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Diffstat (limited to 'library/alloc/src/rc.rs')
| -rw-r--r-- | library/alloc/src/rc.rs | 4145 |
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diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs new file mode 100644 index 00000000000..2a4d4f26444 --- /dev/null +++ b/library/alloc/src/rc.rs @@ -0,0 +1,4145 @@ +//! 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`]. 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 +//! [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")] + +use core::any::Any; +use core::cell::Cell; +#[cfg(not(no_global_oom_handling))] +use core::clone::CloneToUninit; +use core::cmp::Ordering; +use core::hash::{Hash, Hasher}; +use core::intrinsics::abort; +#[cfg(not(no_global_oom_handling))] +use core::iter; +use core::marker::{PhantomData, Unsize}; +use core::mem::{self, ManuallyDrop, align_of_val_raw}; +use core::num::NonZeroUsize; +use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver}; +use core::panic::{RefUnwindSafe, UnwindSafe}; +#[cfg(not(no_global_oom_handling))] +use core::pin::Pin; +use core::pin::PinCoerceUnsized; +use core::ptr::{self, NonNull, drop_in_place}; +#[cfg(not(no_global_oom_handling))] +use core::slice::from_raw_parts_mut; +use core::{borrow, fmt, hint}; +#[cfg(test)] +use std::boxed::Box; + +#[cfg(not(no_global_oom_handling))] +use crate::alloc::handle_alloc_error; +use crate::alloc::{AllocError, Allocator, Global, Layout}; +use crate::borrow::{Cow, ToOwned}; +#[cfg(not(test))] +use crate::boxed::Box; +#[cfg(not(no_global_oom_handling))] +use crate::string::String; +#[cfg(not(no_global_oom_handling))] +use crate::vec::Vec; + +// 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 RcInner<T: ?Sized> { + strong: Cell<usize>, + weak: Cell<usize>, + value: T, +} + +/// Calculate layout for `RcInner<T>` using the inner value's layout +fn rc_inner_layout_for_value_layout(layout: Layout) -> Layout { + // Calculate layout using the given value layout. + // Previously, layout was calculated on the expression + // `&*(ptr as *const RcInner<T>)`, but this created a misaligned + // reference (see #54908). + Layout::new::<RcInner<()>>().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 +#[doc(search_unbox)] +#[cfg_attr(not(test), rustc_diagnostic_item = "Rc")] +#[stable(feature = "rust1", since = "1.0.0")] +#[rustc_insignificant_dtor] +pub struct Rc< + T: ?Sized, + #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global, +> { + ptr: NonNull<RcInner<T>>, + phantom: PhantomData<RcInner<T>>, + alloc: A, +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized, A: Allocator> !Send for Rc<T, A> {} + +// 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, A: Allocator> !Sync for Rc<T, A> {} + +#[stable(feature = "catch_unwind", since = "1.9.0")] +impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Rc<T, A> {} +#[stable(feature = "rc_ref_unwind_safe", since = "1.58.0")] +impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> RefUnwindSafe for Rc<T, A> {} + +#[unstable(feature = "coerce_unsized", issue = "18598")] +impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Rc<U, A>> for Rc<T, A> {} + +#[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] + unsafe fn from_inner(ptr: NonNull<RcInner<T>>) -> Self { + unsafe { Self::from_inner_in(ptr, Global) } + } + + #[inline] + unsafe fn from_ptr(ptr: *mut RcInner<T>) -> Self { + unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) } + } +} + +impl<T: ?Sized, A: Allocator> Rc<T, A> { + #[inline(always)] + fn inner(&self) -> &RcInner<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() } + } + + #[inline] + fn into_inner_with_allocator(this: Self) -> (NonNull<RcInner<T>>, A) { + let this = mem::ManuallyDrop::new(this); + (this.ptr, unsafe { ptr::read(&this.alloc) }) + } + + #[inline] + unsafe fn from_inner_in(ptr: NonNull<RcInner<T>>, alloc: A) -> Self { + Self { ptr, phantom: PhantomData, alloc } + } + + #[inline] + unsafe fn from_ptr_in(ptr: *mut RcInner<T>, alloc: A) -> Self { + unsafe { Self::from_inner_in(NonNull::new_unchecked(ptr), alloc) } + } + + // Non-inlined part of `drop`. + #[inline(never)] + unsafe fn drop_slow(&mut self) { + // Reconstruct the "strong weak" pointer and drop it when this + // variable goes out of scope. This ensures that the memory is + // deallocated even if the destructor of `T` panics. + let _weak = Weak { ptr: self.ptr, alloc: &self.alloc }; + + // Destroy the contained object. + // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed. + unsafe { + ptr::drop_in_place(&mut (*self.ptr.as_ptr()).value); + } + } +} + +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(RcInner { 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 { + /// /// Constructs 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() } + /// }) + /// } + /// + /// /// Returns 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, + { + Self::new_cyclic_in(data_fn, Global) + } + + /// Constructs a new `Rc` with uninitialized contents. + /// + /// # Examples + /// + /// ``` + /// #![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))] + #[stable(feature = "new_uninit", since = "1.82.0")] + #[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), + <*mut u8>::cast, + )) + } + } + + /// 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_zeroed_alloc)] + /// + /// 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_zeroed_alloc", issue = "129396")] + #[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), + <*mut u8>::cast, + )) + } + } + + /// 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(RcInner { + 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)] + /// #![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), + <*mut u8>::cast, + )?)) + } + } + + /// 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)] + /// + /// 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), + <*mut u8>::cast, + )?)) + } + } + /// 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)) } + } +} + +impl<T, A: Allocator> Rc<T, A> { + /// Constructs a new `Rc` in the provided allocator. + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let five = Rc::new_in(5, System); + /// ``` + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + #[inline] + pub fn new_in(value: T, alloc: A) -> Rc<T, A> { + // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable. + // That would make code size bigger. + match Self::try_new_in(value, alloc) { + Ok(m) => m, + Err(_) => handle_alloc_error(Layout::new::<RcInner<T>>()), + } + } + + /// Constructs a new `Rc` with uninitialized contents in the provided allocator. + /// + /// # Examples + /// + /// ``` + /// #![feature(get_mut_unchecked)] + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let mut five = Rc::<u32, _>::new_uninit_in(System); + /// + /// let five = unsafe { + /// // Deferred initialization: + /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5); + /// + /// five.assume_init() + /// }; + /// + /// assert_eq!(*five, 5) + /// ``` + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "new_uninit", issue = "63291")] + #[inline] + pub fn new_uninit_in(alloc: A) -> Rc<mem::MaybeUninit<T>, A> { + unsafe { + Rc::from_ptr_in( + Rc::allocate_for_layout( + Layout::new::<T>(), + |layout| alloc.allocate(layout), + <*mut u8>::cast, + ), + alloc, + ) + } + } + + /// Constructs a new `Rc` with uninitialized contents, with the memory + /// being filled with `0` bytes, in the provided allocator. + /// + /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and + /// incorrect usage of this method. + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let zero = Rc::<u32, _>::new_zeroed_in(System); + /// let zero = unsafe { zero.assume_init() }; + /// + /// assert_eq!