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/*!
* Thread-safe hybrid reference counting pointers
*
* Loosely based on the algorithm described in
* ["Biased reference counting: minimizing atomic operations in garbage collection"][doi:10.1145/3243176.3243195]
* by Jiho Choi et. al. but adapted to Rust's type system and its lack of a managed runtime
* environment.
*
* The type `HybridRc<T, State>` provides thread-safe shared ownership of a value of type `T`
* allocated on the heap, just like `std::sync::Arc<T>` does. The main difference is that one
* thread at a time can use non-atomic reference counting for better performance. That means that
* `HybridRc` is especially suited for workloads where one thread accesses the shared value
* significantly more often than others.
*
* There a two variants of [`HybridRc`]:
* - `HybridRc<T, `[`Local`]`>` (type aliased as [`Rc`]): very fast but only usable on one thread.
* - `HybridRc<T, `[`Shared`]`>` (type aliased as [`Arc`]): slower but universally usable.
*
* Instances of both variants are convertible into each other. Especially, an `Rc` can always be
* converted into an `Arc` using [`HybridRc::to_shared(&rc)`] or [`.into()`].
*
* An `Arc` on the other hand can only be converted into an `Rc` using [`HybridRc::to_local(&arc)`]
* or [`.try_into()`] if no other thread has `Rc`s for the same value. The thread holding `Rc`s to
* a value is called the "owner thread". Once all `Rc`s are dropped, the shared value becomes
* ownerless again.
*
* `HybridRc` is designed as a drop-in replacement for `std::sync::Arc` and `std::rc::Rc`, so except
* for the conversion functionality outlined above the usage is similar to these and other smart
* pointers.
*
* # Thread Safety
*
* `HybridRc` uses two separate reference counters - one modified non-atomically and one using
* atomic operations - and keeps track of a owner thread that is allowed to modify the "local"
* reference counter. This means that it is thread-safe, while one thread is exempted from
* the disadvantage of atomic operations being more expensive than ordinary memory accesses.
*
* # `no_std` Support
*
* This crate provides limited support for `no_std` environments. In this mode `Arc::to_local()` and
* `Weak::upgrade_local()` only succeed if no `Rc` exists on *any* thread, as threads cannot be
* reliably identified without `std`.
*
* To enable `no_std` mode, disable the default enabled `std` feature in Cargo.toml. A global
* allocator is required.
*
* ```toml
* [dependencies]
* hybrid-rc = { version = "…", default-features = false }
* ```
*
* # Examples
*
* Multiple threads need a reference to a shared value while one thread needs to clone references
* to the value significantly more often than the others.
* ```
* use hybrid_rc::{Rc, Arc};
* use std::thread;
* use std::sync::mpsc::channel;
*
* # type SomeComplexType = std::collections::BinaryHeap<()>;
* # fn expensive_computation<T>(x: impl AsRef<T>, i: i32) -> i32 { let _ = x.as_ref(); i }
* # fn do_something<T>(x: impl AsRef<T>, _i: i32) { let _ = x.as_ref(); }
* # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
* let local = Rc::new(SomeComplexType::new());
* let (sender, receiver) = channel();
*
* // Spawn of threads for multiple expensive computations
* for i in 1..=4 {
* let sender = sender.clone();
* let shared = Rc::to_shared(&local);
* thread::spawn(move || {
* sender.send(expensive_computation(shared, i));
* });
* }
*
* // Do something that needs single-thread reference counting
* for i in 1..=1000 {
* do_something(local.clone(), i);
* }
*
* // Collect expensive computation results
* for i in 1..=4 {
* println!("{:?}", receiver.recv().unwrap());
* }
* # Ok(())
* # }
* ```
*
* A library wants to give library consumers flexibility for multithreading but also internally
* have the performance of `std::rc::Rc` for e.g. a complex tree structure that is mutated on
* the main thread.
* ```
* use hybrid_rc::Rc;
* use std::thread;
*
* # fn get_local_hybridrc_from_some_library() -> Rc<()> { Rc::default() }
* # fn do_something(_: &()) { }
* # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
* let reference = get_local_hybridrc_from_some_library();
* let shared = Rc::to_shared(&reference);
*
* // do the work in another thread
* let worker = thread::spawn(move || {
* do_something(&*shared);
* });
*
* // Do something useful with the library
*
* worker.join()?;
* # Ok(())
* # }
* ```
*
* [`HybridRc::to_shared(&rc)`]: HybridRc::to_shared
* [`HybridRc::to_local(&arc)`]: HybridRc::to_local
* [`.into()`]: HybridRc#impl-From<HybridRc<T%2C%20Local>>
* [`.try_into()`]: HybridRc#impl-TryFrom<HybridRc<T%2C%20Shared>>
* [doi:10.1145/3243176.3243195]: https://dl.acm.org/doi/10.1145/3243176.3243195
*/
#![cfg_attr(not(feature = "std"), no_std)]
#![deny(unsafe_op_in_unsafe_fn)]
extern crate alloc;
use alloc::alloc::Layout;
use alloc::borrow::{Cow, ToOwned};
use alloc::boxed::Box;
use alloc::string::String;
use alloc::vec::Vec;
use core::any::Any;
use core::borrow::Borrow;
use core::cell::Cell;
use core::convert::Infallible;
use core::convert::TryFrom;
use core::hash::{Hash, Hasher};
use core::marker::PhantomData;
use core::ops::Deref;
#[cfg(not(feature = "std"))]
use core::panic::{RefUnwindSafe, UnwindSafe};
use core::pin::Pin;
use core::ptr::NonNull;
use core::sync::atomic;
use core::sync::atomic::Ordering;
use core::{cmp, fmt, iter, mem, ptr};
#[cfg(feature = "std")]
use std::panic::{RefUnwindSafe, UnwindSafe};
mod atomic_thread_id;
use atomic_thread_id::{AtomicOptionThreadId, ThreadId};
mod slice_builder;
use slice_builder::SliceBuilder;
mod tests;
mod thread_id;
/// Provides a senitel pointer value for dangling `Weak`s.
///
/// This is not NULL to allow optimizations through [`NonNull`] but cannot ever be a valid pointer
/// to a [`RcBox`].
#[inline]
const fn senitel<T>() -> NonNull<T> {
unsafe { NonNull::new_unchecked(usize::MAX as *mut T) }
}
/// Checks if the provided pointer is the [`senitel`]
#[inline]
fn is_senitel<T: ?Sized>(ptr: *const T) -> bool {
ptr.cast::<()>() == senitel().as_ptr()
}
/// Internal module for non-public definition of `RcState`.
mod state_trait {
use core::fmt::Debug;
/// Internal trait for type-level enumeration of `Shared` and `Local`.
pub trait RcState: Debug {
const SHARED: bool;
}
}
use state_trait::RcState;
/// Marker types for the states of a [`HybridRc`]
pub mod state {
/// Marks a [`HybridRc`] as shared.
///
/// `HybridRc<_, Shared>` atomically updates the shared reference counter.
///
/// # See also
/// - [`Local`]
///
/// [`HybridRc`]: super::HybridRc
#[derive(Debug, Clone, Copy)]
pub enum Shared {}
impl super::RcState for Shared {
const SHARED: bool = true;
}
/// Marks a [`HybridRc`] as local.
///
/// `HybridRc<_, Local>` non-atomically updates the local reference counter.
///
/// # See also
/// - [`Shared`]
///
/// [`HybridRc`]: super::HybridRc
#[derive(Debug, Clone, Copy)]
pub enum Local {}
impl super::RcState for Local {
const SHARED: bool = false;
}
}
use state::{Local, Shared};
/// An enumeration of possible errors when upgrading a [`Weak`].
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum UpgradeError {
/// The referenced value was already dropped because no strong references to it exists anymore.
ValueDropped,
/// The requested action would have created a new [`Rc`] while at least one `Rc` still existed
/// on another thread.
WrongThread,
}
impl fmt::Display for UpgradeError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
Self::ValueDropped => f.write_str("value was already dropped"),
Self::WrongThread => {
f.write_str("tried to get a local reference while another thread was the owner")
}
}
}
}
#[cfg(feature = "std")]
impl std::error::Error for UpgradeError {}
impl From<Infallible> for UpgradeError {
fn from(x: Infallible) -> UpgradeError {
match x {}
}
}
/// The `AllocError` error indicates an allocation failure when using `try_new()` etc.
///
/// Will become a type alias for [`std::alloc::AllocError`] once that is stabilized.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub struct AllocError;
impl fmt::Display for AllocError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("memory allocation failed")
}
}
#[cfg(feature = "std")]
impl std::error::Error for AllocError {}
impl From<Infallible> for AllocError {
fn from(_: Infallible) -> AllocError {
unreachable!();
}
}
/// Reimplementation of `ptr::set_ptr_value` as long as that one is unstable
///
/// Constructs a new pointer to `addr_ptr` with the metadata and type of `meta_ptr`.
#[inline]
fn set_ptr_value<T: ?Sized, U>(mut meta_ptr: *const T, addr_ptr: *mut U) -> *mut T {
let thin = (&mut meta_ptr as *mut *const T).cast::<*const u8>();
// Safety: In case of a thin pointer, this operations is identical
// to a simple assignment. In case of a fat pointer, with the current
// fat pointer layout implementation, the first field of such a
// pointer is always the data pointer, which is likewise assigned.
unsafe { *thin = addr_ptr.cast() };
meta_ptr as *mut T
}
/// Metadata part of a shared allocation.
struct RcMeta {
/// Id for the thread which may use local references
owner: AtomicOptionThreadId,
/// Strong local reference count
strong_local: Cell<usize>,
/// Strong shared reference count (+ 1 for all strong local references combined)
strong_shared: atomic::AtomicUsize,
/// Weak reference count (+ 1 for all strong references combined)
///
/// If `usize::MAX`, the ability to downgrade strong pointers is temporarily locked to avoid
/// races in `get_mut()`.
weak: atomic::AtomicUsize,
}
/// Heap struct for shared allocations of `T`.
///
/// `repr(C)` to future-proof against possible layout optimizations which could interfere with
/// `[into|from]_raw()` of transmutable data types.
#[repr(C)]
struct RcBox<T: ?Sized> {
meta: RcMeta,
data: T,
}
impl<T: ?Sized> RcBox<T> {
/// Deallocates an `RcBox`
///
/// `meta` will be dropped, but `data` must have already been dropped in place.
///
/// # Safety
/// The allocation must have been previously allocated with `RcBox::allocate_*()`.
#[inline]
unsafe fn dealloc(ptr: NonNull<RcBox<T>>) {
unsafe { ptr::addr_of_mut!((*ptr.as_ptr()).meta).drop_in_place() };
let layout = Layout::for_value(unsafe { ptr.as_ref() });
unsafe { alloc::alloc::dealloc(ptr.as_ptr().cast(), layout) };
}
/// Tries to allocate an `RcBox` for a possibly dynamically sized value
///
/// Size and alignment of `example` are used for allocation and if `example` is a fat reference
/// the pointer metadata is copied to the resulting pointer.
