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//! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
//! Counted'.
//!
//! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
//! allocated in the heap. Invoking [`clone`][clone] on [`Rc`] produces a new
//! pointer to the same allocation in the heap. When the last [`Rc`] pointer to a
//! given allocation is destroyed, the value stored in that allocation (often
//! referred to as "inner value") is also dropped.
//!
//! Shared references in Rust disallow mutation by default, and [`Rc`]
//! is no exception: you cannot generally obtain a mutable reference to
//! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
//! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
//! inside an `Rc`][mutability].
//!
//! [`Rc`] uses non-atomic reference counting. This means that overhead is very
//! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
//! does not implement [`Send`][send]. As a result, the Rust compiler
//! will check *at compile time* that you are not sending [`Rc`]s between
//! threads. If you need multi-threaded, atomic reference counting, use
//! [`sync::Arc`][arc].
//!
//! The [`downgrade`][downgrade] method can be used to create a non-owning
//! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
//! to an [`Rc`], but this will return [`None`] if the value stored in the allocation has
//! already been dropped. In other words, `Weak` pointers do not keep the value
//! inside the allocation alive; however, they *do* keep the allocation
//! (the backing store for the inner value) alive.
//!
//! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
//! [`Weak`] is used to break cycles. For example, a tree could have strong
//! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
//! children back to their parents.
//!
//! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
//! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
//! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are associated
//! functions, called using [fully qualified syntax]:
//!
//! ```
//! use cactusref::Rc;
//!
//! let my_rc = Rc::new(());
//! 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 cactusref::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 cactusref::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 cactusref::Rc;
//!
//! struct Owner {
//! name: String,
//! // ...other fields
//! }
//!
//! struct Gadget {
//! id: i32,
//! owner: Rc<Owner>,
//! // ...other fields
//! }
//!
//! // 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 cactusref::Rc;
//! use cactusref::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
//! }
//!
//! // Create a reference-counted `Owner`. Note that we've put the `Owner`'s
//! // vector of `Gadget`s inside a `RefCell` so that we can mutate it through
//! // a shared reference.
//! let gadget_owner: Rc<Owner> = Rc::new(
//! Owner {
//! name: "Gadget Man".to_string(),
//! gadgets: RefCell::new(vec![]),
//! }
//! );
//!
//! // Create `Gadget`s belonging to `gadget_owner`, as before.
//! let gadget1 = Rc::new(
//! Gadget {
//! id: 1,
//! owner: Rc::clone(&gadget_owner),
//! }
//! );
//! let gadget2 = Rc::new(
//! Gadget {
//! id: 2,
//! owner: Rc::clone(&gadget_owner),
//! }
//! );
//!
//! // Add the `Gadget`s to their `Owner`.
//! {
//! let mut gadgets = gadget_owner.gadgets.borrow_mut();
//! gadgets.push(Rc::downgrade(&gadget1));
//! gadgets.push(Rc::downgrade(&gadget2));
//!
//! // `RefCell` dynamic borrow ends here.
//! }
//!
//! // Iterate over our `Gadget`s, printing their details out.
//! for gadget_weak in gadget_owner.gadgets.borrow().iter() {
//!
//! // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
//! // guarantee the allocation still exists, we need to call
//! // `upgrade`, which returns an `Option<Rc<Gadget>>`.
//! //
//! // In this case we know the allocation still exists, so we simply
//! // `unwrap` the `Option`. In a more complicated program, you might
//! // need graceful error handling for a `None` result.
//!
//! let gadget = gadget_weak.upgrade().unwrap();
//! println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
//! }
//!
//! // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
//! // are destroyed. There are now no strong (`Rc`) pointers to the
//! // gadgets, so they are destroyed. This zeroes the reference count on
//! // Gadget Man, so he gets destroyed as well.
//! ```
//!
//! [clone]: Clone::clone
//! [`Cell`]: core::cell::Cell
//! [`RefCell`]: core::cell::RefCell
//! [send]: core::marker::Send
#![cfg_attr(feature = "std", doc = "[arc]: std::sync::Arc")]
#![cfg_attr(
not(feature = "std"),
doc = "[arc]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html"
)]
//! [`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
use core::borrow;
use core::cell::{Cell, RefCell};
use core::cmp::Ordering;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::intrinsics::abort;
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Deref;
use core::pin::Pin;
use core::ptr::{self, NonNull};
use alloc::alloc::handle_alloc_error;
use alloc::alloc::{AllocError, Allocator, Global, Layout};
use alloc::boxed::Box;
use crate::link::Links;
#[cfg(test)]
#[allow(clippy::redundant_clone)]
#[allow(clippy::uninlined_format_args)]
mod tests;
// This is repr(C) to future-proof against possible field-reordering, which
// would interfere with otherwise safe [into|from]_raw() of transmutable
// inner types.