(*zero, 0) + /// ``` + /// + /// [zeroed]: mem::MaybeUninit::zeroed + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "new_uninit", issue = "63291")] + #[inline] + pub fn new_zeroed_in(alloc: A) -> Rc<mem::MaybeUninit<T>, A> { + unsafe { + Rc::from_ptr_in( + Rc::allocate_for_layout( + Layout::new::<T>(), + |layout| alloc.allocate_zeroed(layout), + <*mut u8>::cast, + ), + alloc, + ) + } + } + + /// Constructs a new `Rc<T, A>` in the given allocator while giving you a `Weak<T, A>` 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, A>` is created, such that you can + /// clone and store it inside the `T`. + /// + /// `new_cyclic_in` first allocates the managed allocation for the `Rc<T, A>`, + /// then calls your closure, giving it a `Weak<T, A>` to this allocation, + /// and only afterwards completes the construction of the `Rc<T, A>` by placing + /// the `T` returned from your closure into the allocation. + /// + /// Since the new `Rc<T, A>` is not fully-constructed until `Rc<T, A>::new_cyclic_in` + /// 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, A>`] is dropped normally. + /// + /// # Examples + /// + /// See [`new_cyclic`]. + /// + /// [`new_cyclic`]: Rc::new_cyclic + /// [`upgrade`]: Weak::upgrade + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Rc<T, A> + where + F: FnOnce(&Weak<T, A>) -> T, + { + // Construct the inner in the "uninitialized" state with a single + // weak reference. + let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in( + RcInner { + strong: Cell::new(0), + weak: Cell::new(1), + value: mem::MaybeUninit::<T>::uninit(), + }, + alloc, + )); + let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into(); + let init_ptr: NonNull<RcInner<T>> = uninit_ptr.cast(); + + let weak = Weak { ptr: init_ptr, alloc }; + + // 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(&raw 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); + + // Strong references should collectively own a shared weak reference, + // so don't run the destructor for our old weak reference. + // Calling into_raw_with_allocator has the double effect of giving us back the allocator, + // and forgetting the weak reference. + let alloc = weak.into_raw_with_allocator().1; + + Rc::from_inner_in(init_ptr, alloc) + }; + + strong + } + + /// Constructs a new `Rc<T>` in the provided allocator, returning an error if the allocation + /// fails + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let five = Rc::try_new_in(5, System); + /// # Ok::<(), std::alloc::AllocError>(()) + /// ``` + #[unstable(feature = "allocator_api", issue = "32838")] + #[inline] + pub fn try_new_in(value: T, alloc: A) -> Result<Self, 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. + let (ptr, alloc) = Box::into_unique(Box::try_new_in( + RcInner { strong: Cell::new(1), weak: Cell::new(1), value }, + alloc, + )?); + Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) }) + } + + /// Constructs a new `Rc` with uninitialized contents, in the provided allocator, returning an + /// error if the allocation fails + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// #![feature(get_mut_unchecked)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let mut five = Rc::<u32, _>::try_new_uninit_in(System)?; + /// + /// let five = unsafe { + /// // Deferred initialization: + /// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5); + /// + /// five.assume_init() + /// }; + /// + /// assert_eq!(*five, 5); + /// # Ok::<(), std::alloc::AllocError>(()) + /// ``` + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "new_uninit", issue = "63291")] + #[inline] + pub fn try_new_uninit_in(alloc: A) -> Result<Rc<mem::MaybeUninit<T>, A>, AllocError> { + unsafe { + Ok(Rc::from_ptr_in( + Rc::try_allocate_for_layout( + Layout::new::<T>(), + |layout| alloc.allocate(layout), + <*mut u8>::cast, + )?, + alloc, + )) + } + } + + /// Constructs a new `Rc` with uninitialized contents, with the memory + /// being filled with `0` bytes, in the provided allocator, 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)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let zero = Rc::<u32, _>::try_new_zeroed_in(System)?; + /// 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")] + #[inline] + pub fn try_new_zeroed_in(alloc: A) -> Result<Rc<mem::MaybeUninit<T>, A>, AllocError> { + unsafe { + Ok(Rc::from_ptr_in( + Rc::try_allocate_for_layout( + Layout::new::<T>(), + |layout| alloc.allocate_zeroed(layout), + <*mut u8>::cast, + )?, + alloc, + )) + } + } + + /// Constructs a new `Pin<Rc<T>>` in the provided allocator. If `T` does not implement `Unpin`, then + /// `value` will be pinned in memory and unable to be moved. + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + #[inline] + pub fn pin_in(value: T, alloc: A) -> Pin<Self> + where + A: 'static, + { + unsafe { Pin::new_unchecked(Rc::new_in(value, alloc)) } + } + + /// 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 { + let this = ManuallyDrop::new(this); + + let val: T = unsafe { ptr::read(&**this) }; // copy the contained object + let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator + + // 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, alloc }; + Ok(val) + } else { + Err(this) + } + } + + /// Returns the inner value, if the `Rc` has exactly one strong reference. + /// + /// Otherwise, [`None`] is returned and the `Rc` is dropped. + /// + /// This will succeed even if there are outstanding weak references. + /// + /// If `Rc::into_inner` is called on every clone of this `Rc`, + /// it is guaranteed that exactly one of the calls returns the inner value. + /// This means in particular that the inner value is not dropped. + /// + /// [`Rc::try_unwrap`] is conceptually similar to `Rc::into_inner`. + /// And while they are meant for different use-cases, `Rc::into_inner(this)` + /// is in fact equivalent to <code>[Rc::try_unwrap]\(this).[ok][Result::ok]()</code>. + /// (Note that the same kind of equivalence does **not** hold true for + /// [`Arc`](crate::sync::Arc), due to race conditions that do not apply to `Rc`!) + /// + /// # Examples + /// + /// ``` + /// use std::rc::Rc; + /// + /// let x = Rc::new(3); + /// assert_eq!(Rc::into_inner(x), Some(3)); + /// + /// let x = Rc::new(4); + /// let y = Rc::clone(&x); + /// + /// assert_eq!(Rc::into_inner(y), None); + /// assert_eq!(Rc::into_inner(x), Some(4)); + /// ``` + #[inline] + #[stable(feature = "rc_into_inner", since = "1.70.0")] + pub fn into_inner(this: Self) -> Option<T> { + Rc::try_unwrap(this).ok() + } +} + +impl<T> Rc<[T]> { + /// Constructs a new reference-counted slice with uninitialized contents. + /// + /// # Examples + /// + /// ``` + /// #![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))] + #[stable(feature = "new_uninit", since = "1.82.0")] + #[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_zeroed_alloc)] + /// + /// 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_zeroed_alloc", issue = "129396")] + #[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.cast::<T>(), len) + as *mut RcInner<[mem::MaybeUninit<T>]> + }, + )) + } + } + + /// Converts the reference-counted slice into a reference-counted array. + /// + /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type. + /// + /// If `N` is not exactly equal to the length of `self`, then this method returns `None`. + #[unstable(feature = "slice_as_array", issue = "133508")] + #[inline] + #[must_use] + pub fn into_array<const N: usize>(self) -> Option<Rc<[T; N]>> { + if self.len() == N { + let ptr = Self::into_raw(self) as *const [T; N]; + + // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length. + let me = unsafe { Rc::from_raw(ptr) }; + Some(me) + } else { + None + } + } +} + +impl<T, A: Allocator> Rc<[T], A> { + /// Constructs a new reference-counted slice with uninitialized contents. + /// + /// # Examples + /// + /// ``` + /// #![feature(get_mut_unchecked)] + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let mut values = Rc::<[u32], _>::new_uninit_slice_in(3, System); + /// + /// let values = unsafe { + /// // Deferred initialization: + /// Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1); + /// Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2); + /// Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3); + /// + /// values.assume_init() + /// }; + /// + /// assert_eq!