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1// SPDX-License-Identifier: GPL-2.0
2
3//! Kernel types.
4
5use crate::init::{self, PinInit};
6use alloc::boxed::Box;
7use core::{
8 cell::UnsafeCell,
9 marker::{PhantomData, PhantomPinned},
10 mem::MaybeUninit,
11 ops::{Deref, DerefMut},
12 ptr::NonNull,
13};
14
15/// Used to transfer ownership to and from foreign (non-Rust) languages.
16///
17/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
18/// later may be transferred back to Rust by calling [`Self::from_foreign`].
19///
20/// This trait is meant to be used in cases when Rust objects are stored in C objects and
21/// eventually "freed" back to Rust.
22pub trait ForeignOwnable: Sized {
23 /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
24 /// [`ForeignOwnable::from_foreign`].
25 type Borrowed<'a>;
26
27 /// Converts a Rust-owned object to a foreign-owned one.
28 ///
29 /// The foreign representation is a pointer to void.
30 fn into_foreign(self) -> *const core::ffi::c_void;
31
32 /// Borrows a foreign-owned object.
33 ///
34 /// # Safety
35 ///
36 /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
37 /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
38 unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
39
40 /// Converts a foreign-owned object back to a Rust-owned one.
41 ///
42 /// # Safety
43 ///
44 /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
45 /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
46 /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
47 /// this object must have been dropped.
48 unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
49}
50
51impl<T: 'static> ForeignOwnable for Box<T> {
52 type Borrowed<'a> = &'a T;
53
54 fn into_foreign(self) -> *const core::ffi::c_void {
55 Box::into_raw(self) as _
56 }
57
58 unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
59 // SAFETY: The safety requirements for this function ensure that the object is still alive,
60 // so it is safe to dereference the raw pointer.
61 // The safety requirements of `from_foreign` also ensure that the object remains alive for
62 // the lifetime of the returned value.
63 unsafe { &*ptr.cast() }
64 }
65
66 unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
67 // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
68 // call to `Self::into_foreign`.
69 unsafe { Box::from_raw(ptr as _) }
70 }
71}
72
73impl ForeignOwnable for () {
74 type Borrowed<'a> = ();
75
76 fn into_foreign(self) -> *const core::ffi::c_void {
77 core::ptr::NonNull::dangling().as_ptr()
78 }
79
80 unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}
81
82 unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
83}
84
85/// Runs a cleanup function/closure when dropped.
86///
87/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
88///
89/// # Examples
90///
91/// In the example below, we have multiple exit paths and we want to log regardless of which one is
92/// taken:
93/// ```
94/// # use kernel::types::ScopeGuard;
95/// fn example1(arg: bool) {
96/// let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
97///
98/// if arg {
99/// return;
100/// }
101///
102/// pr_info!("Do something...\n");
103/// }
104///
105/// # example1(false);
106/// # example1(true);
107/// ```
108///
109/// In the example below, we want to log the same message on all early exits but a different one on
110/// the main exit path:
111/// ```
112/// # use kernel::types::ScopeGuard;
113/// fn example2(arg: bool) {
114/// let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
115///
116/// if arg {
117/// return;
118/// }
119///
120/// // (Other early returns...)
121///
122/// log.dismiss();
123/// pr_info!("example2 no early return\n");
124/// }
125///
126/// # example2(false);
127/// # example2(true);
128/// ```
129///
130/// In the example below, we need a mutable object (the vector) to be accessible within the log
131/// function, so we wrap it in the [`ScopeGuard`]:
132/// ```
133/// # use kernel::types::ScopeGuard;
134/// fn example3(arg: bool) -> Result {
135/// let mut vec =
136/// ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len()));
137///
138/// vec.try_push(10u8)?;
139/// if arg {
140/// return Ok(());
141/// }
142/// vec.try_push(20u8)?;
143/// Ok(())
144/// }
145///
146/// # assert_eq!(example3(false), Ok(()));
147/// # assert_eq!(example3(true), Ok(()));
148/// ```
149///
150/// # Invariants
151///
152/// The value stored in the struct is nearly always `Some(_)`, except between
153/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
154/// will have been returned to the caller. Since [`ScopeGuard::dismiss`] consumes the guard,
155/// callers won't be able to use it anymore.
156pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);
157
158impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
159 /// Creates a new guarded object wrapping the given data and with the given cleanup function.
160 pub fn new_with_data(data: T, cleanup_func: F) -> Self {
161 // INVARIANT: The struct is being initialised with `Some(_)`.
162 Self(Some((data, cleanup_func)))
163 }
164
165 /// Prevents the cleanup function from running and returns the guarded data.
166 pub fn dismiss(mut self) -> T {
167 // INVARIANT: This is the exception case in the invariant; it is not visible to callers
168 // because this function consumes `self`.
169 self.0.take().unwrap().0
170 }
171}
172
173impl ScopeGuard<(), fn(())> {
174 /// Creates a new guarded object with the given cleanup function.
175 pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
176 ScopeGuard::new_with_data((), move |_| cleanup())
177 }
178}
179
180impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
181 type Target = T;
182
183 fn deref(&self) -> &T {
184 // The type invariants guarantee that `unwrap` will succeed.
185 &self.0.as_ref().unwrap().0
186 }
187}
188
189impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
190 fn deref_mut(&mut self) -> &mut T {
191 // The type invariants guarantee that `unwrap` will succeed.
192 &mut self.0.as_mut().unwrap().0
193 }
194}
195
196impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
197 fn drop(&mut self) {
198 // Run the cleanup function if one is still present.
199 if let Some((data, cleanup)) = self.0.take() {
200 cleanup(data)
201 }
202 }
203}
204
205/// Stores an opaque value.
206///
207/// This is meant to be used with FFI objects that are never interpreted by Rust code.
208#[repr(transparent)]
209pub struct Opaque<T> {
210 value: UnsafeCell<MaybeUninit<T>>,
211 _pin: PhantomPinned,
212}
213
214impl<T> Opaque<T> {
215 /// Creates a new opaque value.
216 pub const fn new(value: T) -> Self {
217 Self {
218 value: UnsafeCell::new(MaybeUninit::new(value)),
219 _pin: PhantomPinned,
220 }
221 }
222
223 /// Creates an uninitialised value.
224 pub const fn uninit() -> Self {
225 Self {
226 value: UnsafeCell::new(MaybeUninit::uninit()),
227 _pin: PhantomPinned,
228 }
229 }
230
231 /// Creates a pin-initializer from the given initializer closure.
232 ///
233 /// The returned initializer calls the given closure with the pointer to the inner `T` of this
234 /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
235 ///
236 /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
237 /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
238 /// to verify at that point that the inner value is valid.
239 pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
240 // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
241 // initialize the `T`.
242 unsafe {
243 init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| {
244 init_func(Self::raw_get(slot));
245 Ok(())
246 })
247 }
248 }
249
250 /// Returns a raw pointer to the opaque data.
251 pub fn get(&self) -> *mut T {
252 UnsafeCell::get(&self.value).cast::<T>()
253 }
254
255 /// Gets the value behind `this`.
256 ///
257 /// This function is useful to get access to the value without creating intermediate
258 /// references.
259 pub const fn raw_get(this: *const Self) -> *mut T {
260 UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
261 }
262}
263
264/// Types that are _always_ reference counted.
265///
266/// It allows such types to define their own custom ref increment and decrement functions.
267/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
268/// [`ARef<T>`].
269///
270/// This is usually implemented by wrappers to existing structures on the C side of the code. For
271/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
272/// instances of a type.
273///
274/// # Safety
275///
276/// Implementers must ensure that increments to the reference count keep the object alive in memory
277/// at least until matching decrements are performed.
