1#![unstable(feature = "maybe_dangling", issue = "118166")]
2
3use crate::{mem, ptr};
4
5/// Allows wrapped [references] and [boxes] to dangle.
6///
7/// <section class="warning">
8/// This type is not properly implemented yet, and the documentation below is thus not accurate.
9/// </section>
10///
11/// That is, if a reference (or a `Box`) is wrapped in `MaybeDangling` (including when in a
12/// (nested) field of a compound type wrapped in `MaybeDangling`), it does not have to follow
13/// pointer aliasing rules or be dereferenceable.
14///
15/// This can be useful when the value can become dangling while the function holding it is still
16/// executing (particularly in concurrent code). As a somewhat absurd example, consider this code:
17///
18/// ```rust,no_run
19/// #![feature(box_as_ptr)]
20/// # use std::alloc::{dealloc, Layout};
21/// # use std::mem;
22///
23/// let mut boxed = Box::new(0_u32);
24/// let ptr = Box::as_mut_ptr(&mut boxed);
25///
26/// // Safety: the pointer comes from a box and thus was allocated before; `box` is not used afterwards
27/// unsafe { dealloc(ptr.cast(), Layout::new::<u32>()) };
28///
29/// mem::forget(boxed); // <-- this is UB!
30/// ```
31///
32/// Even though the `Box`'s destructor is not run (and thus we don't have a double free bug), this
33/// code is still UB. This is because when moving `boxed` into `forget`, its validity invariants
34/// are asserted, causing UB since the `Box` is dangling. The safety comment is as such wrong, as
35/// moving the `boxed` variable as part of the `forget` call *is* a use.
36///
37/// To fix this we could use `MaybeDangling`:
38///
39// FIXME: remove `no_run` once the semantics are actually implemented
40/// ```rust,no_run
41/// #![feature(maybe_dangling, box_as_ptr)]
42/// # use std::alloc::{dealloc, Layout};
43/// # use std::mem::{self, MaybeDangling};
44///
45/// let mut boxed = MaybeDangling::new(Box::new(0_u32));
46/// let ptr = Box::as_mut_ptr(boxed.as_mut());
47///
48/// // Safety: the pointer comes from a box and thus was allocated before; `box` is not used afterwards
49/// unsafe { dealloc(ptr.cast(), Layout::new::<u32>()) };
50///
51/// mem::forget(boxed); // <-- this is OK!
52/// ```
53///
54/// Note that the bit pattern must still be valid for the wrapped type. That is, [references]
55/// (and [boxes]) still must be aligned and non-null.
56///
57/// Additionally note that safe code can still assume that the inner value in a `MaybeDangling` is
58/// **not** dangling -- functions like [`as_ref`] and [`into_inner`] are safe. It is not sound to
59/// return a dangling reference in a `MaybeDangling` to safe code. However, it *is* sound
60/// to hold such values internally inside your code -- and there's no way to do that without
61/// this type. Note that other types can use this type and thus get the same effect; in particular,
62/// [`ManuallyDrop`] will use `MaybeDangling`.
63///
64/// Note that `MaybeDangling` doesn't prevent drops from being run, which can lead to UB if the
65/// drop observes a dangling value. If you need to prevent drops from being run use [`ManuallyDrop`]
66/// instead.
67///
68/// [references]: prim@reference
69/// [boxes]: ../../std/boxed/struct.Box.html
70/// [`into_inner`]: MaybeDangling::into_inner
71/// [`as_ref`]: MaybeDangling::as_ref
72/// [`ManuallyDrop`]: crate::mem::ManuallyDrop
73#[repr(transparent)]
74#[rustc_pub_transparent]
75#[derive(Debug, Copy, Clone, Default)]
76pub struct MaybeDangling<P: ?Sized>(P);
77
78impl<P: ?Sized> MaybeDangling<P> {
79 /// Wraps a value in a `MaybeDangling`, allowing it to dangle.
80 pub const fn new(x: P) -> Self
81 where
82 P: Sized,
83 {
84 MaybeDangling(x)
85 }
86
87 /// Returns a reference to the inner value.
88 ///
89 /// Note that this is UB if the inner value is currently dangling.
90 pub const fn as_ref(&self) -> &P {
91 &self.0
92 }
93
94 /// Returns a mutable reference to the inner value.
95 ///
96 /// Note that this is UB if the inner value is currently dangling.
97 pub const fn as_mut(&mut self) -> &mut P {
98 &mut self.0
99 }
100
101 /// Extracts the value from the `MaybeDangling` container.
102 ///
103 /// Note that this is UB if the inner value is currently dangling.
104 pub const fn into_inner(self) -> P
105 where
106 P: Sized,
107 {
108 // FIXME: replace this with `self.0` when const checker can figure out that `self` isn't actually dropped
109 // SAFETY: this is equivalent to `self.0`
110 let x = unsafe { ptr::read(&self.0) };
111 mem::forget(self);
112 x
113 }
114}
115