(*values, [1, 2, 3]) + /// ``` + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "new_uninit", issue = "63291")] + #[inline] + pub fn new_uninit_slice_in(len: usize, alloc: A) -> Rc<[mem::MaybeUninit<T>], A> { + unsafe { Rc::from_ptr_in(Rc::allocate_for_slice_in(len, &alloc), alloc) } + } + + /// 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(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let values = Rc::<[u32], _>::new_zeroed_slice_in(3, System); + /// let values = unsafe { values.assume_init() }; + /// + /// assert_eq!(*values, [0, 0, 0]) + /// ``` + /// + /// [zeroed]: mem::MaybeUninit::zeroed + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "new_uninit", issue = "63291")] + #[inline] + pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Rc<[mem::MaybeUninit<T>], A> { + unsafe { + Rc::from_ptr_in( + Rc::allocate_for_layout( + Layout::array::<T>(len).unwrap(), + |layout| alloc.allocate_zeroed(layout), + |mem| { + ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) + as *mut RcInner<[mem::MaybeUninit<T>]> + }, + ), + alloc, + ) + } + } +} + +impl<T, A: Allocator> Rc<mem::MaybeUninit<T>, A> { + /// 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(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) + /// ``` + #[stable(feature = "new_uninit", since = "1.82.0")] + #[inline] + pub unsafe fn assume_init(self) -> Rc<T, A> { + let (ptr, alloc) = Rc::into_inner_with_allocator(self); + unsafe { Rc::from_inner_in(ptr.cast(), alloc) } + } +} + +impl<T, A: Allocator> Rc<[mem::MaybeUninit<T>], A> { + /// 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(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]) + /// ``` + #[stable(feature = "new_uninit", since = "1.82.0")] + #[inline] + pub unsafe fn assume_init(self) -> Rc<[T], A> { + let (ptr, alloc) = Rc::into_inner_with_allocator(self); + unsafe { Rc::from_ptr_in(ptr.as_ptr() as _, alloc) } + } +} + +impl<T: ?Sized> Rc<T> { + /// 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] with the following requirements: + /// + /// * If `U` is sized, it must have the same size and alignment as `T`. This + /// is trivially true if `U` is `T`. + /// * If `U` is unsized, its data pointer must have the same size and + /// alignment as `T`. This is trivially true if `Rc<U>` was constructed + /// through `Rc<T>` and then converted to `Rc<U>` through an [unsized + /// coercion]. + /// + /// Note that if `U` or `U`'s data pointer is not `T` but has the same size + /// and alignment, this is basically like transmuting references of + /// different types. See [`mem::transmute`][transmute] for more information + /// on what restrictions apply in this case. + /// + /// The raw pointer must point to a block of memory allocated by the global allocator + /// + /// 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 + /// [transmute]: core::mem::transmute + /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions + /// + /// # 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! + /// ``` + /// + /// Convert a slice back into its original array: + /// + /// ``` + /// use std::rc::Rc; + /// + /// let x: Rc<[u32]> = Rc::new([1, 2, 3]); + /// let x_ptr: *const [u32] = Rc::into_raw(x); + /// + /// unsafe { + /// let x: Rc<[u32; 3]> = Rc::from_raw(x_ptr.cast::<[u32; 3]>()); + /// assert_eq!(&*x, &[1, 2, 3]); + /// } + /// ``` + #[inline] + #[stable(feature = "rc_raw", since = "1.17.0")] + pub unsafe fn from_raw(ptr: *const T) -> Self { + unsafe { Self::from_raw_in(ptr, Global) } + } + + /// 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`, the + /// associated `Rc` instance must be valid (i.e. the strong count must be at + /// least 1) for the duration of this method, and `ptr` must point to a block of memory + /// allocated by the global allocator. + /// + /// # 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)); + /// # // Prevent leaks for Miri. + /// # Rc::decrement_strong_count(ptr); + /// } + /// ``` + #[inline] + #[stable(feature = "rc_mutate_strong_count", since = "1.53.0")] + pub unsafe fn increment_strong_count(ptr: *const T) { + unsafe { Self::increment_strong_count_in(ptr, Global) } + } + + /// 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`, the + /// associated `Rc` instance must be valid (i.e. the strong count must be at + /// least 1) when invoking this method, and `ptr` must point to a block of memory + /// allocated by the global allocator. 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 { Self::decrement_strong_count_in(ptr, Global) } + } +} + +impl<T: ?Sized, A: Allocator> Rc<T, A> { + /// Returns a reference to the underlying allocator. + /// + /// Note: this is an associated function, which means that you have + /// to call it as `Rc::allocator(&r)` instead of `r.allocator()`. This + /// is so that there is no conflict with a method on the inner type. + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn allocator(this: &Self) -> &A { + &this.alloc + } + + /// 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"); + /// # // Prevent leaks for Miri. + /// # drop(unsafe { Rc::from_raw(x_ptr) }); + /// ``` + #[must_use = "losing the pointer will leak memory"] + #[stable(feature = "rc_raw", since = "1.17.0")] + #[rustc_never_returns_null_ptr] + pub fn into_raw(this: Self) -> *const T { + let this = ManuallyDrop::new(this); + Self::as_ptr(&*this) + } + + /// Consumes the `Rc`, returning the wrapped pointer and allocator. + /// + /// To avoid a memory leak the pointer must be converted back to an `Rc` using + /// [`Rc::from_raw_in`]. + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let x = Rc::new_in("hello".to_owned(), System); + /// let (ptr, alloc) = Rc::into_raw_with_allocator(x); + /// assert_eq!(unsafe { &*ptr }, "hello"); + /// let x = unsafe { Rc::from_raw_in(ptr, alloc) }; + /// assert_eq!(&*x, "hello"); + /// ``` + #[must_use = "losing the pointer will leak memory"] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn into_raw_with_allocator(this: Self) -> (*const T, A) { + let this = mem::ManuallyDrop::new(this); + let ptr = Self::as_ptr(&this); + // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped + let alloc = unsafe { ptr::read(&this.alloc) }; + (ptr, alloc) + } + + /// 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 as there are strong counts in the `Rc`. + /// + /// # Examples + /// + /// ``` + /// use std::rc::Rc; + /// + /// let x = Rc::new(0); + /// 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 }, 0); + /// ``` + #[stable(feature = "weak_into_raw", since = "1.45.0")] + #[rustc_never_returns_null_ptr] + pub fn as_ptr(this: &Self) -> *const T { + let ptr: *mut RcInner<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 { &raw mut (*ptr).value } + } + + /// Constructs an `Rc<T, A>` from a raw pointer in the provided allocator. + /// + /// The raw pointer must have been previously returned by a call to [`Rc<U, + /// A>::into_raw`][into_raw] with the following requirements: + /// + /// * If `U` is sized, it must have the same size and alignment as `T`. This + /// is trivially true if `U` is `T`. + /// * If `U` is unsized, its data pointer must have the same size and + /// alignment as `T`. This is trivially true if `Rc<U>` was constructed + /// through `Rc<T>` and then converted to `Rc<U>` through an [unsized + /// coercion]. + /// + /// Note that if `U` or `U`'s data pointer is not `T` but has the same size + /// and alignment, this is basically like transmuting references of + /// different types. See [`mem::transmute`][transmute] for more information + /// on what restrictions apply in this case. + /// + /// The raw pointer must point to a block of memory allocated by `alloc` + /// + /// 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 + /// [transmute]: core::mem::transmute + /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let x = Rc::new_in("hello".to_owned(), System); + /// let x_ptr = Rc::into_raw(x); + /// + /// unsafe { + /// // Convert back to an `Rc` to prevent leak. + /// let x = Rc::from_raw_in(x_ptr, System); + /// 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! + /// ``` + /// + /// Convert a slice back into its original array: + /// + /// ``` + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let x: Rc<[u32], _> = Rc::new_in([1, 2, 3], System); + /// let x_ptr: *const [u32] = Rc::into_raw(x); + /// + /// unsafe { + /// let x: Rc<[u32; 3], _> = Rc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System); + /// assert_eq!