1use crate::cell::UnsafeCell;
2use crate::marker::{PointerLike, Unpin};
3use crate::ops::{CoerceUnsized, DispatchFromDyn};
4use crate::pin::Pin;
5use crate::{fmt, ptr};
6
7/// This type provides a way to entirely opt-out of typical aliasing rules;
8/// specifically, `&mut UnsafePinned<T>` is not guaranteed to be a unique pointer.
9/// This also subsumes the effects of `UnsafeCell`, i.e., `&UnsafePinned<T>` may point to data
10/// that is being mutated.
11///
12/// However, even if you define your type like `pub struct Wrapper(UnsafePinned<...>)`, it is still
13/// very risky to have an `&mut Wrapper` that aliases anything else. Many functions that work
14/// generically on `&mut T` assume that the memory that stores `T` is uniquely owned (such as
15/// `mem::swap`). In other words, while having aliasing with `&mut Wrapper` is not immediate
16/// Undefined Behavior, it is still unsound to expose such a mutable reference to code you do not
17/// control! Techniques such as pinning via [`Pin`] are needed to ensure soundness.
18///
19/// Similar to [`UnsafeCell`](crate::cell::UnsafeCell), `UnsafePinned` will not usually show up in
20/// the public API of a library. It is an internal implementation detail of libraries that need to
21/// support aliasing mutable references.
22///
23/// This type blocks niches the same way `UnsafeCell` does.
24#[lang = "unsafe_pinned"]
25#[repr(transparent)]
26#[unstable(feature = "unsafe_pinned", issue = "125735")]
27pub struct UnsafePinned<T: ?Sized> {
28 value: UnsafeCell<T>,
29}
30
31// Override the manual `!Sync` in `UnsafeCell`.
32#[unstable(feature = "unsafe_pinned", issue = "125735")]
33unsafe impl<T: ?Sized + Sync> Sync for UnsafePinned<T> {}
34
35/// When this type is used, that almost certainly means safe APIs need to use pinning to avoid the
36/// aliases from becoming invalidated. Therefore let's mark this as `!Unpin`. You can always opt
37/// back in to `Unpin` with an `impl` block, provided your API is still sound while unpinned.
38#[unstable(feature = "unsafe_pinned", issue = "125735")]
39impl<T: ?Sized> !Unpin for UnsafePinned<T> {}
40
41// `Send` and `Sync` are inherited from `T`. This is similar to `SyncUnsafeCell`, since
42// we eventually concluded that `UnsafeCell` implicitly making things `!Sync` is sometimes
43// unergonomic. A type that needs to be `!Send`/`!Sync` should really have an explicit
44// opt-out itself, e.g. via an `PhantomData<*mut T>` or (one day) via `impl !Send`/`impl !Sync`.
45
46impl<T> UnsafePinned<T> {
47 /// Constructs a new instance of `UnsafePinned` which will wrap the specified value.
48 ///
49 /// All access to the inner value through `&UnsafePinned<T>` or `&mut UnsafePinned<T>` or
50 /// `Pin<&mut UnsafePinned<T>>` requires `unsafe` code.
51 #[inline(always)]
52 #[must_use]
53 #[unstable(feature = "unsafe_pinned", issue = "125735")]
54 pub const fn new(value: T) -> Self {
55 UnsafePinned { value: UnsafeCell::new(value) }
56 }
57
58 /// Unwraps the value, consuming this `UnsafePinned`.
59 #[inline(always)]
60 #[must_use]
61 #[unstable(feature = "unsafe_pinned", issue = "125735")]
62 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
63 pub const fn into_inner(self) -> T {
64 self.value.into_inner()
65 }
66}
67
68impl<T: ?Sized> UnsafePinned<T> {
69 /// Get read-write access to the contents of a pinned `UnsafePinned`.
70 #[inline(always)]
71 #[must_use]
72 #[unstable(feature = "unsafe_pinned", issue = "125735")]
73 pub const fn get_mut_pinned(self: Pin<&mut Self>) -> *mut T {
74 // SAFETY: we're not using `get_unchecked_mut` to unpin anything
75 unsafe { self.get_unchecked_mut() }.get_mut_unchecked()
76 }
77
78 /// Get read-write access to the contents of an `UnsafePinned`.
79 ///
80 /// You should usually be using `get_mut_pinned` instead to explicitly track the fact that this
81 /// memory is "pinned" due to there being aliases.
