yoke/
kinda_sorta_dangling.rs

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1// This file is part of ICU4X. For terms of use, please see the file
2// called LICENSE at the top level of the ICU4X source tree
3// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
4
5use core::mem::{ManuallyDrop, MaybeUninit};
6use core::ops::{Deref, DerefMut};
7
8/// This type is intended to be similar to the type `MaybeDangling<T>`
9/// proposed in [RFC 3336].
10///
11/// The effect of this is that in Rust's safety model, types inside here are not
12/// expected to have any memory dependent validity properties (`dereferenceable`, `noalias`).
13///
14/// See [#3696] for a testcase where `Yoke` fails under miri's field-retagging mode if not using
15/// KindaSortaDangling.
16///
17/// This has `T: 'static` since we don't need anything
18/// else and we don't want to have to think (more) about variance over lifetimes or dropck.
19///
20/// After [RFC 3336] lands we can use `MaybeDangling` instead.
21///
22/// Note that a version of this type also exists publicly as the [`maybe_dangling`]
23/// crate; which also exports a patched `ManuallyDrop` with similar semantics and
24/// does not require `T: 'static`. Consider using this if you need something more general
25/// and are okay with adding dependencies.
26///
27/// [RFC 3336]: https://github.com/rust-lang/rfcs/pull/3336
28/// [#3696]: https://github.com/unicode-org/icu4x/issues/3696
29/// [`maybe_dangling`](https://docs.rs/maybe-dangling/0.1.0/maybe_dangling/struct.MaybeDangling.html)
30#[repr(transparent)]
31pub(crate) struct KindaSortaDangling<T: 'static> {
32    /// Safety invariant: This is always an initialized T, never uninit or other
33    /// invalid bit patterns. Its drop glue will execute during Drop::drop rather than
34    /// during the drop glue for KindaSortaDangling, which means that we have to be careful about
35    /// not touching the values as initialized during `drop` after that, but that's a short period of time.
36    dangle: MaybeUninit<T>,
37}
38
39impl<T: 'static> KindaSortaDangling<T> {
40    #[inline]
41    pub(crate) const fn new(dangle: T) -> Self {
42        KindaSortaDangling {
43            dangle: MaybeUninit::new(dangle),
44        }
45    }
46    #[inline]
47    pub(crate) fn into_inner(self) -> T {
48        // Self has a destructor, we want to avoid having it be called
49        let manual = ManuallyDrop::new(self);
50        // Safety:
51        // We can call assume_init_read() due to the library invariant on this type,
52        // however since it is a read() we must be careful about data duplication.
53        // The only code using `self` after this is the drop glue, which we have disabled via
54        // the ManuallyDrop.
55        unsafe { manual.dangle.assume_init_read() }
56    }
57}
58
59impl<T: 'static> Deref for KindaSortaDangling<T> {
60    type Target = T;
61    #[inline]
62    fn deref(&self) -> &T {
63        // Safety: Due to the safety invariant on `dangle`, it is guaranteed to be always
64        // initialized as deref is never called during drop.
65        unsafe { self.dangle.assume_init_ref() }
66    }
67}
68
69impl<T: 'static> DerefMut for KindaSortaDangling<T> {
70    #[inline]
71    fn deref_mut(&mut self) -> &mut T {
72        // Safety: Due to the safety invariant on `dangle`, it is guaranteed to be always
73        // initialized as deref_mut is never called during drop.
74        unsafe { self.dangle.assume_init_mut() }
75    }
76}
77
78impl<T: 'static> Drop for KindaSortaDangling<T> {
79    #[inline]
80    fn drop(&mut self) {
81        // Safety: We are reading and dropping a valid initialized T.
82        //
83        // As `drop_in_place()` is a `read()`-like duplication operation we must be careful that the original value isn't
84        // used afterwards. It won't be because this is drop and the only
85        // code that will run after this is `self`'s drop glue, and that drop glue is empty
86        // because MaybeUninit has no drop.
87        //
88        // We use `drop_in_place()` instead of `let _ = ... .assume_init_read()` to avoid creating a move
89        // of the inner `T` (without `KindaSortaDangling` protection!) type into a local -- we don't want to
90        // assert any of `T`'s memory-related validity properties here.
91        unsafe {
92            self.dangle.as_mut_ptr().drop_in_place();
93        }
94    }
95}
yoke/kinda_sorta_dangling.rs

yoke/
kinda_sorta_dangling.rs