(&*x, &[1, 2, 3]); + /// } + /// ``` + #[unstable(feature = "allocator_api", issue = "32838")] + pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self { + let offset = unsafe { data_offset(ptr) }; + + // Reverse the offset to find the original RcInner. + let rc_ptr = unsafe { ptr.byte_sub(offset) as *mut RcInner<T> }; + + unsafe { Self::from_ptr_in(rc_ptr, alloc) } + } + + /// 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, A> + where + A: Clone, + { + 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, alloc: this.alloc.clone() } + } + + /// 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`, the + /// associated `Rc` instance must be valid (i.e. the strong count must be at + /// least 1) for the duration of this method, and `ptr` must point to a block of memory + /// allocated by `alloc` + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let five = Rc::new_in(5, System); + /// + /// unsafe { + /// let ptr = Rc::into_raw(five); + /// Rc::increment_strong_count_in(ptr, System); + /// + /// let five = Rc::from_raw_in(ptr, System); + /// assert_eq!(2, Rc::strong_count(&five)); + /// # // Prevent leaks for Miri. + /// # Rc::decrement_strong_count_in(ptr, System); + /// } + /// ``` + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A) + where + A: Clone, + { + // Retain Rc, but don't touch refcount by wrapping in ManuallyDrop + let rc = unsafe { mem::ManuallyDrop::new(Rc::<T, A>::from_raw_in(ptr, alloc)) }; + // 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`, the + /// associated `Rc` instance must be valid (i.e. the strong count must be at + /// least 1) when invoking this method, and `ptr` must point to a block of memory + /// allocated by `alloc`. 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 + /// + /// ``` + /// #![feature(allocator_api)] + /// + /// use std::rc::Rc; + /// use std::alloc::System; + /// + /// let five = Rc::new_in(5, System); + /// + /// unsafe { + /// let ptr = Rc::into_raw(five); + /// Rc::increment_strong_count_in(ptr, System); + /// + /// let five = Rc::from_raw_in(ptr, System); + /// assert_eq!(2, Rc::strong_count(&five)); + /// Rc::decrement_strong_count_in(ptr, System); + /// assert_eq!(1, Rc::strong_count(&five)); + /// } + /// ``` + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) { + unsafe { drop(Rc::from_raw_in(ptr, alloc)) }; + } + + /// 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 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(); + /// 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`]. This function ignores the metadata of `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 { + ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr()) + } +} + +#[cfg(not(no_global_oom_handling))] +impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Rc<T, A> { + /// 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()); + /// ``` + #[inline] + #[stable(feature = "rc_unique", since = "1.4.0")] + pub fn make_mut(this: &mut Self) -> &mut T { + let size_of_val = size_of_val::<T>(&**this); + + if Rc::strong_count(this) != 1 { + // Gotta clone the data, there are other Rcs. + + let this_data_ref: &T = &**this; + // `in_progress` drops the allocation if we panic before finishing initializing it. + let mut in_progress: UniqueRcUninit<T, A> = + UniqueRcUninit::new(this_data_ref, this.alloc.clone()); + + // Initialize with clone of this. + let initialized_clone = unsafe { + // Clone. If the clone panics, `in_progress` will be dropped and clean up. + this_data_ref.clone_to_uninit(in_progress.data_ptr().cast()); + // Cast type of pointer, now that it is initialized. + in_progress.into_rc() + }; + + // Replace `this` with newly constructed Rc. + *this = initialized_clone; + } else if Rc::weak_count(this) != 0 { + // Can just steal the data, all that's left is Weaks + + // We don't need panic-protection like the above branch does, but we might as well + // use the same mechanism. + let mut in_progress: UniqueRcUninit<T, A> = + UniqueRcUninit::new(&**this, this.alloc.clone()); + unsafe { + // Initialize `in_progress` with move of **this. + // We have to express this in terms of bytes because `T: ?Sized`; there is no + // operation that just copies a value based on its `size_of_val()`. + ptr::copy_nonoverlapping( + ptr::from_ref(&**this).cast::<u8>(), + in_progress.data_ptr().cast::<u8>(), + size_of_val, + ); + + 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(); + // Replace `this` with newly constructed Rc that has the moved data. + ptr::write(this, in_progress.into_rc()); + } + } + // 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 } + } +} + +impl<T: Clone, A: Allocator> Rc<T, A> { + /// 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 + /// + /// ``` + /// # 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] + #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")] + pub fn unwrap_or_clone(this: Self) -> T { + Rc::try_unwrap(this).unwrap_or_else(|rc| (*rc).clone()) + } +} + +impl<A: Allocator> Rc<dyn Any, A> { + /// Attempts 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, A>, Self> { + if (*self).is::<T>() { + unsafe { + let (ptr, alloc) = Rc::into_inner_with_allocator(self); + Ok(Rc::from_inner_in(ptr.cast(), alloc)) + } + } 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, A> { + unsafe { + let (ptr, alloc) = Rc::into_inner_with_allocator(self); + Rc::from_inner_in(ptr.cast(), alloc) + } + } +} + +impl<T: ?Sized> Rc<T> { + /// Allocates an `RcInner<T>` with sufficient space for + /// a possibly-unsized inner value where the value has the layout provided. + /// + /// The function `mem_to_rc_inner` is called with the data pointer + /// and must return back a (potentially fat)-pointer for the `RcInner<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_rc_inner: impl FnOnce(*mut u8) -> *mut RcInner<T>, + ) -> *mut RcInner<T> { + let layout = rc_inner_layout_for_value_layout(value_layout); + unsafe { + Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rc_inner) + .unwrap_or_else(|_| handle_alloc_error(layout)) + } + } + + /// Allocates an `RcInner<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_rc_inner` is called with the data pointer + /// and must return back a (potentially fat)-pointer for the `RcInner<T>`. + #[inline] + unsafe fn try_allocate_for_layout( + value_layout: Layout, + allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>, + mem_to_rc_inner: impl FnOnce(*mut u8) -> *mut RcInner<T>, + ) -> Result<*mut RcInner<T>, AllocError> { + let layout = rc_inner_layout_for_value_layout(value_layout); + + // Allocate for the layout. + let ptr = allocate(layout)?; + + // Initialize the RcInner + let inner = mem_to_rc_inner(ptr.as_non_null_ptr().as_ptr()); + unsafe { + debug_assert_eq!