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1// SPDX-License-Identifier: GPL-2.0
2
3// Copyright (C) 2024 Google LLC.
4
5//! A field that is exclusively owned by a [`ListArc`].
6//!
7//! This can be used to have reference counted struct where one of the reference counted pointers
8//! has exclusive access to a field of the struct.
9//!
10//! [`ListArc`]: crate::list::ListArc
11
12use core::cell::UnsafeCell;
13
14/// A field owned by a specific [`ListArc`].
15///
16/// [`ListArc`]: crate::list::ListArc
17pub struct ListArcField<T, const ID: u64 = 0> {
18 value: UnsafeCell<T>,
19}
20
21// SAFETY: If the inner type is thread-safe, then it's also okay for `ListArc` to be thread-safe.
22unsafe impl<T: Send + Sync, const ID: u64> Send for ListArcField<T, ID> {}
23// SAFETY: If the inner type is thread-safe, then it's also okay for `ListArc` to be thread-safe.
24unsafe impl<T: Send + Sync, const ID: u64> Sync for ListArcField<T, ID> {}
25
26impl<T, const ID: u64> ListArcField<T, ID> {
27 /// Creates a new `ListArcField`.
28 pub fn new(value: T) -> Self {
29 Self {
30 value: UnsafeCell::new(value),
31 }
32 }
33
34 /// Access the value when we have exclusive access to the `ListArcField`.
35 ///
36 /// This allows access to the field using an `UniqueArc` instead of a `ListArc`.
37 pub fn get_mut(&mut self) -> &mut T {
38 self.value.get_mut()
39 }
40
41 /// Unsafely assert that you have shared access to the `ListArc` for this field.
42 ///
43 /// # Safety
44 ///
45 /// The caller must have shared access to the `ListArc<ID>` containing the struct with this
46 /// field for the duration of the returned reference.
47 pub unsafe fn assert_ref(&self) -> &T {
48 // SAFETY: The caller has shared access to the `ListArc`, so they also have shared access
49 // to this field.
50 unsafe { &*self.value.get() }
51 }
52
53 /// Unsafely assert that you have mutable access to the `ListArc` for this field.
54 ///
55 /// # Safety
56 ///
57 /// The caller must have mutable access to the `ListArc<ID>` containing the struct with this
58 /// field for the duration of the returned reference.
59 #[expect(clippy::mut_from_ref)]
60 pub unsafe fn assert_mut(&self) -> &mut T {
61 // SAFETY: The caller has exclusive access to the `ListArc`, so they also have exclusive
62 // access to this field.
63 unsafe { &mut *self.value.get() }
64 }
65}
66
67/// Defines getters for a [`ListArcField`].
68#[macro_export]
69#[doc(hidden)]
70macro_rules! define_list_arc_field_getter {
71 ($pub:vis fn $name:ident(&self $(<$id:tt>)?) -> &$typ:ty { $field:ident }
72 $($rest:tt)*
73 ) => {
74 $pub fn $name<'a>(self: &'a $crate::list::ListArc<Self $(, $id)?>) -> &'a $typ {
75 let field = &(&**self).$field;
76 // SAFETY: We have a shared reference to the `ListArc`.
77 unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_ref(field) }
78 }
79
80 $crate::list::define_list_arc_field_getter!($($rest)*);
81 };
82
83 ($pub:vis fn $name:ident(&mut self $(<$id:tt>)?) -> &mut $typ:ty { $field:ident }
84 $($rest:tt)*
85 ) => {
86 $pub fn $name<'a>(self: &'a mut $crate::list::ListArc<Self $(, $id)?>) -> &'a mut $typ {
87 let field = &(&**self).$field;
88 // SAFETY: We have a mutable reference to the `ListArc`.
89 unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_mut(field) }
90 }
91
92 $crate::list::define_list_arc_field_getter!($($rest)*);
93 };
94
95 () => {};
96}
97pub use define_list_arc_field_getter;