278///
279/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
280/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
281/// alive.)
282pub unsafe trait AlwaysRefCounted {
283 /// Increments the reference count on the object.
284 fn inc_ref(&self);
285
286 /// Decrements the reference count on the object.
287 ///
288 /// Frees the object when the count reaches zero.
289 ///
290 /// # Safety
291 ///
292 /// Callers must ensure that there was a previous matching increment to the reference count,
293 /// and that the object is no longer used after its reference count is decremented (as it may
294 /// result in the object being freed), unless the caller owns another increment on the refcount
295 /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
296 /// [`AlwaysRefCounted::dec_ref`] once).
297 unsafe fn dec_ref(obj: NonNull<Self>);
298}
299
300/// An owned reference to an always-reference-counted object.
301///
302/// The object's reference count is automatically decremented when an instance of [`ARef`] is
303/// dropped. It is also automatically incremented when a new instance is created via
304/// [`ARef::clone`].
305///
306/// # Invariants
307///
308/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
309/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
310pub struct ARef<T: AlwaysRefCounted> {
311 ptr: NonNull<T>,
312 _p: PhantomData<T>,
313}
314
315// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
316// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
317// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
318// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
319unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
320
321// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
322// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
323// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
324// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
325// example, when the reference count reaches zero and `T` is dropped.
326unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
327
328impl<T: AlwaysRefCounted> ARef<T> {
329 /// Creates a new instance of [`ARef`].
330 ///
331 /// It takes over an increment of the reference count on the underlying object.
332 ///
333 /// # Safety
334 ///
335 /// Callers must ensure that the reference count was incremented at least once, and that they
336 /// are properly relinquishing one increment. That is, if there is only one increment, callers
337 /// must not use the underlying object anymore -- it is only safe to do so via the newly
338 /// created [`ARef`].
339 pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
340 // INVARIANT: The safety requirements guarantee that the new instance now owns the
341 // increment on the refcount.
342 Self {
343 ptr,
344 _p: PhantomData,
345 }
346 }
347}
348
349impl<T: AlwaysRefCounted> Clone for ARef<T> {
350 fn clone(&self) -> Self {
351 self.inc_ref();
352 // SAFETY: We just incremented the refcount above.
353 unsafe { Self::from_raw(self.ptr) }
354 }
355}
356
357impl<T: AlwaysRefCounted> Deref for ARef<T> {
358 type Target = T;
359
360 fn deref(&self) -> &Self::Target {
361 // SAFETY: The type invariants guarantee that the object is valid.
362 unsafe { self.ptr.as_ref() }
363 }
364}
365
366impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
367 fn from(b: &T) -> Self {
368 b.inc_ref();
369 // SAFETY: We just incremented the refcount above.
370 unsafe { Self::from_raw(NonNull::from(b)) }
371 }
372}
373
374impl<T: AlwaysRefCounted> Drop for ARef<T> {
375 fn drop(&mut self) {
376 // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
377 // decrement.
378 unsafe { T::dec_ref(self.ptr) };
379 }
380}
381
382/// A sum type that always holds either a value of type `L` or `R`.
383pub enum Either<L, R> {
384 /// Constructs an instance of [`Either`] containing a value of type `L`.
385 Left(L),
386
387 /// Constructs an instance of [`Either`] containing a value of type `R`.
388 Right(R),
389}
1// SPDX-License-Identifier: GPL-2.0
2
3//! Kernel types.
4
5use crate::init::{self, PinInit};
6use alloc::boxed::Box;
7use core::{
8 cell::UnsafeCell,
9 marker::{PhantomData, PhantomPinned},
10 mem::MaybeUninit,
11 ops::{Deref, DerefMut},
12 ptr::NonNull,
13};
14
15/// Used to transfer ownership to and from foreign (non-Rust) languages.