(Layout::for_value_raw(inner), layout); + + (&raw mut (*inner).strong).write(Cell::new(1)); + (&raw mut (*inner).weak).write(Cell::new(1)); + } + + Ok(inner) + } +} + +impl<T: ?Sized, A: Allocator> Rc<T, A> { + /// Allocates an `RcInner<T>` with sufficient space for an unsized inner value + #[cfg(not(no_global_oom_handling))] + unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut RcInner<T> { + // Allocate for the `RcInner<T>` using the given value. + unsafe { + Rc::<T>::allocate_for_layout( + Layout::for_value_raw(ptr), + |layout| alloc.allocate(layout), + |mem| mem.with_metadata_of(ptr as *const RcInner<T>), + ) + } + } + + #[cfg(not(no_global_oom_handling))] + fn from_box_in(src: Box<T, A>) -> Rc<T, A> { + unsafe { + let value_size = size_of_val(&*src); + let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src)); + + // Copy value as bytes + ptr::copy_nonoverlapping( + (&raw const *src) as *const u8, + (&raw mut (*ptr).value) as *mut u8, + value_size, + ); + + // Free the allocation without dropping its contents + let (bptr, alloc) = Box::into_raw_with_allocator(src); + let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref()); + drop(src); + + Self::from_ptr_in(ptr, alloc) + } + } +} + +impl<T> Rc<[T]> { + /// Allocates an `RcInner<[T]>` with the given length. + #[cfg(not(no_global_oom_handling))] + unsafe fn allocate_for_slice(len: usize) -> *mut RcInner<[T]> { + unsafe { + Self::allocate_for_layout( + Layout::array::<T>(len).unwrap(), + |layout| Global.allocate(layout), + |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut RcInner<[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(), (&raw mut (*ptr).value) 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 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 RcInner 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_raw(ptr); + + // Pointer to first element + let elems = (&raw mut (*ptr).value) 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 RcInner. + mem::forget(guard); + + Self::from_ptr(ptr) + } + } +} + +impl<T, A: Allocator> Rc<[T], A> { + /// Allocates an `RcInner<[T]>` with the given length. + #[inline] + #[cfg(not(no_global_oom_handling))] + unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut RcInner<[T]> { + unsafe { + Rc::<[T]>::allocate_for_layout( + Layout::array::<T>(len).unwrap(), + |layout| alloc.allocate(layout), + |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut RcInner<[T]>, + ) + } + } +} + +#[cfg(not(no_global_oom_handling))] +/// 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, A: Allocator> Deref for Rc<T, A> { + type Target = T; + + #[inline(always)] + fn deref(&self) -> &T { + &self.inner().value + } +} + +#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")] +unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Rc<T, A> {} + +//#[unstable(feature = "unique_rc_arc", issue = "112566")] +#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")] +unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for UniqueRc<T, A> {} + +#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")] +unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {} + +#[unstable(feature = "deref_pure_trait", issue = "87121")] +unsafe impl<T: ?Sized, A: Allocator> DerefPure for Rc<T, A> {} + +//#[unstable(feature = "unique_rc_arc", issue = "112566")] +#[unstable(feature = "deref_pure_trait", issue = "87121")] +unsafe impl<T: ?Sized, A: Allocator> DerefPure for UniqueRc<T, A> {} + +#[unstable(feature = "legacy_receiver_trait", issue = "none")] +impl<T: ?Sized> LegacyReceiver for Rc<T> {} + +#[stable(feature = "rust1", since = "1.0.0")] +unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Rc<T, A> { + /// 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!" + /// ``` + #[inline] + fn drop(&mut self) { + unsafe { + self.inner().dec_strong(); + if self.inner().strong() == 0 { + self.drop_slow(); + } + } + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized, A: Allocator + Clone> Clone for Rc<T, A> { + /// 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) -> Self { + unsafe { + self.inner().inc_strong(); + Self::from_inner_in(self.ptr, self.alloc.clone()) + } + } +} + +#[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> { + unsafe { + Self::from_inner( + Box::leak(Box::write( + Box::new_uninit(), + RcInner { strong: Cell::new(1), weak: Cell::new(1), value: T::default() }, + )) + .into(), + ) + } + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "more_rc_default_impls", since = "1.80.0")] +impl Default for Rc<str> { + /// Creates an empty str inside an Rc + /// + /// This may or may not share an allocation with other Rcs on the same thread. + #[inline] + fn default() -> Self { + let rc = Rc::<[u8]>::default(); + // `[u8]` has the same layout as `str`. + unsafe { Rc::from_raw(Rc::into_raw(rc) as *const str) } + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "more_rc_default_impls", since = "1.80.0")] +impl<T> Default for Rc<[T]> { + /// Creates an empty `[T]` inside an Rc + /// + /// This may or may not share an allocation with other Rcs on the same thread. + #[inline] + fn default() -> Self { + let arr: [T; 0] = []; + Rc::from(arr) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +trait RcEqIdent<T: ?Sized + PartialEq, A: Allocator> { + fn eq(&self, other: &Rc<T, A>) -> bool; + fn ne(&self, other: &Rc<T, A>) -> bool; +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + PartialEq, A: Allocator> RcEqIdent<T, A> for Rc<T, A> { + #[inline] + default fn eq(&self, other: &Rc<T, A>) -> bool { + **self == **other + } + + #[inline] + default fn ne(&self, other: &Rc<T, A>) -> 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, A: Allocator> RcEqIdent<T, A> for Rc<T, A> { + #[inline] + fn eq(&self, other: &Rc<T, A>) -> bool { + Rc::ptr_eq(self, other) || **self == **other + } + + #[inline] + fn ne(&self, other: &Rc<T, A>) -> bool { + !Rc::ptr_eq(self, other) && **self != **other + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Rc<T, A> { + /// 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, A>) -> bool { + RcEqIdent::eq(self, other) + } + + /// Inequality for two `Rc`s. + /// + /// Two `Rc`s are not equal if their inner values are not equal. + /// + /// 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(6)); + /// ``` + #[inline] + fn ne(&self, other: &Rc<T, A>) -> bool { + RcEqIdent::ne(self, other) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + Eq, A: Allocator> Eq for Rc<T, A> {} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Rc<T, A> { + /// 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, A>) -> 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, A>) -> 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, A>) -> 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, A>) -> 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, A>) -> bool { + **self >= **other + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + Ord, A: Allocator> Ord for Rc<T, A> { + /// 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, A>) -> Ordering { + (**self).cmp(&**other) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + Hash, A: Allocator> Hash for Rc<T, A> { + fn hash<H: Hasher>(&self, state: &mut H) { + (**self).hash(state); + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Rc<T, A> { + 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, A: Allocator> fmt::Debug for Rc<T, A> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Debug::fmt(&**self, f) + } +} + +#[stable(feature = "rust1", since = "1.0.0")] +impl<T: ?Sized, A: Allocator> fmt::Pointer for Rc<T, A> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Pointer::fmt(&(&raw const **self), 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_array", since = "1.74.0")] +impl<T, const N: usize> From<[T; N]> for Rc<[T]> { + /// Converts a [`[T; N]`](prim@array) into an `Rc<[T]>`. + /// + /// The conversion moves the array into a newly allocated `Rc`. + /// + /// # Example + /// + /// ``` + /// # use std::rc::Rc; + /// let original: [i32; 3] = [1, 2, 3]; + /// let shared: Rc<[i32]> = Rc::from(original); + /// assert_eq!