#[repr(C)]
pub(crate) struct RcBox<T> {
strong: Cell<usize>,
weak: Cell<usize>,
pub links: MaybeUninit<RefCell<Links<T>>>,
pub value: MaybeUninit<T>,
}
impl<T> RcBox<T> {
/// # Safety
///
/// Callers must ensure this `RcBox` is not dead.
#[inline]
pub(crate) unsafe fn links(&self) -> &RefCell<Links<T>> {
let links = &self.links;
// SAFETY: because callers have ensured the `RcBox` is not dead, `links`
// has not yet been deallocated and the `MaybeUninit` is inhabited.
let pointer_to_links = links as *const MaybeUninit<RefCell<Links<T>>>;
&*(pointer_to_links.cast::<RefCell<Links<T>>>())
}
}
/// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
/// Counted'.
///
/// See the [module-level documentation](./index.html) for more details.
///
/// The inherent methods of `Rc` are all associated functions, which means
/// that you have to call them as e.g., [`Rc::get_mut(&mut value)`][get_mut] instead of
/// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
///
/// [get_mut]: Rc::get_mut
pub struct Rc<T> {
pub(crate) ptr: NonNull<RcBox<T>>,
phantom: PhantomData<RcBox<T>>,
}
/// `Rc` is not `Send`.
///
/// ```compile_fail
/// use cactusref::Rc;
/// fn requires_send<T: Send>(val: T) {}
/// let rc = Rc::<usize>::new(1);
/// requires_send(rc);
/// ```
mod rc_is_not_send {}
/// `Rc` is not `Sync`.
///
/// ```compile_fail
/// use cactusref::Rc;
/// fn requires_sync<T: Sync>(val: T) {}
/// let rc = Rc::<usize>::new(1);
/// requires_sync(rc);
/// ```
mod rc_is_not_sync {}
impl<T> Rc<T> {
#[inline(always)]
pub(crate) fn inner(&self) -> &RcBox<T> {
// This unsafety is ok because while this Rc is alive we're guaranteed
// that the inner pointer is valid.
unsafe { self.ptr.as_ref() }
}
fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
Self {
ptr,
phantom: PhantomData,
}
}
unsafe fn from_ptr(ptr: *mut RcBox<T>) -> Self {
Self::from_inner(NonNull::new_unchecked(ptr))
}
}
impl<T> Rc<T> {
/// Constructs a new `Rc<T>`.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
/// ```
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.
Self::from_inner(
Box::leak(Box::new(RcBox {
strong: Cell::new(1),
weak: Cell::new(1),
links: MaybeUninit::new(RefCell::new(Links::new())),
value: MaybeUninit::new(value),
}))
.into(),
)
}
/// Constructs a new `Rc` with uninitialized contents.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let mut five = Rc::<u32>::new_uninit();
///
/// let five = unsafe {
/// // Deferred initialization:
/// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[must_use]
pub fn new_uninit() -> Rc<MaybeUninit<T>> {
unsafe {
Rc::from_ptr(Rc::allocate_for_layout(
Layout::new::<T>(),
|layout| Global.allocate(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.
pub fn pin(value: T) -> Pin<Rc<T>> {
unsafe { Pin::new_unchecked(Rc::new(value)) }
}
/// Returns the inner value, if the `Rc` has exactly one strong reference.
///
/// Otherwise, an [`Err`] is returned with the same `Rc` that was
/// passed in.
///
/// This will succeed even if there are outstanding weak references.
///
/// # Examples
///
/// ```
/// use cactusref::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);
/// ```
///
/// # Errors
///
/// If the given `Rc` does not have exactly one strong reference, it is
/// returned in the `Err` variant of the returned `Result`.
#[inline]
pub fn try_unwrap(this: Self) -> Result<T, Self> {
if Rc::strong_count(&this) == 1 {
unsafe {
let val = ptr::read(&*this); // copy the contained object
// Indicate to Weaks that they can't be promoted by decrementing
// the strong count, and then remove the implicit "strong weak"
// pointer while also handling drop logic by just crafting a
// fake Weak.
this.inner().dec_strong();
let _weak = Weak {
ptr: this.ptr,
phantom: PhantomData,
};
mem::forget(this);
Ok(val)
}
} else {
Err(this)
}
}
}
impl<T> Rc<MaybeUninit<T>> {
/// Converts to `Rc<T>`.
///
/// # Safety
///
/// As with [`MaybeUninit::assume_init`],
/// it is up to the caller to guarantee that the inner value
/// really is in an initialized state.
/// Calling this when the content is not yet fully initialized
/// causes immediate undefined behavior.
///
/// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let mut five = Rc::<u32>::new_uninit();
///
/// let five = unsafe {
/// // Deferred initialization:
/// Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
///
/// five.assume_init()
/// };
///
/// assert_eq!(*five, 5)
/// ```
#[inline]
#[must_use]
pub unsafe fn assume_init(self) -> Rc<T> {
Rc::from_inner(ManuallyDrop::new(self).ptr.cast())
}
}
impl<T> Rc<T> {
/// Consumes the `Rc`, returning the wrapped pointer.