16///
17/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
18/// later may be transferred back to Rust by calling [`Self::from_foreign`].
19///
20/// This trait is meant to be used in cases when Rust objects are stored in C objects and
21/// eventually "freed" back to Rust.
22pub trait ForeignOwnable: Sized {
23 /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
24 /// [`ForeignOwnable::from_foreign`].
25 type Borrowed<'a>;
26
27 /// Converts a Rust-owned object to a foreign-owned one.
28 ///
29 /// The foreign representation is a pointer to void.
30 fn into_foreign(self) -> *const core::ffi::c_void;
31
32 /// Borrows a foreign-owned object.
33 ///
34 /// # Safety
35 ///
36 /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
37 /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
38 unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
39
40 /// Converts a foreign-owned object back to a Rust-owned one.
41 ///
42 /// # Safety
43 ///
44 /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
45 /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
46 /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for
47 /// this object must have been dropped.
48 unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
49
50 /// Tries to convert a foreign-owned object back to a Rust-owned one.
51 ///
52 /// A convenience wrapper over [`ForeignOwnable::from_foreign`] that returns [`None`] if `ptr`
53 /// is null.
54 ///
55 /// # Safety
56 ///
57 /// `ptr` must either be null or satisfy the safety requirements for
58 /// [`ForeignOwnable::from_foreign`].
59 unsafe fn try_from_foreign(ptr: *const core::ffi::c_void) -> Option<Self> {
60 if ptr.is_null() {
61 None
62 } else {
63 // SAFETY: Since `ptr` is not null here, then `ptr` satisfies the safety requirements
64 // of `from_foreign` given the safety requirements of this function.
65 unsafe { Some(Self::from_foreign(ptr)) }
66 }
67 }
68}
69
70impl<T: 'static> ForeignOwnable for Box<T> {
71 type Borrowed<'a> = &'a T;
72
73 fn into_foreign(self) -> *const core::ffi::c_void {
74 Box::into_raw(self) as _
75 }
76
77 unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
78 // SAFETY: The safety requirements for this function ensure that the object is still alive,
79 // so it is safe to dereference the raw pointer.
80 // The safety requirements of `from_foreign` also ensure that the object remains alive for
81 // the lifetime of the returned value.
82 unsafe { &*ptr.cast() }
83 }
84
85 unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
86 // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
87 // call to `Self::into_foreign`.
88 unsafe { Box::from_raw(ptr as _) }
89 }
90}
91
92impl ForeignOwnable for () {
93 type Borrowed<'a> = ();
94
95 fn into_foreign(self) -> *const core::ffi::c_void {
96 core::ptr::NonNull::dangling().as_ptr()
97 }
98
99 unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {}
100
101 unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {}
102}
103
104/// Runs a cleanup function/closure when dropped.
105///
106/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
107///
108/// # Examples
109///
110/// In the example below, we have multiple exit paths and we want to log regardless of which one is
111/// taken:
112///
113/// ```
114/// # use kernel::types::ScopeGuard;
115/// fn example1(arg: bool) {
116/// let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
117///
118/// if arg {
119/// return;
120/// }
121///
122/// pr_info!("Do something...\n");
123/// }
124///
125/// # example1(false);
126/// # example1(true);
127/// ```
128///
129/// In the example below, we want to log the same message on all early exits but a different one on
130/// the main exit path:
131///
132/// ```
133/// # use kernel::types::ScopeGuard;
134/// fn example2(arg: bool) {
135/// let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
136///
137/// if arg {
138/// return;
139/// }
140///
141/// // (Other early returns...)
