cactusref 0.3.0

//! 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::convert::From;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::intrinsics::abort;
use core::marker::{PhantomData, Unpin};
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)]
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 as *const T).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 as *mut T) {
            // This is a dangling Weak.
            ptr as *mut RcBox<T>
        } else {
            // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
            // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
            let offset = 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 kill(&self) {
        self.strong_ref().set(0);
    }

    #[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);
}