///
/// To avoid a memory leak the pointer must be converted back to an `Rc` using
/// [`Rc::from_raw`][from_raw].
///
/// [from_raw]: Rc::from_raw
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let x = Rc::new("hello".to_owned());
/// let x_ptr = Rc::into_raw(x);
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// // Reconstruct the `Rc` to avoid a leak.
/// let _ = unsafe { Rc::from_raw(x_ptr) };
/// ```
#[must_use]
pub fn into_raw(this: Self) -> *const T {
let ptr = Self::as_ptr(&this);
mem::forget(this);
ptr
}
/// Provides a raw pointer to the data.
///
/// The counts are not affected in any way and the `Rc` is not consumed. The pointer is valid
/// for as long there are strong counts in the `Rc`.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let x = Rc::new("hello".to_owned());
/// let y = Rc::clone(&x);
/// let x_ptr = Rc::as_ptr(&x);
/// assert_eq!(x_ptr, Rc::as_ptr(&y));
/// assert_eq!(unsafe { &*x_ptr }, "hello");
/// ```
#[must_use]
pub fn as_ptr(this: &Self) -> *const T {
let ptr: *mut RcBox<T> = NonNull::as_ptr(this.ptr);
// SAFETY: This cannot go through Deref::deref or Rc::inner because
// this is required to retain raw/mut provenance such that e.g. `get_mut` can
// write through the pointer after the Rc is recovered through `from_raw`.
unsafe {
// SAFETY: we can cast the `MaybeUninit<T>` to a `T` because we are
// calling and associated function with a live `Rc`. If an `Rc` is
// not dead, the inner `MaybeUninit` is inhabited.
ptr::addr_of_mut!((*ptr).value).cast::<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] where `U` must have the same size
/// and alignment as `T`. This is trivially true if `U` is `T`.
/// Note that if `U` is not `T` but has the same size and alignment, this is
/// basically like transmuting references of different types. See
/// [`mem::transmute`][transmute] for more information on what
/// restrictions apply in this case.
///
/// The user of `from_raw` has to make sure a specific value of `T` is only
/// dropped once.
///
/// This function is unsafe because improper use may lead to memory unsafety,
/// even if the returned `Rc<T>` is never accessed.
///
/// [into_raw]: Rc::into_raw
/// [transmute]: core::mem::transmute
///
/// # Examples
///
/// ```
/// use cactusref::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!
/// ```
///
/// # Safety
///
/// Callers must ensure that `ptr` points to a live `Rc` and was created
/// with a call to [`Rc::into_raw`].
pub unsafe fn from_raw(ptr: *const T) -> Self {
let offset = data_offset(ptr);
// Reverse the offset to find the original RcBox.
let rc_ptr = (ptr as *mut u8)
.offset(-offset)
.with_metadata_of(ptr as *mut RcBox<T>);
Self::from_ptr(rc_ptr)
}
/// Creates a new [`Weak`] pointer to this allocation.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// let weak_five = Rc::downgrade(&five);
/// ```
#[must_use]
pub fn downgrade(this: &Self) -> Weak<T> {
this.inner().inc_weak();
// Make sure we do not create a dangling Weak
debug_assert!(!is_dangling(this.ptr.as_ptr()));
Weak {
ptr: this.ptr,
phantom: PhantomData,
}
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
/// let _weak_five = Rc::downgrade(&five);
///
/// assert_eq!(1, Rc::weak_count(&five));
/// ```
#[inline]
#[must_use]
pub fn weak_count(this: &Self) -> usize {
this.inner().weak() - 1
}
/// Gets the number of strong (`Rc`) pointers to this allocation.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
/// let _also_five = Rc::clone(&five);
///
/// assert_eq!(2, Rc::strong_count(&five));
/// ```
#[inline]
#[must_use]
pub fn strong_count(this: &Self) -> usize {
this.inner().strong()
}
/// Increments the strong reference count on the `Rc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Rc::into_raw`, and the
/// associated `Rc` instance must be valid (i.e. the strong count must be at
/// least 1) for the duration of this method.
///
/// # Examples
///
/// ```
/// use cactusref::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));
///
/// // Decrement the strong count to avoid a leak.
/// Rc::decrement_strong_count(ptr);
/// }
/// ```
#[inline]
pub unsafe fn increment_strong_count(ptr: *const T) {
// Retain Rc, but don't touch refcount by wrapping in ManuallyDrop
let rc = ManuallyDrop::new(Rc::<T>::from_raw(ptr));
// Now increase refcount, but don't drop new refcount either
let _rc_clone: ManuallyDrop<_> = rc.clone();
}
/// Decrements the strong reference count on the `Rc<T>` associated with the
/// provided pointer by one.