142///
143/// log.dismiss();
144/// pr_info!("example2 no early return\n");
145/// }
146///
147/// # example2(false);
148/// # example2(true);
149/// ```
150///
151/// In the example below, we need a mutable object (the vector) to be accessible within the log
152/// function, so we wrap it in the [`ScopeGuard`]:
153///
154/// ```
155/// # use kernel::types::ScopeGuard;
156/// fn example3(arg: bool) -> Result {
157/// let mut vec =
158/// ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len()));
159///
160/// vec.try_push(10u8)?;
161/// if arg {
162/// return Ok(());
163/// }
164/// vec.try_push(20u8)?;
165/// Ok(())
166/// }
167///
168/// # assert_eq!(example3(false), Ok(()));
169/// # assert_eq!(example3(true), Ok(()));
170/// ```
171///
172/// # Invariants
173///
174/// The value stored in the struct is nearly always `Some(_)`, except between
175/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
176/// will have been returned to the caller. Since [`ScopeGuard::dismiss`] consumes the guard,
177/// callers won't be able to use it anymore.
178pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);
179
180impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
181 /// Creates a new guarded object wrapping the given data and with the given cleanup function.
182 pub fn new_with_data(data: T, cleanup_func: F) -> Self {
183 // INVARIANT: The struct is being initialised with `Some(_)`.
184 Self(Some((data, cleanup_func)))
185 }
186
187 /// Prevents the cleanup function from running and returns the guarded data.
188 pub fn dismiss(mut self) -> T {
189 // INVARIANT: This is the exception case in the invariant; it is not visible to callers
190 // because this function consumes `self`.
191 self.0.take().unwrap().0
192 }
193}
194
195impl ScopeGuard<(), fn(())> {
196 /// Creates a new guarded object with the given cleanup function.
197 pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
198 ScopeGuard::new_with_data((), move |_| cleanup())
199 }
200}
201
202impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
203 type Target = T;
204
205 fn deref(&self) -> &T {
206 // The type invariants guarantee that `unwrap` will succeed.
207 &self.0.as_ref().unwrap().0
208 }
209}
210
211impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
212 fn deref_mut(&mut self) -> &mut T {
213 // The type invariants guarantee that `unwrap` will succeed.
214 &mut self.0.as_mut().unwrap().0
215 }
216}
217
218impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
219 fn drop(&mut self) {
220 // Run the cleanup function if one is still present.
221 if let Some((data, cleanup)) = self.0.take() {
222 cleanup(data)
223 }
224 }
225}
226
227/// Stores an opaque value.
228///
229/// This is meant to be used with FFI objects that are never interpreted by Rust code.
230#[repr(transparent)]
231pub struct Opaque<T> {
232 value: UnsafeCell<MaybeUninit<T>>,
233 _pin: PhantomPinned,
234}
235
236impl<T> Opaque<T> {
237 /// Creates a new opaque value.
238 pub const fn new(value: T) -> Self {
239 Self {
240 value: UnsafeCell::new(MaybeUninit::new(value)),
241 _pin: PhantomPinned,
242 }
243 }
244
245 /// Creates an uninitialised value.
246 pub const fn uninit() -> Self {
247 Self {
248 value: UnsafeCell::new(MaybeUninit::uninit()),
249 _pin: PhantomPinned,
250 }
251 }
252
253 /// Creates a pin-initializer from the given initializer closure.
254 ///
255 /// The returned initializer calls the given closure with the pointer to the inner `T` of this
256 /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
257 ///
258 /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
259 /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
260 /// to verify at that point that the inner value is valid.
261 pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
262 // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
263 // initialize the `T`.
264 unsafe {
265 init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| {
266 init_func(Self::raw_get(slot));
267 Ok(())
268 })
269 }
270 }
271
272 /// Returns a raw pointer to the opaque data.
273 pub fn get(&self) -> *mut T {
274 UnsafeCell::get(&self.value).cast::<T>()
275 }
276
277 /// Gets the value behind `this`.
278 ///
279 /// This function is useful to get access to the value without creating intermediate
280 /// references.
281 pub const fn raw_get(this: *const Self) -> *mut T {
282 UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
283 }
284}
285
286/// Types that are _always_ reference counted.