(&[1, 2, 3], &shared[..]); + /// ``` + #[inline] + fn from(v: [T; N]) -> Rc<[T]> { + Rc::<[T; N]>::from(v) + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "shared_from_slice", since = "1.21.0")] +impl<T: Clone> From<&[T]> for Rc<[T]> { + /// Allocates a reference-counted slice and fills 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_mut_slice", since = "1.84.0")] +impl<T: Clone> From<&mut [T]> for Rc<[T]> { + /// Allocates a reference-counted slice and fills it by cloning `v`'s items. + /// + /// # Example + /// + /// ``` + /// # use std::rc::Rc; + /// let mut original = [1, 2, 3]; + /// let original: &mut [i32] = &mut original; + /// let shared: Rc<[i32]> = Rc::from(original); + /// assert_eq!(&[1, 2, 3], &shared[..]); + /// ``` + #[inline] + fn from(v: &mut [T]) -> Rc<[T]> { + Rc::from(&*v) + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "shared_from_slice", since = "1.21.0")] +impl From<&str> for Rc<str> { + /// Allocates a reference-counted string slice and copies `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_mut_slice", since = "1.84.0")] +impl From<&mut str> for Rc<str> { + /// Allocates a reference-counted string slice and copies `v` into it. + /// + /// # Example + /// + /// ``` + /// # use std::rc::Rc; + /// let mut original = String::from("statue"); + /// let original: &mut str = &mut original; + /// let shared: Rc<str> = Rc::from(original); + /// assert_eq!("statue", &shared[..]); + /// ``` + #[inline] + fn from(v: &mut str) -> Rc<str> { + Rc::from(&*v) + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "shared_from_slice", since = "1.21.0")] +impl From<String> for Rc<str> { + /// Allocates a reference-counted string slice and copies `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, A: Allocator> From<Box<T, A>> for Rc<T, A> { + /// 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, A>) -> Rc<T, A> { + Rc::from_box_in(v) + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "shared_from_slice", since = "1.21.0")] +impl<T, A: Allocator> From<Vec<T, A>> for Rc<[T], A> { + /// Allocates a reference-counted slice and moves `v`'s items into it. + /// + /// # Example + /// + /// ``` + /// # use std::rc::Rc; + /// let unique: Vec<i32> = vec![1, 2, 3]; + /// let shared: Rc<[i32]> = Rc::from(unique); + /// assert_eq!(&[1, 2, 3], &shared[..]); + /// ``` + #[inline] + fn from(v: Vec<T, A>) -> Rc<[T], A> { + unsafe { + let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc(); + + let rc_ptr = Self::allocate_for_slice_in(len, &alloc); + ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).value) as *mut T, len); + + // Create a `Vec<T, &A>` with length 0, to deallocate the buffer + // without dropping its contents or the allocator + let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc); + + Self::from_ptr_in(rc_ptr, alloc) + } + } +} + +#[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>, +{ + /// Creates 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, A: Allocator, const N: usize> TryFrom<Rc<[T], A>> for Rc<[T; N], A> { + type Error = Rc<[T], A>; + + fn try_from(boxed_slice: Rc<[T], A>) -> Result<Self, Self::Error> { + if boxed_slice.len() == N { + let (ptr, alloc) = Rc::into_inner_with_allocator(boxed_slice); + Ok(unsafe { Rc::from_inner_in(ptr.cast(), alloc) }) + } else { + Err(boxed_slice) + } + } +} + +#[cfg(not(no_global_oom_handling))] +#[stable(feature = "shared_from_iter", since = "1.37.0")] +impl<T> 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: 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")] +#[cfg_attr(not(test), rustc_diagnostic_item = "RcWeak")] +pub struct Weak< + T: ?Sized, + #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global, +> { + // 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 RcInner has alignment at least 2. + // This is only possible when `T: Sized`; unsized `T` never dangle. + ptr: NonNull<RcInner<T>>, + alloc: A, +} + +#[stable(feature = "rc_weak", since = "1.4.0")] +impl<T: ?Sized, A: Allocator> !Send for Weak<T, A> {} +#[stable(feature = "rc_weak", since = "1.4.0")] +impl<T: ?Sized, A: Allocator> !Sync for Weak<T, A> {} + +#[unstable(feature = "coerce_unsized", issue = "18598")] +impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {} + +#[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()); + /// ``` + #[inline] + #[stable(feature = "downgraded_weak", since = "1.10.0")] + #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")] + #[must_use] + pub const fn new() -> Weak<T> { + Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global } + } +} + +impl<T, A: Allocator> Weak<T, A> { + /// Constructs a new `Weak<T>`, without allocating any memory, technically in the provided + /// allocator. + /// 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()); + /// ``` + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn new_in(alloc: A) -> Weak<T, A> { + Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc } + } +} + +pub(crate) fn is_dangling<T: ?Sized>(ptr: *const T) -> bool { + (ptr.cast::<()>()).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> { + /// 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, and `ptr` must point to a block of memory allocated by the global allocator. + /// + /// 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 + #[inline] + #[stable(feature = "weak_into_raw", since = "1.45.0")] + pub unsafe fn from_raw(ptr: *const T) -> Self { + unsafe { Self::from_raw_in(ptr, Global) } + } +} + +impl<T: ?Sized, A: Allocator> Weak<T, A> { + /// Returns a reference to the underlying allocator. + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn allocator(&self) -> &A { + &self.alloc + } + + /// 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 behavior. + /// // 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 RcInner<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 RcInner (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 { &raw 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 = "losing the pointer will leak memory"] + #[stable(feature = "weak_into_raw", since = "1.45.0")] + pub fn into_raw(self) -> *const T { + mem::ManuallyDrop::new(self).as_ptr() + } + + /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator. + /// + /// 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_in`]. + /// + /// The same restrictions of accessing the target of the pointer as with + /// [`as_ptr`] apply. + /// + /// # Examples + /// + /// ``` + /// #![feature(allocator_api)] + /// use std::rc::{Rc, Weak}; + /// use std::alloc::System; + /// + /// let strong = Rc::new_in("hello".to_owned(), System); + /// let weak = Rc::downgrade(&strong); + /// let (raw, alloc) = weak.into_raw_with_allocator(); + /// + /// assert_eq!(1, Rc::weak_count(&strong)); + /// assert_eq!("hello", unsafe { &*raw }); + /// + /// drop(unsafe { Weak::from_raw_in(raw, alloc) }); + /// assert_eq!