///
/// Returns a mutable pointer on success and the memory layout that could not be allocated
/// if the allocation failed.
#[inline]
fn try_allocate_for_val(
meta: RcMeta,
example: &T,
zeroed: bool,
) -> Result<NonNull<RcBox<T>>, Layout> {
let layout = Layout::new::<RcBox<()>>();
let layout = layout
.extend(Layout::for_value(example))
.map_err(|_| layout)?
.0
.pad_to_align();
// Allocate memory
let ptr = unsafe {
if zeroed {
alloc::alloc::alloc_zeroed(layout)
} else {
alloc::alloc::alloc(layout)
}
}
.cast::<RcBox<()>>();
// Write RcMeta fields
// Safety: Freshly allocated, so valid to write to.
unsafe { ptr::addr_of_mut!((*ptr).meta).write(meta) };
// Combine metadata from `example` with new memory
let result = set_ptr_value(example, ptr);
NonNull::new(result as *mut RcBox<T>).ok_or(layout)
}
/// Allocates an `RcBox` for a possibly dynamically sized value
///
/// Size and alignment of `example` are used for allocation and if `example` is a fat reference
/// the pointer metadata is copied to the resulting pointer.
///
/// Returns a mutable pointer on success.
///
/// # Panics
/// Panics or aborts if the allocation failed.
#[inline]
fn allocate_for_val(meta: RcMeta, example: &T, zeroed: bool) -> NonNull<RcBox<T>> {
match Self::try_allocate_for_val(meta, example, zeroed) {
Ok(result) => result,
Err(layout) => alloc::alloc::handle_alloc_error(layout),
}
}
/// Get the pointer to a `RcBox<T>` from a pointer to the data
///
/// # Safety
///
/// The pointer must point to (and have valid metadata for) the data part of a previously
/// valid instance of `RcBox<T>` and it must not be dangling.
#[inline]
unsafe fn ptr_from_data_ptr(ptr: *const T) -> *const RcBox<T> {
// Calculate layout of RcBox<T> without `data` tail, but including padding
let base_layout = Layout::new::<RcBox<()>>();
// Safety: covered by the safety contract above
let value_alignment = mem::align_of_val(unsafe { &*ptr });
let value_offset_layout =
Layout::from_size_align(0, value_alignment).expect("invalid memory layout");
let layout = base_layout
.extend(value_offset_layout)
.expect("invalid memory layout")
.0;
// Move pointer to point to the start of the original RcBox<T>
// Safety: covered by the safety contract above
let rcbox = unsafe { ptr.cast::<u8>().offset(-(layout.size() as isize)) };
set_ptr_value(ptr, rcbox as *mut u8) as *const RcBox<T>
}
}
impl<T> RcBox<T> {
/// Tries to allocate an `RcBox`
///
/// Returns a mutable reference with arbitrary lifetime on success and the memory layout that
/// could not be allocated if the allocation failed.
#[inline]
fn try_allocate(meta: RcMeta) -> Result<NonNull<RcBox<mem::MaybeUninit<T>>>, Layout> {
let layout = Layout::new::<RcBox<T>>();
let ptr = unsafe { alloc::alloc::alloc(layout) }.cast::<RcBox<mem::MaybeUninit<T>>>();
if ptr.is_null() {
Err(layout)
} else {
unsafe { ptr::addr_of_mut!((*ptr).meta).write(meta) };
Ok(unsafe { NonNull::new_unchecked(ptr) })
}
}
/// Allocates an `RcBox`
///
/// Returns a mutable reference with arbitrary lifetime on success.
///
/// # Panics
/// Panics or aborts if the allocation failed.
#[inline]
fn allocate(meta: RcMeta) -> NonNull<RcBox<mem::MaybeUninit<T>>> {
match Self::try_allocate(meta) {
Ok(result) => result,
Err(layout) => alloc::alloc::handle_alloc_error(layout),
}
}
/// Tries to allocate an `RcBox` for a slice.
///
/// Returns a mutable reference with arbitrary lifetime on success and the memory layout that
/// could not be allocated if the allocation failed or the layout calculation overflowed.
#[inline]
fn try_allocate_slice<'a>(
meta: RcMeta,
len: usize,
zeroed: bool,
) -> Result<&'a mut RcBox<[mem::MaybeUninit<T>]>, Layout> {
// Calculate memory layout
let layout = Layout::new::<RcBox<[T; 0]>>();
let payload_layout = Layout::array::<T>(len).map_err(|_| layout)?;
let layout = layout
.extend(payload_layout)
.map_err(|_| layout)?
.0
.pad_to_align();
// Allocate memory
let ptr = unsafe {
if zeroed {
alloc::alloc::alloc_zeroed(layout)
} else {
alloc::alloc::alloc(layout)
}
};
// Build a fat pointer
// The immediate slice reference [MaybeUninit<u8>] *should* be sound
let ptr = ptr::slice_from_raw_parts_mut(ptr.cast::<mem::MaybeUninit<u8>>(), len)
as *mut RcBox<[mem::MaybeUninit<T>]>;
if ptr.is_null() {
// Allocation failed
Err(layout)
} else {
// Initialize metadata field and return result
unsafe { ptr::addr_of_mut!((*ptr).meta).write(meta) };
Ok(unsafe { ptr.as_mut().unwrap() })
}
}
/// Allocates an `RcBox` for a slice
///
/// Returns a mutable reference with arbitrary lifetime on success.
///
/// # Panics
/// Panics or aborts if the allocation failed or the memory layout calculation overflowed.
#[inline]
fn allocate_slice<'a>(
meta: RcMeta,
len: usize,
zeroed: bool,
) -> &'a mut RcBox<[mem::MaybeUninit<T>]> {
match Self::try_allocate_slice(meta, len, zeroed) {
Ok(result) => result,
Err(layout) => alloc::alloc::handle_alloc_error(layout),
}
}
}
impl<T> RcBox<mem::MaybeUninit<T>> {
/// Converts to a mutable reference without the `MaybeUninit` wrapper.
///
/// # Safety
/// The payload must have been fully initialized or this causes immediate undefined behaviour.
#[inline]
unsafe fn assume_init(&mut self) -> &mut RcBox<T> {
unsafe { (self as *mut Self).cast::<RcBox<T>>().as_mut() }.unwrap()
}
}
impl<T> RcBox<[mem::MaybeUninit<T>]> {
/// Converts to a mutable reference without the `MaybeUninit` wrapper.
///
/// # Safety
/// The payload slice must have been fully initialized or this causes immediate undefined
/// behaviour.
#[inline]
unsafe fn assume_init(&mut self) -> &mut RcBox<[T]> {
unsafe { (self as *mut _ as *mut RcBox<[T]>).as_mut() }.unwrap()
}
}
impl RcMeta {
/// Increments the local reference counter unconditionally.
///
/// *Only safe to use on the owner thread and as long as at least one local reference exists.*
///
/// # Panics
/// Panics if the counter overflowed.
#[inline(always)]
fn inc_strong_local(&self) {
let counter = self.strong_local.get();
if counter == usize::MAX {
panic!("reference counter overflow");
}
self.strong_local.set(counter + 1);
}
/// Increment the local reference counter.
///
/// Also adjusts the shared reference counter if neccessary.
///
/// Fails if this would resurrect an already dropped
/// value.
///
/// *Only safe to use on the owner thread.*
///
/// # Panics
/// Panics if one of the counters overflowed.
#[inline]
fn try_inc_strong_local(&self) -> Result<(), ()> {
let counter = self.strong_local.get();
if counter == usize::MAX {
panic!("reference counter overflow");
} else if counter == 0 {
self.try_inc_strong_shared()?;
}
self.strong_local.set(counter + 1);
Ok(())
}
/// Decrements the local reference counter.
///
/// Also adjusts the shared reference counter and
/// the `owner` if neccessary.
///
/// Returns **true** if no strong references remain at all.
///
/// *Only safe to use on the owner thread.*
///
/// # Panics
/// Panics if the shared reference counter was already zero.
#[inline(always)]
fn dec_strong_local(&self) -> bool {
let counter = self.strong_local.get();
self.strong_local.set(counter - 1);
if counter == 1 {
self.remove_last_local_reference()
} else {
false
}
}
/// Decrements the shared counter and sets the `owner` to `None`.
///
/// Used internally by `dec_strong_local()`
///
/// # Panics
/// Panics if the counter was already zero.
fn remove_last_local_reference(&self) -> bool {
let old_shared = self.strong_shared.fetch_sub(1, Ordering::Release);
if old_shared == 0 {
panic!("reference counter underflow");
}
self.owner.store(None, Ordering::Release);
old_shared == 1
}
/// Increments the shared reference counter unconditionally.
///
/// *Only safe to use as long as at least one shared reference exists.*
///
/// # Panics
/// Panics if the counter overflowed.
#[inline]
fn inc_strong_shared(&self) {
let old_counter = self.strong_shared.fetch_add(1, Ordering::Relaxed);
if old_counter == usize::MAX {
panic!("reference counter overflow");
}
}
/// Increments the shared reference counter.
///
/// Also adjusts the shared reference counter and the `owner` if neccessary.
///
/// Fails if this would resurrect an already dropped value.
///
/// # Panics
/// Panics if the counter overflowed.
#[inline]
fn try_inc_strong_shared(&self) -> Result<(), ()> {
self.strong_shared
.fetch_update(
Ordering::Relaxed,
Ordering::Relaxed,
|old_counter| match old_counter {
0 => None,
usize::MAX => panic!("reference counter overflow"),
_ => Some(old_counter + 1),
},
)
.map(|_| ())
.map_err(|_| ())
}
/// Decrements the shared reference counter.
///
/// Returns **true** if no strong references remain at all.
///
/// # Panics
/// Panics if the counter was already zero.
#[inline]
fn dec_strong_shared(&self) -> bool {
let old_counter = self.strong_shared.fetch_sub(1, Ordering::Release);
if old_counter == 0 {
panic!("reference counter underflow");
}
old_counter == 1 && self.owner.load(Ordering::Relaxed).is_none()
}
/// Increments the weak reference counter.
///
/// # Panics
/// Panics if the counter overflowed or was already zero.
#[inline]
fn inc_weak(&self) {
const MAX_COUNT: usize = usize::MAX - 1;
let mut counter = self.weak.load(Ordering::Relaxed);
// CAS loop
loop {
match counter {
usize::MAX => {
core::hint::spin_loop();
counter = self.weak.load(Ordering::Relaxed);
continue;
}
MAX_COUNT => panic!("weak counter overflow"),
0 => panic!("BUG: weak resurrection of dead counted reference"),
_ => {
let result = self.weak.compare_exchange_weak(
counter,
counter + 1,
Ordering::Acquire,
Ordering::Relaxed,
);
match result {
Ok(_) => break,
Err(old) => counter = old,
}
}
}
}
}
/// Increments the weak reference counter (without a spin loop).