///
/// # Safety
///
/// The pointer must have been obtained through `Rc::into_raw`, and the
/// associated `Rc` instance must be valid (i.e. the strong count must be at
/// least 1) when invoking this method. This method can be used to release
/// the final `Rc` and backing storage, but **should not** be called after
/// the final `Rc` has been released.
///
/// # Examples
///
/// ```
/// use cactusref::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]
pub unsafe fn decrement_strong_count(ptr: *const T) {
drop(Rc::from_raw(ptr));
}
/// Returns `true` if there are no other `Rc` or [`Weak`] pointers to
/// this allocation.
#[inline]
fn is_unique(this: &Self) -> bool {
Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
}
/// Returns a mutable reference into the given `Rc`, if there are
/// no other `Rc` or [`Weak`] pointers to the same allocation.
///
/// Returns [`None`] otherwise, because it is not safe to
/// mutate a shared value.
///
/// See also [`make_mut`][make_mut], which will [`clone`][clone]
/// the inner value when there are other pointers.
///
/// [make_mut]: Rc::make_mut
/// [clone]: Clone::clone
///
/// # Examples
///
/// ```
/// use cactusref::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]
pub fn get_mut(this: &mut Self) -> Option<&mut T> {
if Rc::is_unique(this) {
unsafe { Some(Rc::get_mut_unchecked(this)) }
} else {
None
}
}
/// Returns a mutable reference into the given `Rc`,
/// without any check.
///
/// See also [`get_mut`], which is safe and does appropriate checks.
///
/// [`get_mut`]: Rc::get_mut
///
/// # Safety
///
/// Any other `Rc` or [`Weak`] pointers to the same allocation must not be dereferenced
/// for the duration of the returned borrow.
/// This is trivially the case if no such pointers exist,
/// for example immediately after `Rc::new`.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let mut x = Rc::new(String::new());
/// unsafe {
/// Rc::get_mut_unchecked(&mut x).push_str("foo")
/// }
/// assert_eq!(*x, "foo");
/// ```
#[inline]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
debug_assert!(!this.inner().is_dead());
// 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`).
//
// Safety: If we have an `Rc`, then the allocation is not dead so the `MaybeUninit`
// is inhabited.
let value = &mut (*this.ptr.as_ptr()).value;
// SAFETY: we can cast the `MaybeUninit<T>` to a `T` because we are
// calling and associated function with a live `Rc`. If an `Rc` is not
// dead, the inner `MaybeUninit` is inhabited.
let pointer_to_value = (value as *mut MaybeUninit<T>).cast::<T>();
&mut *(pointer_to_value)
}
/// Returns `true` if the two `Rc`s point to the same allocation
/// (in a vein similar to [`ptr::eq`]).
///
/// # Examples
///
/// ```
/// use cactusref::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));
/// ```
///
/// [`ptr::eq`]: core::ptr::eq
#[inline]
#[must_use]
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
this.ptr.as_ptr() == other.ptr.as_ptr()
}
}
impl<T: Clone> Rc<T> {
/// Makes a mutable reference into the given `Rc`.
///
/// If there are other `Rc` pointers to the same allocation, then `make_mut` will
/// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
/// referred to as clone-on-write.
///
/// If there are no other `Rc` pointers to this allocation, then [`Weak`]
/// pointers to this allocation will be disassociated.
///
/// See also [`get_mut`], which will fail rather than cloning.
///
/// [`clone`]: Clone::clone
/// [`get_mut`]: Rc::get_mut
///
/// # Examples
///
/// ```
/// use cactusref::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 cactusref::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]
pub fn make_mut(this: &mut Self) -> &mut T {
if Rc::strong_count(this) != 1 {
// Gotta clone the data, there are other Rcs.
// Pre-allocate memory to allow writing the cloned value directly.
let mut rc = Self::new_uninit();
unsafe {
let data = Rc::get_mut_unchecked(&mut rc);
data.as_mut_ptr().write((**this).clone());
*this = rc.assume_init();
}
} else if Rc::weak_count(this) != 0 {
// Can just steal the data, all that's left is Weaks
let mut rc = Self::new_uninit();
unsafe {
let data: &mut MaybeUninit<T> = mem::transmute(Rc::get_mut_unchecked(&mut rc));
data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
this.inner().dec_strong();
// Remove implicit strong-weak ref (no need to craft a fake
// Weak here -- we know other Weaks can clean up for us)
this.inner().dec_weak();
ptr::write(this, rc.assume_init());
}
}
// This unsafety is ok because we're guaranteed that the pointer
// returned is the *only* pointer that will ever be returned to T. Our
// reference count is guaranteed to be 1 at this point, and we required
// the `Rc<T>` itself to be `mut`, so we're returning the only possible
// reference to the allocation.
unsafe {
let value = &mut this.ptr.as_mut().value;
// SAFETY: we can cast the `MaybeUninit<T>` to a `T` because we are
// calling and associated function with a live `Rc`. If an `Rc` is
// not dead, the inner `MaybeUninit` is inhabited.
let pointer_to_value = (value as *mut MaybeUninit<T>).cast::<T>();
&mut *(pointer_to_value)
}
}
}
impl<T> Rc<T> {
/// Allocates an `RcBox<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided.