(0, Rc::weak_count(&strong)); + /// ``` + /// + /// [`from_raw_in`]: Weak::from_raw_in + /// [`as_ptr`]: Weak::as_ptr + #[must_use = "losing the pointer will leak memory"] + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub fn into_raw_with_allocator(self) -> (*const T, A) { + let this = mem::ManuallyDrop::new(self); + let result = this.as_ptr(); + // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped + let alloc = unsafe { ptr::read(&this.alloc) }; + (result, alloc) + } + + /// 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, and `ptr` must point to a block of memory allocated by `alloc`. + /// + /// 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 + #[inline] + #[unstable(feature = "allocator_api", issue = "32838")] + pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self { + // See Weak::as_ptr for context on how the input pointer is derived. + + let ptr = if is_dangling(ptr) { + // This is a dangling Weak. + ptr as *mut RcInner<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 RcInner. + // SAFETY: the pointer originated from a Weak, so this offset is safe. + unsafe { ptr.byte_sub(offset) as *mut RcInner<T> } + }; + + // SAFETY: we now have recovered the original Weak pointer, so can create the Weak. + Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc } + } + + /// 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, A>> + where + A: Clone, + { + let inner = self.inner()?; + + if inner.strong() == 0 { + None + } else { + unsafe { + inner.inc_strong(); + Some(Rc::from_inner_in(self.ptr, self.alloc.clone())) + } + } + } + + /// 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 { + if let Some(inner) = self.inner() { + if inner.strong() > 0 { + inner.weak() - 1 // subtract the implicit weak ptr + } else { + 0 + } + } else { + 0 + } + } + + /// Returns `None` when the pointer is dangling and there is no allocated `RcInner`, + /// (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()`). However, + /// this function ignores the metadata of `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 { + ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr()) + } +} + +#[stable(feature = "rc_weak", since = "1.4.0")] +unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> { + /// 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 { + self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())); + } + } + } +} + +#[stable(feature = "rc_weak", since = "1.4.0")] +impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> { + /// 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, A> { + if let Some(inner) = self.inner() { + inner.inc_weak() + } + Weak { ptr: self.ptr, alloc: self.alloc.clone() } + } +} + +#[stable(feature = "rc_weak", since = "1.4.0")] +impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> { + 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 { + hint::assert_unchecked(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 { + hint::assert_unchecked(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 RcInner<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, A: Allocator> borrow::Borrow<T> for Rc<T, A> { + fn borrow(&self) -> &T { + &**self + } +} + +#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")] +impl<T: ?Sized, A: Allocator> AsRef<T> for Rc<T, A> { + fn as_ref(&self) -> &T { + &**self + } +} + +#[stable(feature = "pin", since = "1.33.0")] +impl<T: ?Sized, A: Allocator> Unpin for Rc<T, A> {} + +/// Gets the offset within an `RcInner` 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 RcInner. + // Because RcInner 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::<RcInner<()>>(); + layout.size() + layout.padding_needed_for(align) +} + +/// A uniquely owned [`Rc`]. +/// +/// This represents an `Rc` that is known to be uniquely owned -- that is, have exactly one strong +/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong +/// references will fail unless the `UniqueRc` they point to has been converted into a regular `Rc`. +/// +/// Because they are uniquely owned, the contents of a `UniqueRc` can be freely mutated. A common +/// use case is to have an object be mutable during its initialization phase but then have it become +/// immutable and converted to a normal `Rc`. +/// +/// This can be used as a flexible way to create cyclic data structures, as in the example below. +/// +/// ``` +/// #![feature(unique_rc_arc)] +/// use std::rc::{Rc, Weak, UniqueRc}; +/// +/// struct Gadget { +/// #[allow(dead_code)] +/// me: Weak<Gadget>, +/// } +/// +/// fn create_gadget() -> Option<Rc<Gadget>> { +/// let mut rc = UniqueRc::new(Gadget { +/// me: Weak::new(), +/// }); +/// rc.me = UniqueRc::downgrade(&rc); +/// Some(UniqueRc::into_rc(rc)) +/// } +/// +/// create_gadget().unwrap(); +/// ``` +/// +/// An advantage of using `UniqueRc` over [`Rc::new_cyclic`] to build cyclic data structures is that +/// [`Rc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the +/// previous example, `UniqueRc` allows for more flexibility in the construction of cyclic data, +/// including fallible or async constructors. +#[unstable(feature = "unique_rc_arc", issue = "112566")] +pub struct UniqueRc< + T: ?Sized, + #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global, +> { + ptr: NonNull<RcInner<T>>, + // Define the ownership of `RcInner<T>` for drop-check + _marker: PhantomData<RcInner<T>>, + // Invariance is necessary for soundness: once other `Weak` + // references exist, we already have a form of shared mutability! + _marker2: PhantomData<*mut T>, + alloc: A, +} + +// Not necessary for correctness since `UniqueRc` contains `NonNull`, +// but having an explicit negative impl is nice for documentation purposes +// and results in nicer error messages. +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> !Send for UniqueRc<T, A> {} + +// Not necessary for correctness since `UniqueRc` contains `NonNull`, +// but having an explicit negative impl is nice for documentation purposes +// and results in nicer error messages. +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> !Sync for UniqueRc<T, A> {} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueRc<U, A>> + for UniqueRc<T, A> +{ +} + +//#[unstable(feature = "unique_rc_arc", issue = "112566")] +#[unstable(feature = "dispatch_from_dyn", issue = "none")] +impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueRc<U>> for UniqueRc<T> {} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for UniqueRc<T, A> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Display::fmt(&**self, f) + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for UniqueRc<T, A> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Debug::fmt(&**self, f) + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> fmt::Pointer for UniqueRc<T, A> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Pointer::fmt(&(&raw const **self), f) + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for UniqueRc<T, A> { + fn borrow(&self) -> &T { + &**self + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for UniqueRc<T, A> { + fn borrow_mut(&mut self) -> &mut T { + &mut **self + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueRc<T, A> { + fn as_ref(&self) -> &T { + &**self + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueRc<T, A> { + fn as_mut(&mut self) -> &mut T { + &mut **self + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> Unpin for UniqueRc<T, A> {} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueRc<T, A> { + /// Equality for two `UniqueRc`s. + /// + /// Two `UniqueRc`s are equal if their inner values are equal. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five == UniqueRc::new(5)); + /// ``` + #[inline] + fn eq(&self, other: &Self) -> bool { + PartialEq::eq(&**self, &**other) + } + + /// Inequality for two `UniqueRc`s. + /// + /// Two `UniqueRc`s are not equal if their inner values are not equal. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five != UniqueRc::new(6)); + /// ``` + #[inline] + fn ne(&self, other: &Self) -> bool { + PartialEq::ne(&**self, &**other) + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueRc<T, A> { + /// Partial comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `partial_cmp()` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// use std::cmp::Ordering; + /// + /// let five = UniqueRc::new(5); + /// + /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueRc::new(6))); + /// ``` + #[inline(always)] + fn partial_cmp(&self, other: &UniqueRc<T, A>) -> Option<Ordering> { + (**self).partial_cmp(&**other) + } + + /// Less-than comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `<` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five < UniqueRc::new(6)); + /// ``` + #[inline(always)] + fn lt(&self, other: &UniqueRc<T, A>) -> bool { + **self < **other + } + + /// 'Less than or equal to' comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `<=` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five <= UniqueRc::new(5)); + /// ``` + #[inline(always)] + fn le(&self, other: &UniqueRc<T, A>) -> bool { + **self <= **other + } + + /// Greater-than comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `>` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five > UniqueRc::new(4)); + /// ``` + #[inline(always)] + fn gt(&self, other: &UniqueRc<T, A>) -> bool { + **self > **other + } + + /// 'Greater than or equal to' comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `>=` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// + /// let five = UniqueRc::new(5); + /// + /// assert!