287///
288/// It allows such types to define their own custom ref increment and decrement functions.
289/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference
290/// [`ARef<T>`].
291///
292/// This is usually implemented by wrappers to existing structures on the C side of the code. For
293/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted
294/// instances of a type.
295///
296/// # Safety
297///
298/// Implementers must ensure that increments to the reference count keep the object alive in memory
299/// at least until matching decrements are performed.
300///
301/// Implementers must also ensure that all instances are reference-counted. (Otherwise they
302/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object
303/// alive.)
304pub unsafe trait AlwaysRefCounted {
305 /// Increments the reference count on the object.
306 fn inc_ref(&self);
307
308 /// Decrements the reference count on the object.
309 ///
310 /// Frees the object when the count reaches zero.
311 ///
312 /// # Safety
313 ///
314 /// Callers must ensure that there was a previous matching increment to the reference count,
315 /// and that the object is no longer used after its reference count is decremented (as it may
316 /// result in the object being freed), unless the caller owns another increment on the refcount
317 /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls
318 /// [`AlwaysRefCounted::dec_ref`] once).
319 unsafe fn dec_ref(obj: NonNull<Self>);
320}
321
322/// An owned reference to an always-reference-counted object.
323///
324/// The object's reference count is automatically decremented when an instance of [`ARef`] is
325/// dropped. It is also automatically incremented when a new instance is created via
326/// [`ARef::clone`].
327///
328/// # Invariants
329///
330/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In
331/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count.
332pub struct ARef<T: AlwaysRefCounted> {
333 ptr: NonNull<T>,
334 _p: PhantomData<T>,
335}
336
337// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because
338// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
339// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a
340// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
341unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {}
342
343// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync`
344// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
345// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an
346// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for
347// example, when the reference count reaches zero and `T` is dropped.
348unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {}
349
350impl<T: AlwaysRefCounted> ARef<T> {
351 /// Creates a new instance of [`ARef`].
352 ///
353 /// It takes over an increment of the reference count on the underlying object.
354 ///
355 /// # Safety
356 ///
357 /// Callers must ensure that the reference count was incremented at least once, and that they
358 /// are properly relinquishing one increment. That is, if there is only one increment, callers
359 /// must not use the underlying object anymore -- it is only safe to do so via the newly
360 /// created [`ARef`].
361 pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
362 // INVARIANT: The safety requirements guarantee that the new instance now owns the
363 // increment on the refcount.
364 Self {
365 ptr,
366 _p: PhantomData,
367 }
368 }
369}
370
371impl<T: AlwaysRefCounted> Clone for ARef<T> {
372 fn clone(&self) -> Self {
373 self.inc_ref();
374 // SAFETY: We just incremented the refcount above.
375 unsafe { Self::from_raw(self.ptr) }
376 }
377}
378
379impl<T: AlwaysRefCounted> Deref for ARef<T> {
380 type Target = T;
381
382 fn deref(&self) -> &Self::Target {
383 // SAFETY: The type invariants guarantee that the object is valid.
384 unsafe { self.ptr.as_ref() }
385 }
386}
387
388impl<T: AlwaysRefCounted> From<&T> for ARef<T> {
389 fn from(b: &T) -> Self {
390 b.inc_ref();
391 // SAFETY: We just incremented the refcount above.
392 unsafe { Self::from_raw(NonNull::from(b)) }
393 }
394}
395
396impl<T: AlwaysRefCounted> Drop for ARef<T> {
397 fn drop(&mut self) {
398 // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
399 // decrement.
400 unsafe { T::dec_ref(self.ptr) };
401 }
402}
403
404/// A sum type that always holds either a value of type `L` or `R`.
405pub enum Either<L, R> {
406 /// Constructs an instance of [`Either`] containing a value of type `L`.
407 Left(L),
408
409 /// Constructs an instance of [`Either`] containing a value of type `R`.
410 Right(R),
411}