///
/// # Panics
/// Panics if the counter is locked, overflowed or was already zero.
#[inline]
fn inc_weak_nolock(&self) {
const MAX_COUNT: usize = usize::MAX - 1;
match self.weak.fetch_add(1, Ordering::Relaxed) {
usize::MAX => panic!("BUG: weak counter locked"),
MAX_COUNT => panic!("weak counter overflow"),
0 => panic!("BUG: weak resurrection of dead counted reference"),
_ => (),
}
}
/// Decrements the weak reference counter.
///
/// Returns **true** if the counter reached zero.
///
/// # Panics
/// Panics if the counter was already zero.
#[inline]
fn dec_weak(&self) -> bool {
let old_counter = self.weak.fetch_sub(1, Ordering::Release);
if old_counter == 0 {
panic!("weak counter underflow");
}
old_counter == 1
}
/// Checks if there is only one unique reference.
///
/// If `is_local` is true, it is assumed that we can access the local reference counter
///
/// Temporarily locks the weak reference counter to prevent race conditions.
#[inline]
fn has_unique_ref(&self, is_local: bool) -> bool {
let result =
self.weak
.compare_exchange(1, usize::MAX, Ordering::Acquire, Ordering::Relaxed);
if result.is_ok() {
let mut count = self.strong_shared.load(Ordering::Acquire);
if count == 1 {
let owner = self.owner.load(Ordering::Relaxed);
match owner {
None => {}
Some(tid) if is_local || tid == ThreadId::current_thread() => {
count = self.strong_local.get();
}
Some(_) => {
count = 2;
}
}
}
self.weak.store(1, Ordering::Release);
count == 1
} else {
false
}
}
}
/// A hybrid reference-counting pointer.
///
/// - [`HybridRc<T, Shared>`][Arc] behaves mostly like [`std::sync::Arc`]
/// - [`HybridRc<T, Local>`][Rc] behaves mostly like [`std::rc::Rc`].
///
/// See the [module-level documentation][crate] for more details.
///
/// The inherent methods of `HybridRc` are all associated functions, which means that you have to
/// call them as e.g. [`HybridRc::get_mut(&mut x)`] instead of `x.get_mut()`. This avoids conflicts
/// with methods of the inner type `T`.
///
/// [`HybridRc::get_mut(&mut x)`]: Self::get_mut
#[must_use]
pub struct HybridRc<T: ?Sized, State: RcState> {
ptr: NonNull<RcBox<T>>,
phantom: PhantomData<State>,
phantom2: PhantomData<RcBox<T>>,
}
/// Type alias for a local reference counting pointer.
///
/// Provided to ease migrating from [`std::rc::Rc`].
///
/// See the [module-level documentation][crate] for more details.
///
/// The inherent methods of `Rc` are all associated functions, which means that you have to call
/// them as e.g. [`Rc::to_shared(&x)`] instead of `x.to_shared()`. This avoids conflicts with
/// methods of the inner type `T`.
///
/// [`Rc::to_shared(&x)`]: Self::to_shared
pub type Rc<T> = HybridRc<T, Local>;
/// Type alias for a shared reference counting pointer.
///
/// Provided to ease migrating from [`std::sync::Arc`].
///
/// See the [module-level documentation] for more details.
///
/// The inherent methods of `Arc` are all associated functions, which means that you have to call
/// them as e.g. [`Arc::to_local(&x)`] instead of `x.to_local()`. This avoids conflicts with
/// methods of the inner type `T`.
///
/// [`Arc::to_local(&x)`]: Self::to_local
/// [module-level documentation]: crate
pub type Arc<T> = HybridRc<T, Shared>;
impl<T: ?Sized, State: RcState> HybridRc<T, State> {
/// Creates a new `HybridRc` from a pointer to a shared allocation.
///
/// The reference counters must have been updated by the caller.
#[inline(always)]
fn from_inner(ptr: NonNull<RcBox<T>>) -> Self {
Self {
ptr,
phantom: PhantomData,
phantom2: PhantomData,
}
}
/// Provides a reference to the inner value.
#[inline(always)]
fn data(&self) -> &T {
// Safety: as long as one HybridRc or Weak for this item exists, the memory stays allocated.
unsafe { &(*self.ptr.as_ptr()).data }
}
/// Provides a reference to the shared metadata.
#[inline(always)]
fn meta(&self) -> &RcMeta {
// Safety: as long as one HybridRc or Weak for this item exists, the memory stays allocated.
unsafe { &(*self.ptr.as_ptr()).meta }
}
/// Provides a reference to the inner `HybridRc` of a `Pin<HybridRc<T>>`
///
/// # Safety
/// The caller must ensure that the reference is not used to move the value out of self.
#[inline(always)]
unsafe fn pin_get_ref(this: &Pin<Self>) -> &Self {
// SAFETY: Pin is repr(transparent) and by contract the caller doesn't use the reference
// to move the value.
unsafe { &*(this as *const Pin<Self>).cast::<Self>() }
}
/// Returns a mutable reference to the value, without checking for uniqueness.
///
/// # See also
/// - [`get_mut()`], which is safe.
///
/// # Safety
/// No other `HybridRc` or [`Weak`] for the same value must be dereferenced for the duration of
/// the returned borrow.
///
/// # Example
/// ```
/// use hybrid_rc::Rc;
///
/// let mut a = Rc::new([1, 2, 3]);
/// // We know that there can't be any other references yet, so getting a mutable reference
/// // is safe here:
/// let mut_ref = unsafe { Rc::get_mut_unchecked(&mut a) };
/// mut_ref[0] = 42;
///
/// assert_eq!(a[..], [42, 2, 3]);
/// ```
/// [`get_mut()`]: Self::get_mut
#[must_use]
#[inline]
pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
unsafe { &mut (*this.ptr.as_ptr()).data }
}
/// Returns a mutable reference to the value, iff the value is not shared
/// with another `HybridRc` or [`Weak`].
///
/// Returns `None` otherwise.
#[must_use]
#[inline]
pub fn get_mut(this: &mut Self) -> Option<&mut T> {
if this.meta().has_unique_ref(!State::SHARED) {
unsafe { Some(Self::get_mut_unchecked(this)) }
} else {
None
}
}
/// Provides a raw pointer to the referenced value
///
/// The counts are not affected in any way and the `HybridRc` is not consumed.
/// The pointer is valid for as long there exists at least one `HybridRc` for the value.
#[must_use]
#[inline]
pub fn as_ptr(this: &Self) -> *const T {
let ptr = this.ptr.as_ptr();
// Safety: Neccessary for `from_raw()` (when implemented), retains provenance.
// Besides that, does basically the same thing as `data()` or `get_mut_unchecked()`.
unsafe { ptr::addr_of_mut!((*ptr).data) }
}
/// Consumes the `HybridRc<T, State>`, returning the wrapped pointer.
///
/// To avoid a memory leak the pointer must be converted back to a `HybridRc` using
/// [`HybridRc<T, State>::from_raw()`].
#[must_use = "Memory will leak if the result is not used"]
pub fn into_raw(this: Self) -> *const T {
let ptr = Self::as_ptr(&this);
mem::forget(this);
ptr
}
/// Reconstructs a `HybridRc<T, State>` from a raw pointer.
///
/// Creates a `HybridRc<T, State>` from a pointer that has been previously returned by
/// a call to [`into_raw()`].
///
/// # Safety
///
/// The raw pointer must have been previously returned by a call to
/// [`HybridRc<T, State>`][`into_raw()`] for the same `State` *and* the same `T` or another
/// compatible type that has the same size and alignment. The latter case amounts to
/// [`mem::transmute()`] and is likely to produce undefined behaviour if not handled correctly.
///
/// The value must not have been dropped yet.
///
/// [`into_raw()`]: Self::into_raw
pub unsafe fn from_raw(ptr: *const T) -> Self {
// Safety: covered by the safety contract for this function
let box_ptr = unsafe { RcBox::<T>::ptr_from_data_ptr(ptr) };
Self::from_inner(NonNull::new(box_ptr as *mut _).expect("invalid pointer"))
}
/// Creates a new [`Weak`] for the referenced value.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let strong = Rc::new(42i32);
/// let weak = Rc::downgrade(&strong);
///
/// assert_eq!(Rc::as_ptr(&strong), Weak::as_ptr(&weak));
/// ```
#[inline]
pub fn downgrade(this: &Self) -> Weak<T> {
this.meta().inc_weak();
Weak { ptr: this.ptr }
}
/// Creates a new [`PinWeak`] for the referenced value.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let strong = Rc::pin(42i32);
/// let weak = Rc::downgrade_pin(&strong);
/// ```
#[inline]
pub fn downgrade_pin(this: &Pin<Self>) -> PinWeak<T> {
// Safety: We are not moving anything and we don't expose a non-pinned pointer.
let this = unsafe { Self::pin_get_ref(this) };
PinWeak(Self::downgrade(this))
}
/// Checks if two `HybridRc`s point to the same allocation.
#[inline]
pub fn ptr_eq<S: RcState>(this: &Self, other: &HybridRc<T, S>) -> bool {
this.ptr.as_ptr() == other.ptr.as_ptr()
}
/// Checks if two pinned `HybridRc`s point to the same allocation.
#[inline]
pub fn ptr_eq_pin<S: RcState>(this: &Pin<Self>, other: &Pin<HybridRc<T, S>>) -> bool {
// SAFETY: we are not moving anything and we don't expose any pointers.
let this = unsafe { Self::pin_get_ref(this) };
let other = unsafe { HybridRc::<T, S>::pin_get_ref(other) };
this.ptr.as_ptr() == other.ptr.as_ptr()
}
/// Gets the approximate number of strong pointers to the inner value.
///
/// As shared pointers cannot access the local reference counter, `Arc::strong_count()` only
/// provides a lower bound on the reference count at the moment of the call.
///
/// Please also understand that, if the count is greater than one, another thread might change
/// the count at any time, including potentially between calling this method and acting on the
/// result.
///
/// # Examples
///
/// ```
/// use hybrid_rc::{Rc, Arc};
///
/// let reference = Rc::new(42);
/// let _2nd_ref = Rc::clone(&reference);
/// let shared_ref = Rc::to_shared(&reference);
/// let _2nd_shared_ref = Arc::clone(&shared_ref);
///
/// assert_eq!(Rc::strong_count(&reference), 4);
/// // shared_ref only knows the count of shared references and that there is at least one
/// // local reference, so it will show 3 instead of 4:
/// assert_eq!(Arc::strong_count(&shared_ref), 3);
/// ```
#[inline]
pub fn strong_count(this: &Self) -> usize {
let meta = this.meta();
meta.strong_shared.load(Ordering::SeqCst)
+ if State::SHARED {
0
} else {
meta.strong_local.get() - 1
}
}
/// Gets the approximate number of strong pointers to the pinned inner value.
