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Diffstat (limited to 'library/alloc/src/rc.rs')
| -rw-r--r-- | library/alloc/src/rc.rs | 2700 |
1 files changed, 2700 insertions, 0 deletions
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs new file mode 100644 index 000000000..b89b03683 --- /dev/null +++ b/library/alloc/src/rc.rs @@ -0,0 +1,2700 @@ +//! 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, +} + +/// 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 + /// + /// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced + /// for the duration of the returned borrow. + /// 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"); + /// ``` + #[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`]). + /// + /// # 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 diassociating [`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> { + // 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). + let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align(); + 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> { + // 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). + let layout = Layout::new::<RcBox<()>>().extend(value_layout).unwrap().0.pad_to_align(); + + // 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 *mut 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()`). + /// + /// # 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) +} |