(five >= UniqueRc::new(5)); + /// ``` + #[inline(always)] + fn ge(&self, other: &UniqueRc<T, A>) -> bool { + **self >= **other + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueRc<T, A> { + /// Comparison for two `UniqueRc`s. + /// + /// The two are compared by calling `cmp()` on their inner values. + /// + /// # Examples + /// + /// ``` + /// #![feature(unique_rc_arc)] + /// use std::rc::UniqueRc; + /// use std::cmp::Ordering; + /// + /// let five = UniqueRc::new(5); + /// + /// assert_eq!(Ordering::Less, five.cmp(&UniqueRc::new(6))); + /// ``` + #[inline] + fn cmp(&self, other: &UniqueRc<T, A>) -> Ordering { + (**self).cmp(&**other) + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueRc<T, A> {} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueRc<T, A> { + fn hash<H: Hasher>(&self, state: &mut H) { + (**self).hash(state); + } +} + +// Depends on A = Global +impl<T> UniqueRc<T> { + /// Creates a new `UniqueRc`. + /// + /// Weak references to this `UniqueRc` can be created with [`UniqueRc::downgrade`]. Upgrading + /// these weak references will fail before the `UniqueRc` has been converted into an [`Rc`]. + /// After converting the `UniqueRc` into an [`Rc`], any weak references created beforehand will + /// point to the new [`Rc`]. + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "unique_rc_arc", issue = "112566")] + pub fn new(value: T) -> Self { + Self::new_in(value, Global) + } +} + +impl<T, A: Allocator> UniqueRc<T, A> { + /// Creates a new `UniqueRc` in the provided allocator. + /// + /// Weak references to this `UniqueRc` can be created with [`UniqueRc::downgrade`]. Upgrading + /// these weak references will fail before the `UniqueRc` has been converted into an [`Rc`]. + /// After converting the `UniqueRc` into an [`Rc`], any weak references created beforehand will + /// point to the new [`Rc`]. + #[cfg(not(no_global_oom_handling))] + #[unstable(feature = "unique_rc_arc", issue = "112566")] + pub fn new_in(value: T, alloc: A) -> Self { + let (ptr, alloc) = Box::into_unique(Box::new_in( + RcInner { + strong: Cell::new(0), + // keep one weak reference so if all the weak pointers that are created are dropped + // the UniqueRc still stays valid. + weak: Cell::new(1), + value, + }, + alloc, + )); + Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc } + } +} + +impl<T: ?Sized, A: Allocator> UniqueRc<T, A> { + /// Converts the `UniqueRc` into a regular [`Rc`]. + /// + /// This consumes the `UniqueRc` and returns a regular [`Rc`] that contains the `value` that + /// is passed to `into_rc`. + /// + /// Any weak references created before this method is called can now be upgraded to strong + /// references. + #[unstable(feature = "unique_rc_arc", issue = "112566")] + pub fn into_rc(this: Self) -> Rc<T, A> { + let mut this = ManuallyDrop::new(this); + + // Move the allocator out. + // SAFETY: `this.alloc` will not be accessed again, nor dropped because it is in + // a `ManuallyDrop`. + let alloc: A = unsafe { ptr::read(&this.alloc) }; + + // SAFETY: This pointer was allocated at creation time so we know it is valid. + unsafe { + // Convert our weak reference into a strong reference + this.ptr.as_mut().strong.set(1); + Rc::from_inner_in(this.ptr, alloc) + } + } +} + +impl<T: ?Sized, A: Allocator + Clone> UniqueRc<T, A> { + /// Creates a new weak reference to the `UniqueRc`. + /// + /// Attempting to upgrade this weak reference will fail before the `UniqueRc` has been converted + /// to a [`Rc`] using [`UniqueRc::into_rc`]. + #[unstable(feature = "unique_rc_arc", issue = "112566")] + pub fn downgrade(this: &Self) -> Weak<T, A> { + // SAFETY: This pointer was allocated at creation time and we guarantee that we only have + // one strong reference before converting to a regular Rc. + unsafe { + this.ptr.as_ref().inc_weak(); + } + Weak { ptr: this.ptr, alloc: this.alloc.clone() } + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> Deref for UniqueRc<T, A> { + type Target = T; + + fn deref(&self) -> &T { + // SAFETY: This pointer was allocated at creation time so we know it is valid. + unsafe { &self.ptr.as_ref().value } + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +impl<T: ?Sized, A: Allocator> DerefMut for UniqueRc<T, A> { + fn deref_mut(&mut self) -> &mut T { + // SAFETY: This pointer was allocated at creation time so we know it is valid. We know we + // have unique ownership and therefore it's safe to make a mutable reference because + // `UniqueRc` owns the only strong reference to itself. + unsafe { &mut (*self.ptr.as_ptr()).value } + } +} + +#[unstable(feature = "unique_rc_arc", issue = "112566")] +unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueRc<T, A> { + fn drop(&mut self) { + unsafe { + // destroy the contained object + drop_in_place(DerefMut::deref_mut(self)); + + // remove the implicit "strong weak" pointer now that we've destroyed the contents. + self.ptr.as_ref().dec_weak(); + + if self.ptr.as_ref().weak() == 0 { + self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr())); + } + } + } +} + +/// A unique owning pointer to a [`RcInner`] **that does not imply the contents are initialized,** +/// but will deallocate it (without dropping the value) when dropped. +/// +/// This is a helper for [`Rc::make_mut()`] to ensure correct cleanup on panic. +/// It is nearly a duplicate of `UniqueRc<MaybeUninit<T>, A>` except that it allows `T: !Sized`, +/// which `MaybeUninit` does not. +#[cfg(not(no_global_oom_handling))] +struct UniqueRcUninit<T: ?Sized, A: Allocator> { + ptr: NonNull<RcInner<T>>, + layout_for_value: Layout, + alloc: Option<A>, +} + +#[cfg(not(no_global_oom_handling))] +impl<T: ?Sized, A: Allocator> UniqueRcUninit<T, A> { + /// Allocates a RcInner with layout suitable to contain `for_value` or a clone of it. + fn new(for_value: &T, alloc: A) -> UniqueRcUninit<T, A> { + let layout = Layout::for_value(for_value); + let ptr = unsafe { + Rc::allocate_for_layout( + layout, + |layout_for_rc_inner| alloc.allocate(layout_for_rc_inner), + |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const RcInner<T>), + ) + }; + Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) } + } + + /// Returns the pointer to be written into to initialize the [`Rc`]. + fn data_ptr(&mut self) -> *mut T { + let offset = data_offset_align(self.layout_for_value.align()); + unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T } + } + + /// Upgrade this into a normal [`Rc`]. + /// + /// # Safety + /// + /// The data must have been initialized (by writing to [`Self::data_ptr()`]). + unsafe fn into_rc(self) -> Rc<T, A> { + let mut this = ManuallyDrop::new(self); + let ptr = this.ptr; + let alloc = this.alloc.take().unwrap(); + + // SAFETY: The pointer is valid as per `UniqueRcUninit::new`, and the caller is responsible + // for having initialized the data. + unsafe { Rc::from_ptr_in(ptr.as_ptr(), alloc) } + } +} + +#[cfg(not(no_global_oom_handling))] +impl<T: ?Sized, A: Allocator> Drop for UniqueRcUninit<T, A> { + fn drop(&mut self) { + // SAFETY: + // * new() produced a pointer safe to deallocate. + // * We own the pointer unless into_rc() was called, which forgets us. + unsafe { + self.alloc.take().unwrap().deallocate( + self.ptr.cast(), + rc_inner_layout_for_value_layout(self.layout_for_value), + ); + } + } +} |