///
#[inline]
pub fn strong_count_pin(this: &Pin<Self>) -> usize {
// SAFETY: We are not moving anything and we don't expose any pointers.
let this = unsafe { Self::pin_get_ref(this) };
Self::strong_count(this)
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// Please understand that another thread may change the weak count at any time, including
/// potentially between calling this method and acting on the result.
///
/// # Examples
///
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let reference = Rc::new(42);
/// let weak = Rc::downgrade(&reference);
/// let _weak_2 = weak.clone();
///
/// assert_eq!(Rc::weak_count(&reference), 2);
/// ```
#[inline]
pub fn weak_count(this: &Self) -> usize {
match this.meta().weak.load(Ordering::SeqCst) {
// Lock value => there were zero weak references apart from the implicit one.
usize::MAX => 0,
count => count - 1,
}
}
/// Gets the number of [`PinWeak`] pointers to the pinned inner value.
///
#[inline]
pub fn weak_count_pin(this: &Pin<Self>) -> usize {
// SAFETY: We are not moving anything and we don't expose any pointers.
let this = unsafe { Self::pin_get_ref(this) };
Self::weak_count(this)
}
// Constructs an `RcMeta` structure for a new `HybridRc` allocation
#[inline]
fn build_new_meta() -> RcMeta {
RcMeta {
owner: if State::SHARED {
None.into()
} else {
ThreadId::current_thread().into()
},
strong_local: Cell::new(if State::SHARED { 0 } else { 1 }),
strong_shared: 1.into(),
weak: 1.into(),
}
}
/// Drops the contained value and also drops the shared `RcBox` if there are no other `Weak`
/// references.
///
/// # Safety
/// Only safe to use in `drop()` or a consuming function after verifying that no other strong
/// reference exists. Otherwise after calling this e.g. dereferencing the `HybridRc` WILL
/// cause undefined behaviour and even dropping it MAY cause undefined behaviour.
unsafe fn drop_contents_and_maybe_box(&mut self) {
// Safety: only called if this was the last strong reference
unsafe {
ptr::drop_in_place(Self::get_mut_unchecked(self));
}
if self.meta().dec_weak() {
// Safety: only called if this was the last (weak) reference
unsafe {
RcBox::dealloc(self.ptr);
}
}
}
}
impl<T, State: RcState> HybridRc<T, State> {
/// Creates a new `Rc<T>`, moving `data` into a reference counted allocation.
///
/// If `State` is `Local`, the shared value is initially owned by the calling thread, so
/// for another thread to assume ownership [`to_shared()`] must be used and all `Rc`s for
/// the value must be dropped.
///
/// If `State` is `Shared`, initially the shared value has no owner thread, so any thread may
/// call [`to_local()`] to assume ownership.
///
/// # Examples
/// ```
/// use hybrid_rc::Rc;
///
/// let rc = Rc::new(42i32);
/// ```
/// ```compile_fail
/// # let rc = hybrid_rc::Rc::new(42i32);
/// // Cannot be used in another thread without using rc.to_shared()
/// std::thread::spawn(move || *rc).join(); // does not compile
/// ```
///
/// ```
/// use hybrid_rc::Arc;
/// # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
///
/// let arc = Arc::new(42i32);
///
/// std::thread::spawn(move || assert!(*arc == 42)).join()?;
/// # Ok(())
/// # }
/// ```
///
/// [`to_shared()`]: Self::to_shared
/// [`to_local()`]: Self::to_local
#[inline]
pub fn new(data: T) -> Self {
let mut inner = RcBox::allocate(Self::build_new_meta());
let inner = unsafe { inner.as_mut() };
inner.data.write(data);
Self::from_inner(unsafe { inner.assume_init() }.into())
}
/// Creates a new `HybridRc` with uninitialized contents.
#[inline]
pub fn new_uninit() -> HybridRc<mem::MaybeUninit<T>, State> {
let inner = RcBox::allocate(Self::build_new_meta());
HybridRc::from_inner(inner)
}
/// Creates a new `HybridRc` with uninitialized contents, with the memory being filled with
/// 0 bytes.
///
/// See [`MaybeUninit::zeroed()`] for examples of correct and incorrect usage of this method.
///
/// [`MaybeUninit::zeroed()`]: mem::MaybeUninit::zeroed
#[inline]
pub fn new_zeroed() -> HybridRc<mem::MaybeUninit<T>, State> {
let mut inner = RcBox::allocate(Self::build_new_meta());
unsafe { inner.as_mut() }.data = mem::MaybeUninit::zeroed();
HybridRc::from_inner(inner)
}
/// Creates a new `HybridRc` with a possibly cyclic reference.
///
/// For this a reference to a [`Weak`] is passed to the closure that – after this function
/// returns – will point to the new value itself. Attempting to upgrade the weak reference
/// before `new_cyclic` returns will result in a `ValueDropped` error. However, the weak
/// reference may be cloned freely and stored for use at a later time.
#[inline]
pub fn new_cyclic(data_fn: impl FnOnce(&Weak<T>) -> T) -> HybridRc<T, State> {
// Construct metadata for an initially non-upgradable RcBox
let meta = RcMeta {
owner: if State::SHARED {
None.into()
} else {
ThreadId::current_thread().into()
},
strong_local: Cell::new(0),
strong_shared: 0.into(),
weak: 1.into(),
};
// Allocate memory (uninitialized)
let inner = RcBox::<T>::allocate(meta);
// Construct `Weak`
let weak: Weak<T> = Weak { ptr: NonNull::from(inner).cast() };
// Run data function, keeping the ownership of the weak reference.
let data = data_fn(&weak);
// Initialize data in our box
// Not creating an immediate &mut of the whole box to not invalidate the
// weak pointer under Stacked Borrows rules.
unsafe { &mut *ptr::addr_of_mut!((*inner.as_ptr()).data) }.write(data);
// Don't run `Weak`s destructor. The value we just initialized should keep existing and we
// need a weak count of 1 for the strong reference that we are currently constructing.
mem::forget(weak);
// Fix the reference counts
{
let meta = unsafe { &*ptr::addr_of!((*inner.as_ptr()).meta) };
if !State::SHARED {
meta.inc_strong_local()
}
// Must be at least `Release`, so that all threads see the initialized data before
// they can observe a non-zero reference count.
meta.strong_shared.fetch_add(1, Ordering::Release);
}
Self::from_inner(inner.cast())
}
/// Creates a new `Pin<HybridRc<T>>`. If `T` does not implement `Unpin`, then `data` will be
/// pinned in memory and unable to be moved.
#[inline]
pub fn pin(data: T) -> Pin<Self> {
unsafe { Pin::new_unchecked(Self::new(data)) }
}
/// Tries to creates a new `Rc<T>`, moving `data` into a reference counted allocation.
///
/// # Errors
/// Will drop `data` and return `Err(`[`AllocError`]`)` if the allocation fails.
///
/// Please note that the global allocator on some systems may instead abort the process if an
/// allocation failure happens.
#[inline]
pub fn try_new(data: T) -> Result<Self, AllocError> {
let mut inner = RcBox::try_allocate(Self::build_new_meta()).map_err(|_| AllocError)?;
let inner = unsafe { inner.as_mut() };
inner.data.write(data);
Ok(Self::from_inner(unsafe { inner.assume_init() }.into()))
}
/// Tries to construct a new `HybridRc` with uninitialized contents.
///
/// # Errors
/// Will return `Err(`[`AllocError`]`)` if the allocation fails.
///
/// Please note that the global allocator on some systems may instead abort the process if an
/// allocation failure happens.
#[inline]
pub fn try_new_uninit() -> Result<HybridRc<mem::MaybeUninit<T>, State>, AllocError> {
let inner = RcBox::try_allocate(Self::build_new_meta()).map_err(|_| AllocError)?;
Ok(HybridRc::from_inner(inner.into()))
}
/// Tries to construct a new `HybridRc` with uninitialized contents, with the memory being
/// filled with 0 bytes.
///
/// See [`MaybeUninit::zeroed()`] for examples of correct and incorrect usage of this method.
///
/// # Errors
/// Will return `Err(`[`AllocError`]`)` if the allocation fails.
///
/// Please note that the global allocator on some systems may instead abort the process if an
/// allocation failure happens.
///
/// [`MaybeUninit::zeroed()`]: mem::MaybeUninit::zeroed
#[inline]
pub fn try_new_zeroed() -> Result<HybridRc<mem::MaybeUninit<T>, State>, AllocError> {
let mut inner = RcBox::try_allocate(Self::build_new_meta()).map_err(|_| AllocError)?;
unsafe { inner.as_mut() }.data = mem::MaybeUninit::zeroed();
Ok(HybridRc::from_inner(inner))
}
/// Returns the inner value, if this `HybridRc` is the only strong reference to it.
///
/// Any outstanding [`Weak`] references won't be able to upgrade anymore when this succeeds.
///
/// # Errors
/// If this is not the only strong reference to the shared value, an [`Err`] is returned with
/// the same `HybridRc` that was passed in.
///
/// # Examples
///
/// ```
/// use hybrid_rc::Rc;
///
/// let reference = Rc::new(42);
/// let weak = Rc::downgrade(&reference);
///
/// let value = Rc::try_unwrap(reference).unwrap();
/// assert_eq!(value, 42);
/// assert!(weak.upgrade().is_err()); // Weaks cannot upgrade anymore.
/// ```
#[inline]
pub fn try_unwrap(this: Self) -> Result<T, Self> {
if State::SHARED {
Self::try_unwrap_internal(this)
} else {
// If we may access the local counter, first check and decrement that one.
let local_count = this.meta().strong_local.get();
if local_count == 1 {
this.meta().strong_local.set(0);
match Self::try_unwrap_internal(this) {
Ok(result) => Ok(result),
Err(this) => {
this.meta().strong_local.set(local_count);
Err(this)
}
}
} else {
Err(this)
}
}
}
/// Returns the inner value, if this `HybridRc` is the only strong reference to it, assuming
/// that there are no (other) local references to the value.
///
/// Used internally by `try_unwrap()`.
#[inline]
fn try_unwrap_internal(this: Self) -> Result<T, Self> {
let meta = this.meta();
// There is one implicit shared reference for all local references, so if there are no other
// local references or we are a shared shared and the shared counter is 1, we are the only
// strong reference left.
if meta
.strong_shared
.compare_exchange(1, 0, Ordering::AcqRel, Ordering::Relaxed)
.is_err()
{
Err(this)
} else {
// Relaxed should be enough, as `strong_shared` already hit 0, so no more
// Weak upgrading is possible.
meta.owner.store(None, Ordering::Relaxed);
let copy = unsafe { ptr::read(Self::as_ptr(&this)) };
// Make a weak pointer to clean up the remaining implicit weak reference
let _weak = Weak { ptr: this.ptr };
mem::forget(this);
Ok(copy)
}
}
}
impl<T, State: RcState> HybridRc<[T], State> {
/// Creates a new reference-counted slice with uninitialized contents.