///
/// The function `mem_to_rcbox` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
unsafe fn allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
) -> *mut RcBox<T> {
// Calculate layout using the given value layout.
// Previously, layout was calculated on the expression
// `&*(ptr as *const RcBox<T>)`, but this created a misaligned
// reference (see #54908).
let layout = Layout::new::<RcBox<()>>()
.extend(value_layout)
.unwrap()
.0
.pad_to_align();
Rc::try_allocate_for_layout(value_layout, allocate, mem_to_rcbox)
.unwrap_or_else(|_| handle_alloc_error(layout))
}
/// Allocates an `RcBox<T>` with sufficient space for
/// a possibly-unsized inner value where the value has the layout provided,
/// returning an error if allocation fails.
///
/// The function `mem_to_rcbox` is called with the data pointer
/// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
#[inline]
unsafe fn try_allocate_for_layout(
value_layout: Layout,
allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>,
) -> Result<*mut RcBox<T>, AllocError> {
// Calculate layout using the given value layout.
// Previously, layout was calculated on the expression
// `&*(ptr as *const RcBox<T>)`, but this created a misaligned
// reference (see #54908).
let layout = Layout::new::<RcBox<()>>()
.extend(value_layout)
.unwrap()
.0
.pad_to_align();
// Allocate for the layout.
let ptr = allocate(layout)?;
// Initialize the RcBox
let inner = mem_to_rcbox(ptr.as_non_null_ptr().as_ptr());
debug_assert_eq!(Layout::for_value(&*inner), layout);
ptr::write(&mut (*inner).strong, Cell::new(1));
ptr::write(&mut (*inner).weak, Cell::new(1));
ptr::write(
&mut (*inner).links,
MaybeUninit::new(RefCell::new(Links::new())),
);
Ok(inner)
}
/// Allocates an `RcBox<T>` with sufficient space for an unsized inner value
unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
// Allocate for the `RcBox<T>` using the given value.
Self::allocate_for_layout(
Layout::for_value(&*ptr),
|layout| Global.allocate(layout),
|mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
)
}
fn from_box(v: Box<T>) -> Rc<T> {
unsafe {
let (box_unique, alloc) = Box::into_raw_with_allocator(v);
// SAFETY: Pointers obtained from `Box::into_raw` are always
// non-null.
let box_unique = NonNull::new_unchecked(box_unique);
let box_ptr = box_unique.as_ptr();
let value_size = mem::size_of_val(&*box_ptr);
let ptr = Self::allocate_for_ptr(box_ptr);
// Copy value as bytes
ptr::copy_nonoverlapping(
box_ptr.cast_const().cast::<u8>(),
ptr::addr_of_mut!((*ptr).value).cast::<u8>(),
value_size,
);
// Free the allocation without dropping its contents
box_free(box_unique, alloc);
Self::from_ptr(ptr)
}
}
}
impl<T> Deref for Rc<T> {
type Target = T;
#[inline(always)]
fn deref(&self) -> &T {
unsafe {
let value = &self.inner().value;
// SAFETY: we can cast the `MaybeUninit<T>` to a `T` because we are
// calling and associated function with a live `Rc`. If an `Rc` is
// not dead, the inner `MaybeUninit` is inhabited.
let pointer_to_value = (value as *const MaybeUninit<T>).cast::<T>();
&*(pointer_to_value)
}
}
}
impl<T> Clone for Rc<T> {
/// Makes a clone of the `Rc` pointer.
///
/// This creates another pointer to the same allocation, increasing the
/// strong reference count.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// let _ = Rc::clone(&five);
/// ```
#[inline]
fn clone(&self) -> Rc<T> {
self.inner().inc_strong();
Self::from_inner(self.ptr)
}
}
impl<T: Default> Default for Rc<T> {
/// Creates a new `Rc<T>`, with the `Default` value for `T`.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let x: Rc<i32> = Default::default();
/// assert_eq!(*x, 0);
/// ```
#[inline]
fn default() -> Rc<T> {
Rc::new(Default::default())
}
}
impl<T: PartialEq> PartialEq for Rc<T> {
/// Equality for two `Rc`s.
///
/// Two `Rc`s are equal if their inner values are equal, even if they are
/// stored in different allocation.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Rc`s that point to the same allocation are
/// always equal.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five == Rc::new(5));
/// ```
#[inline]
fn eq(&self, other: &Rc<T>) -> bool {
**self == **other
}
/// Inequality for two `Rc`s.
