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
12 use core::cell::UnsafeCell;
13
14 /// A field owned by a specific [`ListArc`].
15 ///
16 /// [`ListArc`]: crate::list::ListArc
17 pub 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.
22 unsafe 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.
24 unsafe impl<T: Send + Sync, const ID: u64> Sync for ListArcField<T, ID> {}
25
26 impl<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 #[allow(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 macro_rules! define_list_arc_field_getter {
70 ($pub:vis fn $name:ident(&self $(<$id:tt>)?) -> &$typ:ty { $field:ident }
71 $($rest:tt)*
72 ) => {
73 $pub fn $name<'a>(self: &'a $crate::list::ListArc<Self $(, $id)?>) -> &'a $typ {
74 let field = &(&**self).$field;
75 // SAFETY: We have a shared reference to the `ListArc`.
76 unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_ref(field) }
77 }
78
79 $crate::list::define_list_arc_field_getter!($($rest)*);
80 };
81
82 ($pub:vis fn $name:ident(&mut self $(<$id:tt>)?) -> &mut $typ:ty { $field:ident }
83 $($rest:tt)*
84 ) => {
85 $pub fn $name<'a>(self: &'a mut $crate::list::ListArc<Self $(, $id)?>) -> &'a mut $typ {
86 let field = &(&**self).$field;
87 // SAFETY: We have a mutable reference to the `ListArc`.
88 unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_mut(field) }
89 }
90
91 $crate::list::define_list_arc_field_getter!($($rest)*);
92 };
93
94 () => {};
95 }
96 pub use define_list_arc_field_getter;
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