#[inline]
pub fn new_uninit_slice(len: usize) -> HybridRc<[mem::MaybeUninit<T>], State> {
let inner = RcBox::allocate_slice(Self::build_new_meta(), len, false);
HybridRc::from_inner(inner.into())
}
/// Creates a new reference-counted slice with uninitialized contents, with the memory being
/// filled with 0 bytes.
#[inline]
pub fn new_zeroed_slice(len: usize) -> HybridRc<[mem::MaybeUninit<T>], State> {
let inner = RcBox::allocate_slice(Self::build_new_meta(), len, true);
HybridRc::from_inner(inner.into())
}
/// Copies the contents of a slice into a new `HybridRc`
///
/// # Safety
/// Either `T` is `Copy` or the caller must guarantee that the the source doesn't drop its
/// contents.
#[inline]
unsafe fn copy_from_slice_unchecked(src: &[T]) -> Self {
let len = src.len();
let inner = RcBox::allocate_slice(Self::build_new_meta(), len, false);
let dest = ptr::addr_of_mut!((*inner).data).cast();
// Safety: The freshly allocated `RcBox` can't alias `src` and the payload can be fully
// initialized by copying the slice memory. The copying is also safe as long as the safety
// requirements for calling this are fulfilled.
unsafe {
src.as_ptr().copy_to_nonoverlapping(dest, src.len());
HybridRc::from_inner(inner.assume_init().into())
}
}
}
impl<T: Copy, State: RcState> HybridRc<[T], State> {
/// Copies the contents of a slice into a new `HybridRc`
///
/// Optimization for copyable types. Will become deprecated once specialization is stablilized.
#[inline]
pub fn copy_from_slice(src: &[T]) -> Self {
// Safety: `T` is `Copy`.
unsafe { Self::copy_from_slice_unchecked(src) }
}
}
impl<T: ?Sized> Rc<T> {
/// Creates a new shared reference (`Arc`) for the referenced value.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Arc};
/// # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
///
/// let local = Rc::new(42i32);
/// let shared = Rc::to_shared(&local);
///
/// // `shared` can be safely transferred to another thread
/// std::thread::spawn(move || assert_eq!(*shared, 42i32)).join()?;
/// # Ok(())
/// # }
/// ```
#[inline]
pub fn to_shared(this: &Self) -> Arc<T> {
this.meta().inc_strong_shared();
Arc::from_inner(this.ptr)
}
/// Creates a new pinned shared reference for the referenced value.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let strong = Rc::pin(42i32);
/// let shared = Rc::to_shared_pin(&strong);
/// assert!(Rc::ptr_eq_pin(&strong, &shared));
/// ```
#[inline]
pub fn to_shared_pin(this: &Pin<Self>) -> Pin<Arc<T>> {
// SAFETY: We are not moving anything, we don't expose a non-pinned pointer,
// and we create a Pin-wrapper only for a pinned value.
unsafe {
let this = Self::pin_get_ref(this);
Pin::new_unchecked(Self::to_shared(this))
}
}
/// Increments the local strong reference count on the `Rc<T>` associated by the given pointer
///
/// Increases the local strong reference count as if a new `Rc` was cloned and kept alive.
/// May panic in the unlikely case the platform-specific maximum for the reference count is
/// reached.
///
/// # Safety
/// The pointer must have been obtained through [`HybridRc<T, Local>::into_raw()`], the value
/// must still be live and have a local strong count of at least 1 when this method is invoked
/// and this call must be performed on the same thread as where the original `Rc` was created.
///
/// [`HybridRc<T, Local>::into_raw()`]: `Rc::into_raw`
#[inline]
pub unsafe fn increment_local_strong_count(ptr: *const T) {
unsafe {
let box_ptr = RcBox::<T>::ptr_from_data_ptr(ptr as *mut T);
(*box_ptr).meta.inc_strong_local();
}
}
/// Decrements the local strong reference count on the `Rc<T>` associated by the given pointer
///
/// If the local strong reference counter reaches 0, the value is no longer considered owned
/// by the calling thread and if there are no shared strong references to keep the value alive,
/// it will be dropped.
///
/// # Safety
/// The pointer must have been obtained through [`HybridRc<T, Local>::into_raw()`], the value
/// must still be live and have a local strong count of at least 1 when this method is invoked
/// and this call must be performed on the same thread as where the original `Rc` was created.
///
/// [`HybridRc<T, Local>::into_raw()`]: `Rc::into_raw`
#[inline]
pub unsafe fn decrement_local_strong_count(ptr: *const T) {
mem::drop(unsafe { Rc::from_raw(ptr) });
}
}
impl<T: ?Sized> Arc<T> {
/// Creates a new local reference (`Rc`) for the referenced value.
///
/// Returns `None` if at least one `Rc` already exists on another thread.
///
/// **Note:** In `no_std` environments `None` is returned if at least one `Rc` exists on *any*
/// thread.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Arc};
/// # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
///
/// let local = Rc::new(42i32);
/// let shared = Rc::to_shared(&local);
///
/// // `shared` can be safely transferred to another thread
/// std::thread::spawn(move || assert_eq!(*shared, 42i32)).join()?;
/// # Ok(())
/// # }
/// ```
#[must_use]
#[inline]
pub fn to_local(this: &Self) -> Option<Rc<T>> {
let meta = this.meta();
let current_thread = ThreadId::current_thread();
let owner = match meta.owner.store_if_none(
Some(current_thread),
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => None,
Err(owner) => owner,
};
match owner {
None => {
meta.try_inc_strong_local()
.expect("inconsistent reference count (shared == 0)");
Some(Rc::from_inner(this.ptr))
}
Some(v) if v == current_thread => {
meta.inc_strong_local();
Some(Rc::from_inner(this.ptr))
}
Some(_) => None,
}
}
/// Creates a new pinned local reference for the referenced value.
///
/// Returns `None` if at least one `Rc` already exists on another thread.
///
/// **Note:** In `no_std` environments `None` is returned if at least one `Rc` exists on *any*
/// thread.
///
/// # Example
/// ```
/// use hybrid_rc::{Arc, Weak};
///
/// let strong = Arc::pin(42i32);
/// let local = Arc::to_local_pin(&strong).unwrap();
/// assert!(Arc::ptr_eq_pin(&strong, &local));
/// ```
#[must_use]
#[inline]
pub fn to_local_pin(this: &Pin<Self>) -> Option<Pin<Rc<T>>> {
// SAFETY: We are not moving anything, we don't expose a non-pinned pointer,
// and we create a Pin-wrapper only for a pinned value.
unsafe {
let this = Self::pin_get_ref(this);
Some(Pin::new_unchecked(Self::to_local(this)?))
}
}
/// Increments the shared strong reference count on the `Arc<T>` associated by the given pointer
///
/// Increases the shared strong reference count as if a new `Arc` was cloned and kept alive.
/// May panic in the unlikely case the platform-specific maximum for the reference count is
/// reached.
///
/// # Safety
/// The pointer must have been obtained through [`HybridRc<T, Shared>::into_raw()`] and the
/// value must still be live when this method is invoked.
///
/// [`HybridRc<T, Shared>::into_raw()`]: `Arc::into_raw`
#[inline]
pub unsafe fn increment_shared_strong_count(ptr: *const T) {
unsafe {
let box_ptr = RcBox::<T>::ptr_from_data_ptr(ptr);
(*box_ptr).meta.inc_strong_shared();
}
}
/// Decrements the shared strong reference count on the `Arc<T>` associated by the given pointer
///
/// If the shared strong reference counter (including the implicit shared reference for local
/// strong references) reaches 0, the value will be dropped.
///
/// # Safety
/// The pointer must have been obtained through [`HybridRc<T, Shared>::into_raw()`] and the
/// value must still be live when this method is invoked.
///
/// [`HybridRc<T, Shared>::into_raw()`]: `Arc::into_raw`
#[inline]
pub unsafe fn decrement_shared_strong_count(ptr: *const T) {
mem::drop(unsafe { Arc::from_raw(ptr) });
}
}
impl<T: Clone, State: RcState> HybridRc<T, State> {
/// Makes a mutable reference into the given `HybridRc`.
///
/// If there are other strong references to the same value, then `make_mut()` will [`clone`] the
/// inner value to a new allocation to ensure unique ownership. This is also referred to as
/// clone-on-write.
///
/// However, if there are no other strong references to this allocation, but some [`Weak`]
/// pointers, then the [`Weak`]s will be disassociated and the inner value will not be cloned.
///
/// See also: [`get_mut()`], which will fail rather than cloning the inner value
/// or diassociating [`Weak`]s.
///
/// [`clone`]: Clone::clone
/// [`get_mut()`]: HybridRc::get_mut
///
/// # Example
///
/// ```
/// use hybrid_rc::Rc;
///
/// let mut reference = Rc::new(42);
///
/// *Rc::make_mut(&mut reference) += 2; // Won't clone anything
/// let mut reference_2 = Rc::clone(&reference); // Won't clone inner value
/// *Rc::make_mut(&mut reference) += 1; // Clones inner data
/// *Rc::make_mut(&mut reference) *= 2; // Won't clone anything
/// *Rc::make_mut(&mut reference_2) /= 4; // Won't clone anything
///
/// // Now `reference` and `reference_2` point to different allocations.
/// assert_eq!(*reference, 90);
/// assert_eq!(*reference_2, 11);
/// ```
#[inline]
pub fn make_mut(this: &mut Self) -> &mut T {
let meta = this.meta();
if State::SHARED {
Self::make_mut_internal(this, false)
} else {
let local_count = meta.strong_local.get();
Self::make_mut_internal(this, local_count > 1)
}
}
/// Makes a mutable reference into the given `HybridRc`, assuming that only the shared strong
/// counter needs to be checked.
///
/// If `force_clone` is true, the counters are ignored and uniqueness will always be ensured
/// by cloning the shared allocation.
///
/// Used internally by `make_mut()`.
#[inline]
fn make_mut_internal(this: &mut Self, force_clone: bool) -> &mut T {
let meta = this.meta();
// There is one implicit shared reference for all local references, so if there are no other
// local references or we are a shared shared and the shared counter is 1, we are the only
// strong reference left.
if force_clone
|| meta
.strong_shared
.compare_exchange(1, 0, Ordering::Acquire, Ordering::Relaxed)
.is_err()
{
// Clone the allocation and make `this` point to the new clone
let mut donor = this.clone_allocation();
mem::swap(&mut this.ptr, &mut donor.ptr);
} else {
// Check if there are Weak references left.