///
/// Two `Rc`s are unequal if their inner values are unequal.
///
/// If `T` also implements `Eq` (implying reflexivity of equality),
/// two `Rc`s that point to the same allocation are
/// never unequal.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five != Rc::new(6));
/// ```
#[inline]
#[allow(clippy::partialeq_ne_impl)]
fn ne(&self, other: &Rc<T>) -> bool {
**self != **other
}
}
impl<T: Eq> Eq for Rc<T> {}
impl<T: PartialOrd> PartialOrd for Rc<T> {
/// Partial comparison for two `Rc`s.
///
/// The two are compared by calling `partial_cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
/// use std::cmp::Ordering;
///
/// let five = Rc::new(5);
///
/// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
/// ```
#[inline(always)]
fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
(**self).partial_cmp(&**other)
}
/// Less-than comparison for two `Rc`s.
///
/// The two are compared by calling `<` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five < Rc::new(6));
/// ```
#[inline(always)]
fn lt(&self, other: &Rc<T>) -> bool {
**self < **other
}
/// 'Less than or equal to' comparison for two `Rc`s.
///
/// The two are compared by calling `<=` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five <= Rc::new(5));
/// ```
#[inline(always)]
fn le(&self, other: &Rc<T>) -> bool {
**self <= **other
}
/// Greater-than comparison for two `Rc`s.
///
/// The two are compared by calling `>` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five > Rc::new(4));
/// ```
#[inline(always)]
fn gt(&self, other: &Rc<T>) -> bool {
**self > **other
}
/// 'Greater than or equal to' comparison for two `Rc`s.
///
/// The two are compared by calling `>=` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
///
/// let five = Rc::new(5);
///
/// assert!(five >= Rc::new(5));
/// ```
#[inline(always)]
fn ge(&self, other: &Rc<T>) -> bool {
**self >= **other
}
}
impl<T: Ord> Ord for Rc<T> {
/// Comparison for two `Rc`s.
///
/// The two are compared by calling `cmp()` on their inner values.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
/// use std::cmp::Ordering;
///
/// let five = Rc::new(5);
///
/// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
/// ```
#[inline]
fn cmp(&self, other: &Rc<T>) -> Ordering {
(**self).cmp(&**other)
}
}
impl<T: Hash> Hash for Rc<T> {
fn hash<H: Hasher>(&self, state: &mut H) {
(**self).hash(state);
}
}
impl<T: fmt::Display> fmt::Display for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
impl<T: fmt::Debug> fmt::Debug for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T> fmt::Pointer for Rc<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Pointer::fmt(&ptr::addr_of!(**self), f)
}
}
impl<T> From<T> for Rc<T> {
/// Converts a generic type `T` into a `Rc<T>`
///
/// The conversion allocates on the heap and moves `t`
/// from the stack into it.
///
/// # Example
/// ```rust
/// # use cactusref::Rc;
/// let x = 5;
/// let rc = Rc::new(5);
///
/// assert_eq!(Rc::from(x), rc);
/// ```
fn from(t: T) -> Self {
Rc::new(t)
}
}
impl<T> From<Box<T>> for Rc<T> {
/// Move a boxed object to a new, reference counted, allocation.
///
/// # Example
///
/// ```
/// # use cactusref::Rc;
/// let original: Box<i32> = Box::new(1);
/// let shared: Rc<i32> = Rc::from(original);
/// assert_eq!(1, *shared);
/// ```
#[inline]
fn from(v: Box<T>) -> Rc<T> {
Rc::from_box(v)
}
}
/// `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
pub struct Weak<T> {
// This is a `NonNull` to allow optimizing the size of this type in enums,
// but it is not necessarily a valid pointer.
// `Weak::new` sets this to `usize::MAX` so that it doesn’t need
// to allocate space on the heap. That's not a value a real pointer
// will ever have because RcBox has alignment at least 2.
// This is only possible when `T: Sized`; unsized `T` never dangle.
ptr: NonNull<RcBox<T>>,
phantom: PhantomData<RcBox<T>>,
}
/// `Weak` is not `Send`.
///
/// ```compile_fail
/// use cactusref::Weak;
/// fn requires_send<T: Send>(val: T) {}
/// let weak = Weak::<usize>::new();
/// requires_send(weak);
/// ```
mod weak_is_not_send {}
/// `Weak` is not `Sync`.
///
/// ```compile_fail
/// use cactusref::Weak;
/// fn requires_sync<T: Sync>(val: T) {}
/// let weak = Weak::<usize>::new();
/// requires_sync(weak);
/// ```
mod weak_is_not_sync {}
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 cactusref::Weak;
///
/// let empty: Weak<i64> = Weak::new();
/// assert!(empty.upgrade().is_none());
/// ```
#[must_use]
pub fn new() -> Weak<T> {
Weak {
ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
phantom: PhantomData,
}
}
}
pub(crate) fn is_dangling<T: ?Sized>(ptr: *mut T) -> bool {
let address = ptr.cast::<()>() as usize;
address == 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> Weak<T> {
/// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. The pointer may be dangling,
/// unaligned or even [`null`] otherwise.
