if core::intrinsics::unlikely(weak == 0) {
abort();
}
}
#[inline]
fn dec_weak(&self) {
self.weak_ref().set(self.weak() - 1);
}
}
impl<T: ?Sized> RcInnerPtr for RcBox<T> {
#[inline(always)]
fn weak_ref(&self) -> &Cell<usize> {
&self.weak
}
#[inline(always)]
fn strong_ref(&self) -> &Cell<usize> {
&self.strong
}
}
impl<'a> RcInnerPtr for WeakInner<'a> {
#[inline(always)]
fn weak_ref(&self) -> &Cell<usize> {
self.weak
}
#[inline(always)]
fn strong_ref(&self) -> &Cell<usize> {
self.strong
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
fn borrow(&self) -> &T {
&**self
}
}
#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized> AsRef<T> for Rc<T> {
fn as_ref(&self) -> &T {
&**self
}
}
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Rc<T> {}
/// Get the offset within an `RcBox` for the payload behind a pointer.
///
/// # Safety
///
/// The pointer must point to (and have valid metadata for) a previously
/// valid instance of T, but the T is allowed to be dropped.
unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
// Align the unsized value to the end of the RcBox.
// Because RcBox is repr(C), it will always be the last field in memory.
// SAFETY: since the only unsized types possible are slices, trait objects,
// and extern types, the input safety requirement is currently enough to
// satisfy the requirements of align_of_val_raw; this is an implementation
// detail of the language that must not be relied upon outside of std.
unsafe { data_offset_align(align_of_val_raw(ptr)) }
}
#[inline]
fn data_offset_align(align: usize) -> usize {
let layout = Layout::new::<RcBox<()>>();
layout.size() + layout.padding_needed_for(align)
}