// Relaxed suffices, as if there is a race with a dropping Weak, then it's only a
// missing optimization, but the code keeps being sound.
if meta.weak.load(Ordering::Relaxed) != 1 {
// Acts as a guard to decrement the weak counter
let _weak = Weak { ptr: this.ptr };
// Steal the payload data
let mut donor = Self::new_uninit();
unsafe {
let uninit = HybridRc::get_mut_unchecked(&mut donor);
uninit.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
let donor = donor.assume_init();
this.ptr = donor.ptr;
mem::forget(donor);
}
} else {
// There were no Weak references, so we are the unique reference. Bump the counter
// back up.
meta.strong_shared.store(1, Ordering::Release);
}
}
// Safe, because by now we are the only reference to the allocation in `this.ptr`, either
// to begin with, by swapping or by stealing.
unsafe { Self::get_mut_unchecked(this) }
}
/// Clones the shared allocation and returns a `HybridRc` pointing to the clone.
#[inline]
fn clone_allocation(&self) -> Self {
let mut result = Self::new_uninit();
let uninit = unsafe { HybridRc::get_mut_unchecked(&mut result) };
uninit.write((*self.data()).clone());
unsafe { result.assume_init() }
}
}
impl<T, State: RcState> HybridRc<mem::MaybeUninit<T>, State> {
/// Assumes the value is initialized and converts to `HybridRc<T, State>`.
///
/// # Safety
///
/// You need to provide the same guarantees as for [`MaybeUninit::assume_init()`].
/// Calling this when the value is not yet fully initialized causes immediate undefined
/// behavior.
///
/// # Examples
///
/// ```
/// use hybrid_rc::Rc;
///
/// let mut reference = Rc::<i64>::new_uninit();
///
/// let reference = unsafe {
/// // Deferred initialization
/// Rc::get_mut_unchecked(&mut reference).as_mut_ptr().write(1337);
/// reference.assume_init()
/// };
///
/// assert_eq!(*reference, 1337)
/// ```
///
/// [`MaybeUninit::assume_init()`]: mem::MaybeUninit::assume_init
#[inline]
pub unsafe fn assume_init(self) -> HybridRc<T, State> {
HybridRc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
}
}
impl<T, State: RcState> HybridRc<[mem::MaybeUninit<T>], State> {
/// Assumes the values are initialized and converts to `HybridRc<[T], State>`.
///
/// # Safety
///
/// You need to provide the same guarantees as for [`MaybeUninit::assume_init()`].
/// Calling this when the whole slice is not yet fully initialized causes immediate undefined
/// behavior.
///
/// [`MaybeUninit::assume_init()`]: mem::MaybeUninit::assume_init
#[inline]
pub unsafe fn assume_init(self) -> HybridRc<[T], State> {
HybridRc::from_inner(unsafe {
mem::ManuallyDrop::new(self)
.ptr
.as_mut()
.assume_init()
.into()
})
}
}
impl<State: RcState> HybridRc<dyn Any, State> {
/// Tries to downcast the `HybridRc<dyn Any, _>` to a concrete type.
///
/// # Errors
/// If a downcast failed, the original `HybridRc` is returned in `Err`
///
/// # Example
///
/// ```
/// use std::any::Any;
/// use std::mem::drop;
/// use hybrid_rc::Rc;
///
/// let value = 42i32;
/// let concrete = Rc::new(value);
/// let any: Rc<dyn Any> = Rc::into(concrete);
///
/// let any = any.downcast::<String>().unwrap_err();
///
/// assert_eq!(*any.downcast::<i32>().unwrap(), 42);
/// ```
#[inline]
pub fn downcast<T: Any>(self) -> Result<HybridRc<T, State>, Self> {
if (*self).is::<T>() {
let ptr = self.ptr.cast::<RcBox<T>>();
mem::forget(self);
Ok(HybridRc::from_inner(ptr))
} else {
Err(self)
}
}
}
impl<State: RcState> HybridRc<dyn Any + Sync + Send, State> {
/// Tries to downcast the `HybridRc<dyn Any + Sync + Send, _>` to a concrete type.
///
/// # Errors
/// If a downcast failed, the original `HybridRc` is returned in `Err`
///
/// # Example
///
/// ```
/// use std::any::Any;
/// use std::mem::drop;
/// use hybrid_rc::Rc;
///
/// let value = 42i32;
/// let concrete = Rc::new(value);
/// let any: Rc<dyn Any + Sync + Send> = Rc::into(concrete);
///
/// let any = any.downcast::<String>().unwrap_err();
///
/// assert_eq!(*any.downcast::<i32>().unwrap(), 42);
/// ```
#[inline]
pub fn downcast<T: Any + Sync + Send>(self) -> Result<HybridRc<T, State>, Self> {
if (*self).is::<T>() {
let ptr = self.ptr.cast::<RcBox<T>>();
mem::forget(self);
Ok(HybridRc::from_inner(ptr))
} else {
Err(self)
}
}
}
impl<T: ?Sized> Clone for HybridRc<T, Local> {
/// Creates another `Rc` for the same value.
///
/// # Example
/// ```
/// use hybrid_rc::Rc;
///
/// let first = Rc::new(42i32);
/// let second = Rc::clone(&first);
///
/// assert_eq!(Rc::as_ptr(&first), Rc::as_ptr(&second));
/// ```
#[inline]
fn clone(&self) -> Self {
self.meta().inc_strong_local();
Self::from_inner(self.ptr)
}
}
impl<T: ?Sized> Clone for HybridRc<T, Shared> {
/// Creates another `Arc` for the same value.
///
/// # Example
/// ```
/// use hybrid_rc::Arc;
/// # fn main() -> Result<(), Box<dyn std::any::Any + Send + 'static>> {
///
/// let first = Arc::new(42i32);
/// let second = Arc::clone(&first);
///
/// assert_eq!(Arc::as_ptr(&first), Arc::as_ptr(&second));
///
/// let value = std::thread::spawn(move || *second)
/// .join()?;
/// assert_eq!(*first, value);
/// # Ok(())
/// # }
/// ```
#[inline]
fn clone(&self) -> Self {
self.meta().inc_strong_shared();
Self::from_inner(self.ptr)
}
}
impl<T: ?Sized, State: RcState> Drop for HybridRc<T, State> {
/// Drops the `HybridRc`.
///
/// This will decrement the appropriate reference count depending on `State`. If both strong
/// reference counts reach zero then the only other references (if any) are [`Weak`]. In that
/// case the inner value is dropped.
#[inline]
fn drop(&mut self) {
let no_more_strong_refs = if State::SHARED {
self.meta().dec_strong_shared()
} else {
self.meta().dec_strong_local()
};
if no_more_strong_refs {
unsafe {
self.drop_contents_and_maybe_box();
}
}
}
}
// Dereferencing traits
impl<T: ?Sized, State: RcState> Deref for HybridRc<T, State> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
self.data()
}
}
impl<T: ?Sized, State: RcState> Borrow<T> for HybridRc<T, State> {
#[inline]
fn borrow(&self) -> &T {
&**self
}
}
impl<T: ?Sized, State: RcState> AsRef<T> for HybridRc<T, State> {
#[inline]
fn as_ref(&self) -> &T {
&**self
}
}
// Safety: T: Sync implies that dereferencing the Arc<T> on multiple threads is sound and T: Send
// implies that dropping T on another thread is sound. So T: Sync + Send gives all guarantees we
// need to make Arc Sync + Send.
unsafe impl<T: ?Sized + Sync + Send> Send for HybridRc<T, Shared> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for HybridRc<T, Shared> {}
// Unwind safety: A HybridRc can only be UnwindSafe if the inner type is RefUnwindSafe.
impl<T: RefUnwindSafe + ?Sized, State: RcState> UnwindSafe for HybridRc<T, State> {}
// Unwind safety: An Arc is always RefUnwindSafe because of its use of atomics.
impl<T: RefUnwindSafe> RefUnwindSafe for HybridRc<T, Shared> {}
// Conversions between different HybridRc variants
impl<T: Any + 'static, State: RcState> From<HybridRc<T, State>>
for HybridRc<dyn Any + 'static, State>
{
/// Upcasts a `HybridRc<T, State>` into a `HybridRc<dyn Any, State>`
#[inline]
fn from(src: HybridRc<T, State>) -> Self {
let ptr = src.ptr.as_ptr() as *mut RcBox<dyn Any>;
mem::forget(src);
Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
}
}
impl<T: Any + Sync + Send + 'static, State: RcState> From<HybridRc<T, State>>
for HybridRc<dyn Any + Sync + Send + 'static, State>
{
/// Upcasts a `HybridRc<T, State>` into a `HybridRc<dyn Any + Sync + Send, State>`
#[inline]
fn from(src: HybridRc<T, State>) -> Self {
let ptr = src.ptr.as_ptr() as *mut RcBox<dyn Any + Sync + Send>;
mem::forget(src);
Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
}
}
impl<T, State: RcState, const N: usize> From<HybridRc<[T; N], State>> for HybridRc<[T], State> {
/// Converts a `HybridRc<[T; N], State>` into a `HybridRc<[T], State>`
///
/// Workaround for coercion as long as `CoerceUnsized` is unstable.
#[inline]
fn from(src: HybridRc<[T; N], State>) -> Self {
let ptr = src.ptr.as_ptr() as *mut RcBox<[T]>;
mem::forget(src);
Self::from_inner(unsafe { NonNull::new_unchecked(ptr) })
}
}
impl<T: ?Sized> From<Rc<T>> for HybridRc<T, Shared> {
/// Converts an `Rc<T>` into an `Arc<T>`.
///
/// See [`to_shared()`].
///
/// [`to_shared()`]: HybridRc::to_shared
#[inline]
fn from(src: Rc<T>) -> Self {
HybridRc::to_shared(&src)
}
}
impl<T: ?Sized> TryFrom<Arc<T>> for HybridRc<T, Local> {
type Error = Arc<T>;
/// Tries to convert an `Arc<T>` into an `Rc<T>`.
///
/// See [`to_local()`].
///
/// [`to_local()`]: HybridRc::to_local
#[inline]
fn try_from(src: Arc<T>) -> Result<Self, Self::Error> {
match HybridRc::to_local(&src) {
Some(result) => Ok(result),
None => Err(src),
}
}
}
impl<T, State: RcState, const N: usize> TryFrom<HybridRc<[T], State>> for HybridRc<[T; N], State> {
type Error = HybridRc<[T], State>;
/// Tries to convert a `HybridRc<[T], State>` into a `HybridRc<[T; N], State>`
///
/// Only succeeds if the length matches exactly.
#[inline]
fn try_from(src: HybridRc<[T], State>) -> Result<Self, Self::Error> {
if src.len() == N {
let ptr = src.ptr.as_ptr().cast();
mem::forget(src);
Ok(Self::from_inner(unsafe { NonNull::new_unchecked(ptr) }))
} else {
Err(src)
}
}
}
// Conversions into HybridRc
impl<T, State: RcState> From<T> for HybridRc<T, State> {
/// Moves a `T` into an `HybridRc<T, State>`
///
/// Equivalent to calling [`HybridRc::new(src)`].
///
/// [`HybridRc::new(t)`]: Self::new
#[inline]
fn from(src: T) -> Self {
Self::new(src)
}
}
impl<T: Clone, State: RcState> From<&[T]> for HybridRc<[T], State> {
/// Allocate a reference-counted slice and clone the elements of `src` into it.