///
/// # Examples
///
/// ```
/// use cactusref::Rc;
/// use std::ptr;
///
/// let strong = Rc::new("hello".to_owned());
/// let weak = Rc::downgrade(&strong);
/// // Both point to the same object
/// assert!(ptr::eq(&*strong, weak.as_ptr()));
/// // The strong here keeps it alive, so we can still access the object.
/// assert_eq!("hello", unsafe { &*weak.as_ptr() });
///
/// drop(strong);
/// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
/// // undefined behaviour.
/// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
/// ```
///
/// [`null`]: core::ptr::null
#[must_use]
pub fn as_ptr(&self) -> *const T {
let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
if is_dangling(ptr) {
// If the pointer is dangling, we return the sentinel directly. This cannot be
// a valid payload address, as the payload is at least as aligned as RcBox (usize).
ptr as *const T
} else {
// SAFETY: if is_dangling returns false, then the pointer is dereferencable.
// The payload may be dropped at this point, and we have to maintain provenance,
// so use raw pointer manipulation.
//
// SAFETY: Because we are a live `Rc`, the `MaybeUninit` `value` is
// inhabited and can be transmuted to an initialized `T`.
unsafe { ptr::addr_of_mut!((*ptr).value) as *const T }
}
}
/// 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 cactusref::{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]
pub fn into_raw(self) -> *const T {
let result = self.as_ptr();
mem::forget(self);
result
}
/// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
///
/// This can be used to safely get a strong reference (by calling [`upgrade`]
/// later) or to deallocate the weak count by dropping the `Weak<T>`.
///
/// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
/// as these don't own anything; the method still works on them).
///
/// # Safety
///
/// The pointer must have originated from the [`into_raw`] and must still own its potential
/// weak reference.
///
/// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
/// takes ownership of one weak reference currently represented as a raw pointer (the weak
/// count is not modified by this operation) and therefore it must be paired with a previous
/// call to [`into_raw`].
///
/// # Examples
///
/// ```
/// use cactusref::{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
pub unsafe fn from_raw(ptr: *const T) -> Self {
// See Weak::as_ptr for context on how the input pointer is derived.
let ptr = if is_dangling(ptr.cast_mut()) {
// This is a dangling Weak.
ptr as *mut RcBox<T>
} else {
// Otherwise, we're guaranteed the pointer came from a nondangling Weak.
// SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
let offset = data_offset(ptr);
// Thus, we reverse the offset to get the whole RcBox.
// SAFETY: the pointer originated from a Weak, so this offset is safe.
(ptr as *mut u8)
.offset(-offset)
.with_metadata_of(ptr as *mut RcBox<T>)
};
// SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
Weak {
ptr: NonNull::new_unchecked(ptr),
phantom: PhantomData,
}
}
/// 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 cactusref::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]
pub fn upgrade(&self) -> Option<Rc<T>> {
let inner = self.inner()?;
if inner.is_dead() {
None
} else {
inner.inc_strong();
Some(Rc::from_inner(self.ptr))
}
}
/// Gets the number of strong (`Rc`) pointers pointing to this allocation.
///
/// If `self` was created using [`Weak::new`], this will return 0.
#[must_use]
pub fn strong_count(&self) -> usize {
if let Some(inner) = self.inner() {
if inner.is_uninit() {
0
} else {
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]
pub fn weak_count(&self) -> usize {
self.inner().map_or(0, |inner| {
if inner.is_uninit() {
0
} else if inner.strong() > 0 {
inner.weak() - 1 // subtract the implicit weak ptr
} else {
0
}
})
}
/// Returns `None` when the pointer is dangling and there is no allocated `RcBox`,
/// (i.e., when this `Weak` was created by `Weak::new`).
#[inline]
#[must_use]
fn inner(&self) -> Option<WeakInner<'_>> {
if is_dangling(self.ptr.as_ptr()) {
None
} else {
// We are careful to *not* create a reference covering the "data" field, as
// the field may be mutated concurrently (for example, if the last `Rc`
// is dropped, the data field will be dropped in-place).
Some(unsafe {
let ptr = self.ptr.as_ptr();
WeakInner {
strong: &(*ptr).strong,
weak: &(*ptr).weak,
}
})
}
}
/// Returns `true` if the two `Weak`s point to the same allocation (similar to
/// [`ptr::eq`]), or if both don't point to any allocation
/// (because they were created with `Weak::new()`).
///
/// # Notes
///
/// Since this compares pointers it means that `Weak::new()` will equal each
/// other, even though they don't point to any allocation.