82 #[inline(always)]
83 #[must_use]
84 #[unstable(feature = "unsafe_pinned", issue = "125735")]
85 pub const fn get_mut_unchecked(&mut self) -> *mut T {
86 ptr::from_mut(self) as *mut T
87 }
88
89 /// Get mutable access to the contents of a shared `UnsafePinned`.
90 ///
91 /// This can be cast to a pointer of any kind. When creating references, you must uphold the
92 /// aliasing rules; see [`UnsafeCell`] for more discussion and caveats.
93 ///
94 /// [`UnsafeCell`]: crate::cell::UnsafeCell#aliasing-rules
95 ///
96 /// ```rust,no_run
97 /// #![feature(unsafe_pinned)]
98 /// use std::pin::UnsafePinned;
99 ///
100 /// unsafe {
101 /// let mut x = UnsafePinned::new(0);
102 /// let ptr = x.get();
103 /// x.get_mut_unchecked().write(1);
104 /// assert_eq!(ptr.read(), 1);
105 /// }
106 /// ```
107 #[inline(always)]
108 #[must_use]
109 #[unstable(feature = "unsafe_pinned", issue = "125735")]
110 pub const fn get(&self) -> *mut T {
111 self.value.get()
112 }
113
114 /// Gets an immutable pointer to the wrapped value.
115 ///
116 /// The difference from [`get`] is that this function accepts a raw pointer, which is useful to
117 /// avoid the creation of temporary references.
118 ///
119 /// [`get`]: UnsafePinned::get
120 #[inline(always)]
121 #[must_use]
122 #[unstable(feature = "unsafe_pinned", issue = "125735")]
123 pub const fn raw_get(this: *const Self) -> *const T {
124 this as *const T
125 }
126
127 /// Gets a mutable pointer to the wrapped value.
128 ///
129 /// The difference from [`get_mut_pinned`] and [`get_mut_unchecked`] is that this function
130 /// accepts a raw pointer, which is useful to avoid the creation of temporary references.
131 ///
132 /// [`get_mut_pinned`]: UnsafePinned::get_mut_pinned
133 /// [`get_mut_unchecked`]: UnsafePinned::get_mut_unchecked
134 #[inline(always)]
135 #[must_use]
136 #[unstable(feature = "unsafe_pinned", issue = "125735")]
137 pub const fn raw_get_mut(this: *mut Self) -> *mut T {
138 this as *mut T
139 }
140}
141
142#[unstable(feature = "unsafe_pinned", issue = "125735")]
143impl<T: Default> Default for UnsafePinned<T> {
144 /// Creates an `UnsafePinned`, with the `Default` value for T.
145 fn default() -> Self {
146 UnsafePinned::new(T::default())
147 }
148}
149
150#[unstable(feature = "unsafe_pinned", issue = "125735")]
151impl<T> From<T> for UnsafePinned<T> {
152 /// Creates a new `UnsafePinned<T>` containing the given value.
153 fn from(value: T) -> Self {
154 UnsafePinned::new(value)
155 }
156}
157
158#[unstable(feature = "unsafe_pinned", issue = "125735")]
159impl<T: ?Sized> fmt::Debug for UnsafePinned<T> {
160 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
161 f.debug_struct(name:"UnsafePinned").finish_non_exhaustive()
162 }
163}
164
165#[unstable(feature = "coerce_unsized", issue = "18598")]
166// #[unstable(feature = "unsafe_pinned", issue = "125735")]
167impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafePinned<U>> for UnsafePinned<T> {}
168
169// Allow types that wrap `UnsafePinned` to also implement `DispatchFromDyn`
170// and become dyn-compatible method receivers.
171// Note that currently `UnsafePinned` itself cannot be a method receiver
172// because it does not implement Deref.
173// In other words:
174// `self: UnsafePinned<&Self>` won't work
175// `self: UnsafePinned<Self>` becomes possible
176// FIXME(unsafe_pinned) this logic is copied from UnsafeCell, is it still sound?
177#[unstable(feature = "dispatch_from_dyn", issue = "none")]
178// #[unstable(feature = "unsafe_pinned", issue = "125735")]
179impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafePinned<U>> for UnsafePinned<T> {}
180
181#[unstable(feature = "pointer_like_trait", issue = "none")]
182// #[unstable(feature = "unsafe_pinned", issue = "125735")]
183impl<T: PointerLike> PointerLike for UnsafePinned<T> {}
184
185// FIXME(unsafe_pinned): impl PinCoerceUnsized for UnsafePinned<T>?
186