///
/// # Example
///
/// ```
/// use hybrid_rc::Rc;
///
/// let vecs = [
/// vec![1,2,3],
/// vec![4,5,6],
/// ];
/// let rc: Rc<[_]> = Rc::from(&vecs[..]);
/// assert_eq!(&rc[..], &vecs);
/// ```
#[inline]
fn from(src: &[T]) -> Self {
let mut builder = SliceBuilder::new(Self::build_new_meta(), src.len());
for item in src {
builder.append(Clone::clone(item));
}
Self::from_inner(builder.finish().into())
}
}
impl<T, State: RcState> From<Vec<T>> for HybridRc<[T], State> {
/// Allocate a reference-counted slice and move `src`'s items into it.
///
/// # Example
///
/// ```
/// use hybrid_rc::Rc;
///
/// let vec = vec!["a","b","c"];
/// let rc: Rc<[_]> = Rc::from(vec);
/// assert_eq!(&rc[..], &["a", "b", "c"]);
/// ```
#[inline]
fn from(mut src: Vec<T>) -> Self {
unsafe {
let result = HybridRc::<_, State>::copy_from_slice_unchecked(&src[..]);
// Set the length of `src`, so that the moved items are not dropped.
src.set_len(0);
result
}
}
}
impl<State: RcState> From<&str> for HybridRc<str, State> {
/// Allocate a reference-counted `str` and copy `src` into it.
///
/// # Example
///
/// ```
/// use hybrid_rc::Rc;
///
/// let a: Rc<str> = Rc::from("foobar");
/// assert_eq!(&a[..], "foobar");
/// ```
#[inline]
fn from(src: &str) -> Self {
let bytes = HybridRc::<_, State>::copy_from_slice(src.as_bytes());
let inner = unsafe { (bytes.ptr.as_ptr() as *mut _ as *mut RcBox<str>).as_mut() }.unwrap();
mem::forget(bytes);
Self::from_inner(inner.into())
}
}
impl<State: RcState> From<String> for HybridRc<str, State> {
/// Allocate a reference-counted `str` and copy `src` into it.
///
/// # Example
///
/// ```
/// use hybrid_rc::Rc;
///
/// let string: String = "foobar".to_owned();
/// let a: Rc<str> = Rc::from(string);
/// assert_eq!(&a[..], "foobar");
/// ```
#[inline]
fn from(src: String) -> Self {
Self::from(&src[..])
}
}
impl<'a, T: ToOwned + ?Sized, State: RcState> From<Cow<'a, T>> for HybridRc<T, State>
where
HybridRc<T, State>: From<&'a T> + From<T::Owned>,
{
/// Creates a new `HybridRc<T, State>` from a clone-on-write pointer by copying its content.
///
/// # Example
///
/// ```rust
/// use hybrid_rc::Rc;
/// use std::borrow::Cow;
///
/// let cow: Cow<str> = Cow::Borrowed("foobar");
/// let a: Rc<str> = Rc::from(cow);
/// assert_eq!(&a[..], "foobar");
/// ```
#[inline]
fn from(src: Cow<'a, T>) -> HybridRc<T, State> {
match src {
Cow::Borrowed(value) => HybridRc::from(value),
Cow::Owned(value) => HybridRc::from(value),
}
}
}
impl<T: ?Sized, State: RcState> From<Box<T>> for HybridRc<T, State> {
#[inline]
fn from(src: Box<T>) -> HybridRc<T, State> {
let len = mem::size_of_val(&*src);
let inner = RcBox::allocate_for_val(Self::build_new_meta(), &*src, false);
let dest = unsafe { ptr::addr_of_mut!((*inner.as_ptr()).data) }.cast();
// Safety: The freshly allocated `RcBox` can't alias `src` and the payload can be fully
// moved by copying the memory, because it's not Pin<Box<T>>. `allocate_for_val` ensures
// the destination payload buffer is big enough for the value.
unsafe {
(&*src as *const T)
.cast::<u8>()
.copy_to_nonoverlapping(dest, len);
}
// Drop original box without running the destructor
// Safety: This *should* be sound, as ManuallyDrop<T> has the same layout as T.
mem::drop(unsafe { mem::transmute::<Box<T>, Box<mem::ManuallyDrop<T>>>(src) });
HybridRc::from_inner(inner)
}
}
impl<T, State: RcState> iter::FromIterator<T> for HybridRc<[T], State> {
/// Takes each element in the `Iterator` and collects it into an `HybridRc<[T], State>`.
///
/// # Performance characteristics
///
/// Collecion is done by first collecting into a `Vec<T>`.
///
/// This will allocate as many times as needed for constructing the `Vec<T>`
/// and then it will allocate once for turning the `Vec<T>` into the `HybridRc<[T], State>`.
///
/// Once specialization is stablilized this will be optimized for [`TrustedLen`] iterators.
///
/// [`TrustedLen`]: core::iter::TrustedLen
fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
let vec: Vec<T> = iter.into_iter().collect();
vec.into()
}
}
// Propagate some useful traits implemented by the inner type
impl<T: Default, State: RcState> Default for HybridRc<T, State> {
/// Creates a new `HybridRc`, with the `Default` value for `T`.
#[inline]
fn default() -> Self {
Self::new(Default::default())
}
}
impl<T: ?Sized + PartialEq, S1: RcState, S2: RcState> PartialEq<HybridRc<T, S2>>
for HybridRc<T, S1>
{
/// Equality for `HybridRc`s.
///
/// Two `HybridRc`s are equal if their inner values are equal, independent of if they are
/// stored in the same allocation.
#[inline]
fn eq(&self, other: &HybridRc<T, S2>) -> bool {
**self == **other
}
}
impl<T: ?Sized + Eq, State: RcState> Eq for HybridRc<T, State> {}
impl<T: ?Sized + Hash, State: RcState> Hash for HybridRc<T, State> {
#[inline]
fn hash<H: Hasher>(&self, state: &mut H) {
Self::data(self).hash(state);
}
}
impl<T: ?Sized + PartialOrd, S1: RcState, S2: RcState> PartialOrd<HybridRc<T, S2>>
for HybridRc<T, S1>
{
/// Partial comparison for `HybridRc`s.
///
/// The two are compared by calling `partial_cmp()` on their inner values.
#[inline]
fn partial_cmp(&self, other: &HybridRc<T, S2>) -> Option<cmp::Ordering> {
(**self).partial_cmp(&**other)
}
}
impl<T: ?Sized + Ord, State: RcState> Ord for HybridRc<T, State> {
/// Comparison for `HybridRc`s.
///
/// The two are compared by calling `cmp()` on their inner values.
#[inline]
fn cmp(&self, other: &Self) -> cmp::Ordering {
(**self).cmp(&**other)
}
}
impl<T: ?Sized + fmt::Display, State: RcState> fmt::Display for HybridRc<T, State> {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&Self::data(self), f)
}
}
impl<T: ?Sized + fmt::Debug, State: RcState> fmt::Debug for HybridRc<T, State> {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&Self::data(self), f)
}
}
// `HybridRc` can be formatted as a pointer.
impl<T: ?Sized, State: RcState> fmt::Pointer for HybridRc<T, State> {
/// Formats the value using the given formatter.
///
/// If the `#` flag is used, the state (shared/local) is written after the address.
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if f.alternate() {
fmt::Pointer::fmt(&Self::as_ptr(self), f)?;
f.write_str(if State::SHARED {
" [shared]"
} else {
" [local]"
})
} else {
fmt::Pointer::fmt(&Self::as_ptr(self), f)
}
}
}
/// `HybridRc<T>` is always `Unpin` itself, because the data value is on the heap,
/// so moving `HybridRc<T>` doesn't move the content even if `T` is not `Unpin`.
///
/// This allows unpinning e.g. `Pin<Box<HybridRc<T>>>` but not any `Pin<HybridRc<T>>`!
impl<T: ?Sized, State: RcState> Unpin for HybridRc<T, State> {}
/// `Weak<T>` represents a non-owning reference to a value managed by a [`HybridRc<T, _>`].
/// The value is accessed by calling [`upgrade()`] or [`upgrade_local()`] on `Weak`.
///
/// `Weak` references are typically used to prevent circular references that would keep
/// the shared value alive indefinitely.
///
/// The typical way to obtain a `Weak<T>` is to call [`HybridRc::downgrade()`].
///
/// [`upgrade()`]: Weak::upgrade
/// [`upgrade_local()`]: Weak::upgrade_local
#[must_use]
pub struct Weak<T: ?Sized> {
ptr: NonNull<RcBox<T>>,
}
impl<T: ?Sized> Weak<T> {
/// Accesses the metadata area of the shared allocation.
///
/// `None` for instances created through `Weak::new()`.
#[inline]
fn meta(&self) -> Option<&RcMeta> {
if is_senitel(self.ptr.as_ptr()) {
None
} else {
// Safety: as long as one Rc or Weak
// for this item exists, the memory stays
// allocated.
Some(unsafe { &(*self.ptr.as_ptr()).meta })
}
}
/// Returns a raw pointer to the value referenced by this `Weak<T>`.
///
/// The pointer is valid only if there are some strong references. It may be dangling,
/// unaligned or even null otherwise.
///
/// # Example
/// ```
/// use hybrid_rc::Rc;
///
/// let strong = Rc::new(42i32);
/// let weak = Rc::downgrade(&strong);
/// {
/// let pointer = weak.as_ptr();
/// // As long as strong is not dropped, the pointer stays valid
/// assert_eq!(42, unsafe { *pointer });
/// }
/// drop(strong);
/// {
/// // Calling weak.as_ptr() is still safe, but dereferencing it would lead
/// // to undefined behaviour.
/// let pointer = weak.as_ptr();
/// // assert_eq!(42, unsafe { &*pointer }); // undefined behaviour
/// }
#[must_use]
#[inline]
pub fn as_ptr(&self) -> *const T {
let ptr: *mut RcBox<T> = self.ptr.as_ptr();
if is_senitel(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 ArcInner (usize).
ptr as *const T
} else {
// Safety: raw pointer manipulation like in sync::Weak, as the payload may have been
// dropped at this point and to keep provenance.
unsafe { ptr::addr_of_mut!((*ptr).data) }
}
}
/// Attempts to upgrade the Weak pointer to an [`Rc`].
///
/// **Note:** Only one thread can have `Rc`s for a value at any point in time.
/// See [`upgrade()`] to upgrade to an [`Arc`].
///
/// In `no_std` environments this will only succeed if no `Rc` exists on *any* thread.
///
/// # Errors
/// - [`ValueDropped`]: the referenced value has already been dropped.
/// - [`WrongThread`]: another thread currently holds `Rc`s for the value.