///
/// # Examples
///
/// ```
/// use cactusref::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 cactusref::{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));
/// ```
///
/// [`ptr::eq`]: core::ptr::eq
#[inline]
#[must_use]
pub fn ptr_eq(&self, other: &Self) -> bool {
self.ptr.as_ptr() == other.ptr.as_ptr()
}
}
unsafe impl<#[may_dangle] T> Drop for Weak<T> {
/// Drops the `Weak` pointer.
///
/// # Examples
///
/// ```
/// use cactusref::{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 {
// SAFETY: `T` is `Sized`, which means `Layout::for_value_raw`
// is always safe to call.
let layout = Layout::for_value_raw(self.ptr.as_ptr());
Global.deallocate(self.ptr.cast(), layout);
}
}
}
}
impl<T> Clone for Weak<T> {
/// Makes a clone of the `Weak` pointer that points to the same allocation.
///
/// # Examples
///
/// ```
/// use cactusref::{Rc, Weak};
///
/// let weak_five = Rc::downgrade(&Rc::new(5));
///
/// let _ = Weak::clone(&weak_five);
/// ```
#[inline]
fn clone(&self) -> Weak<T> {
if let Some(inner) = self.inner() {
inner.inc_weak();
}
Weak {
ptr: self.ptr,
phantom: PhantomData,
}
}
}
impl<T: fmt::Debug> fmt::Debug for Weak<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "(Weak)")
}
}
impl<T> Default for Weak<T> {
/// Constructs a new `Weak<T>`, without allocating any memory.
/// Calling [`upgrade`] on the return value always gives [`None`].
///
/// [`None`]: Option
/// [`upgrade`]: Weak::upgrade
///
/// # Examples
///
/// ```
/// use cactusref::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)]
pub(crate) 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 want to abort on overflow instead of dropping the value.
// The reference count will never be zero when this is called;
// nevertheless, we insert an abort here to hint LLVM at
// an otherwise missed optimization.
if strong == 0 || strong == usize::MAX {
abort();
}
// `usize::MAX` is used to mark the `Rc` as uninitialized, so disallow
// incrementing the strong count to prevent a memory leak and type
// confusion in `Drop::drop`.
if strong + 1 == usize::MAX {
abort();
}
self.strong_ref().set(strong + 1);
}
#[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 want to abort on overflow instead of dropping the value.
// The reference count will never be zero when this is called;
// nevertheless, we insert an abort here to hint LLVM at
// an otherwise missed optimization.
if weak == 0 || weak == usize::MAX {
abort();
}
self.weak_ref().set(weak + 1);
}
#[inline]
fn dec_weak(&self) {
self.weak_ref().set(self.weak() - 1);
}
#[inline]
fn is_dead(&self) -> bool {
self.strong() == 0 || self.is_uninit()
}
#[inline]
fn is_uninit(&self) -> bool {
self.strong() == usize::MAX
}
#[inline]
fn make_uninit(&self) {
self.strong_ref().set(usize::MAX);
}
}
impl<T> RcInnerPtr for RcBox<T> {
#[inline(always)]
fn weak_ref(&self) -> &Cell<usize> {
&self.weak
}
#[inline(always)]
fn strong_ref(&self) -> &Cell<usize> {
&self.strong
}
}
impl<'a> RcInnerPtr for WeakInner<'a> {
#[inline(always)]
fn weak_ref(&self) -> &Cell<usize> {
self.weak
}
#[inline(always)]
fn strong_ref(&self) -> &Cell<usize> {
self.strong
}
}
impl<T> borrow::Borrow<T> for Rc<T> {
fn borrow(&self) -> &T {
self
}
}
impl<T> AsRef<T> for Rc<T> {
fn as_ref(&self) -> &T {
self
}
}
impl<T> Unpin for Rc<T> {}
/// Get the offset within an `RcBox` for the payload behind a pointer.
///
/// # Safety
///
/// The pointer must point to (and have valid metadata for) a previously
/// valid instance of T, but the T is allowed to be dropped.
unsafe fn data_offset<T>(ptr: *const T) -> isize {
let _ = ptr;
let rcbox = MaybeUninit::<RcBox<T>>::uninit();
let base_ptr = rcbox.as_ptr();
let base_ptr = base_ptr as usize;
let field_ptr = ptr::addr_of!((*(base_ptr as *const RcBox<T>)).value);
let field_ptr = field_ptr as usize;
(field_ptr - base_ptr) as isize
}
// Deallocate a `Box` without destroying the inner `T`.
//
// # Safety
//
// Callers must ensure that `ptr` was allocated by `Box::new` with the global allocator.
//
// Callers must ensure that `T` is not dropped.
#[inline]
unsafe fn box_free<T, A: Allocator>(ptr: NonNull<T>, alloc: A) {
// SAFETY: `T` is `Sized`, which means `Layout::for_value_raw` is always
// safe to call.
let layout = Layout::for_value_raw(ptr.as_ptr());
alloc.deallocate(ptr.cast(), layout);
}