///
/// # Example
/// ```
/// use hybrid_rc::{Arc, Rc, Weak, UpgradeError};
/// # fn main() -> Result<(), UpgradeError> {
/// let strong = Arc::new(42i32);
/// let weak = Arc::downgrade(&strong);
///
/// {
/// let strong2 = weak.upgrade_local()?;
/// assert_eq!(Arc::as_ptr(&strong), Rc::as_ptr(&strong2));
/// }
///
/// std::mem::drop(strong);
///
/// let error = Weak::upgrade_local(&weak).unwrap_err();
/// assert_eq!(error, UpgradeError::ValueDropped);
/// # Ok(())
/// # }
/// ```
///
/// [`upgrade()`]: Weak::upgrade
/// [`ValueDropped`]: UpgradeError::ValueDropped
/// [`WrongThread`]: UpgradeError::WrongThread
#[inline]
pub fn upgrade_local(&self) -> Result<Rc<T>, UpgradeError> {
let meta = self.meta().ok_or(UpgradeError::ValueDropped)?;
let current_thread = ThreadId::current_thread();
let owner = match meta.owner.store_if_none(
Some(current_thread),
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => None,
Err(owner) => owner,
};
if owner == None || owner == Some(current_thread) {
if meta.try_inc_strong_local().is_ok() {
Ok(HybridRc::<T, Local>::from_inner(self.ptr))
} else {
// Relaxed is enough, as try_inc_strong_local failing means that
// the value was already dropped.
meta.owner.store(None, Ordering::Relaxed);
Err(UpgradeError::ValueDropped)
}
} else {
Err(UpgradeError::WrongThread)
}
}
/// Attempts to upgrade the Weak pointer to an [`Arc`].
///
/// Also see [`upgrade_local()`] to upgrade to an [`Rc`].
///
/// # Errors
/// - [`ValueDropped`]: the referenced value has already been dropped.
///
/// [`upgrade_local()`]: Weak::upgrade_local
/// [`ValueDropped`]: UpgradeError::ValueDropped
#[inline]
pub fn upgrade(&self) -> Result<Arc<T>, UpgradeError> {
let meta = self.meta().ok_or(UpgradeError::ValueDropped)?;
meta.try_inc_strong_shared()
.map_err(|_| UpgradeError::ValueDropped)?;
Ok(HybridRc::<T, Shared>::from_inner(self.ptr))
}
/// Gets a lower bound to the number of strong pointers to the inner value.
///
/// If `self` was created using [`Weak::new`], this will return 0.
///
/// Please understand that another thread might change the count at any time, including
/// potentially between calling this method and acting on the result.
///
/// # Examples
///
/// ```
/// use hybrid_rc::{Arc, Rc, Weak};
///
/// let reference = Rc::new(42);
/// let _2nd_ref = Rc::clone(&reference);
/// let shared_ref = Rc::to_shared(&reference);
/// let _2nd_shared_ref = Arc::clone(&shared_ref);
/// let weak = Rc::downgrade(&reference);
///
/// // shared_ref only knows the count of shared references and that there is at least one
/// // local reference, so it will show 3 instead of 4:
/// assert_eq!(Weak::strong_count(&weak), 3);
/// ```
#[inline]
pub fn strong_count(&self) -> usize {
if let Some(meta) = self.meta() {
meta.strong_shared.load(Ordering::SeqCst)
} else {
0
}
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// Please understand that another thread may change the count at any time, including
/// potentially between calling this method and acting on the result. Also there might by
/// off-by-one errors when other threads concurrently upgrade or downgrade pointers.
///
/// # Examples
///
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let reference = Rc::new(42);
/// let weak = Rc::downgrade(&reference);
/// let _weak_2 = weak.clone();
///
/// assert_eq!(Weak::weak_count(&weak), 2);
/// ```
#[inline]
pub fn weak_count(&self) -> usize {
if let Some(meta) = self.meta() {
let weak = meta.weak.load(Ordering::SeqCst);
if weak == usize::MAX {
0
} else if meta.strong_shared.load(Ordering::SeqCst) > 0 {
weak - 1
} else {
weak
}
} else {
0
}
}
}
impl<T> Weak<T> {
/// Constructs a dummy `Weak<T>`, without allocating any memory.
///
/// Trying to upgrade the result will always result in a [`ValueDropped`] error.
///
/// [`ValueDropped`]: UpgradeError::ValueDropped
pub fn new() -> Weak<T> {
Self { ptr: senitel() }
}
}
impl<T: ?Sized> fmt::Debug for Weak<T> {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "(Weak)")
}
}
impl<T: ?Sized> fmt::Pointer for Weak<T> {
/// Formats the value using the given formatter.
///
/// If the `#` flag is used, the state (weak) is written after the address.
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if f.alternate() {
fmt::Pointer::fmt(&Self::as_ptr(self), f)?;
f.write_str(" [weak]")
} else {
fmt::Pointer::fmt(&Self::as_ptr(self), f)
}
}
}
impl<T> Default for Weak<T> {
/// Constructs a dummy `Weak<T>`, without allocating any memory.
///
/// See [`Weak<T>::new()`].
#[inline]
fn default() -> Self {
Self::new()
}
}
impl<T: ?Sized> Clone for Weak<T> {
/// Creates another `Weak` reference for the same value.
///
/// # Example
/// ```
/// use hybrid_rc::{Rc, Weak};
///
/// let strong = Rc::new(42i32);
/// let weak = Rc::downgrade(&strong);
/// let weak2 = Weak::clone(&weak);
///
/// assert_eq!(weak.as_ptr(), weak2.as_ptr());
/// ```
#[inline]
fn clone(&self) -> Self {
if let Some(meta) = self.meta() {
// We can ignore the lock in Weak::clone() as the counter is only locked by HybridRc
// when there are no Weak instances/ (meta.weak == 1).
meta.inc_weak_nolock();
}
Self { ptr: self.ptr }
}
}
impl<T: ?Sized> Drop for Weak<T> {
/// Drops the `Weak` reference.
///
/// Once all `HybridRc` and `Weak` references to a shared value are dropped, the shared
/// allocation is fully released.
#[inline]
fn drop(&mut self) {
if let Some(meta) = self.meta() {
let last_reference = meta.dec_weak();
if last_reference {
unsafe {
// Safety: only called if this was the last (weak) reference
RcBox::dealloc(self.ptr);
}
}
}
}
}
// Safety: Like for Arc<T> T: Send + Sync gives all guarantees we need to make Weak Send + Sync.
unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
/// `PinWeak<T>` represents a non-owning reference to a pinned value managed by a
/// [`Pin`]`<`[`HybridRc<T, _>`]`>`.
///
/// The typical way to obtain a `PinWeak<T>` is to call [`HybridRc::downgrade_pin()`].
///
/// See [`Weak<T>`] for more information about weak references.
///
/// [`upgrade()`]: PinWeak::upgrade
/// [`upgrade_local()`]: PinWeak::upgrade_local
#[repr(transparent)]
pub struct PinWeak<T: ?Sized>(Weak<T>);
impl<T: ?Sized> PinWeak<T> {
/// Attempts to upgrade the pinned weak pointer to a pinned [`Rc`].
///
/// See [`Weak::upgrade_local()`] for more information.
///
/// # Errors
/// - [`ValueDropped`]: the referenced value has already been dropped.
/// - [`WrongThread`]: another thread currently holds `Rc`s for the value.
///
/// [`ValueDropped`]: UpgradeError::ValueDropped
/// [`WrongThread`]: UpgradeError::WrongThread
#[inline]
pub fn upgrade_local(&self) -> Result<Pin<Rc<T>>, UpgradeError> {
Ok(unsafe { Pin::new_unchecked(self.0.upgrade_local()?) })
}
/// Attempts to upgrade the pinned weak pointer to a pinned [`Arc`].
///
/// See [`Weak::upgrade()`] for more information.
///
/// # Errors
/// - [`ValueDropped`]: the referenced value has already been dropped.
///
/// [`ValueDropped`]: UpgradeError::ValueDropped
#[inline]
pub fn upgrade(&self) -> Result<Pin<Arc<T>>, UpgradeError> {
Ok(unsafe { Pin::new_unchecked(self.0.upgrade()?) })
}
/// Gets a lower bound to the number of strong pointers to the inner value.
///
/// See [`Weak::strong_count()`] for more information.
#[inline]
pub fn strong_count(&self) -> usize {
self.0.strong_count()
}
/// Gets the number of [`Weak`] pointers to this allocation.
///
/// See [`Weak::strong_count()`] for more information.
#[inline]
pub fn weak_count(&self) -> usize {
self.0.weak_count()
}
/// Transforms this `PinWeak<T>` into a [`Weak<T>`]
///
/// # Safety
/// This function is unsafe. You must guarantee that you will continue to treat the `Weak` as
/// pinned after you call this function. Not maintaining the pinning invariants that is a
/// violation of the API contract and may lead to undefined behavior in later (safe) operations.
///
/// If the underlying data is [`Unpin`], [`PinWeak::into_inner()`] should be used instead.
#[inline]
pub unsafe fn into_inner_unchecked(self) -> Weak<T> {
self.0
}
}
impl<T> PinWeak<T> {
/// Constructs a dummy `PinWeak<T>`, without allocating any memory.
///
/// Trying to upgrade the result will always result in a [`ValueDropped`] error.
///
/// [`ValueDropped`]: UpgradeError::ValueDropped
pub fn new() -> PinWeak<T> {
Self(Weak::new())
}
}
impl<T: ?Sized + Unpin> PinWeak<T> {
/// Transforms this `PinWeak<T>` into a [`Weak<T>`]
///
/// This requires that the data inside the shared allocation is [`Unpin`], so that we
/// can ignore the pinning invariants when unwrapping it.
#[inline]
pub fn into_inner(self) -> Weak<T> {
self.0
}
}
impl<T: ?Sized> fmt::Debug for PinWeak<T> {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "Pin<(Weak)>")
}
}
impl<T: ?Sized> fmt::Pointer for PinWeak<T> {
/// Formats the value using the given formatter.
///
/// If the `#` flag is used, the state (weak) is written after the address.
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if f.alternate() {
fmt::Pointer::fmt(&self.0.as_ptr(), f)?;
f.write_str(" [weak]")
} else {
fmt::Pointer::fmt(&self.0.as_ptr(), f)
}
}
}
impl<T: ?Sized> Clone for PinWeak<T> {
/// Creates another pinned weak reference for the same value.
///
/// See [`Weak::clone()`] for more information.
#[inline]
fn clone(&self) -> Self {
Self(self.0.clone())
}
}
impl<T> Default for PinWeak<T> {
/// Constructs a dummy `PinWeak<T>`, without allocating any memory.
///
/// See [`PinWeak<T>::new()`].
#[inline]
fn default() -> Self {
Self::new()
}
}
// Safety: Like for Weak<T> T: Send + Sync gives all guarantees we need to make PinWeak Send + Sync.
unsafe impl<T: ?Sized + Sync + Send> Send for PinWeak<T> {}
unsafe impl<T: ?Sized + Sync + Send> Sync for PinWeak<T> {}