// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at:
https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
use core::mem::{ManuallyDrop, MaybeUninit};
use core::ops::{Deref, DerefMut};
/// This type is intended to be similar to the type `MaybeDangling<T>`
/// proposed in [RFC 3336].
///
/// The effect of this is that in Rust's safety model, types inside here are not
/// expected to have any memory dependent validity properties (`dereferenceable`, `noalias
`).
///
/// See [#3696] for a testcase where `Yoke` fails this under miri's field-retagging mode.
///
/// This has `T: 'static` since we don't need anything
/// else and we don't want to have to think (more) about variance over lifetimes or dropck.
///
/// After [RFC 3336] lands we can use `MaybeDangling` instead.
///
/// Note that a version of this type also exists publicly as the [`maybe_dangling`]
/// crate; which also exports a patched `ManuallyDrop` with similar semantics and
/// does not require `T: 'static`. Consider using this if you need something more general
/// and are okay with adding dependencies.
///
/// [RFC 3336]: https://github.com/rust-lang/rfcs/pull/3336
/// [#3696]: https://github.com/unicode-org/icu4x/issues/3696
/// [`maybe_dangling`](https://docs.rs/maybe-dangling/0.1.0/maybe_dangling/struct.MaybeDangling.html)
#[repr(transparent)]
pub(crate) struct KindaSortaDangling<T: 'static> {
/// Safety invariant: This is always an initialized T, never uninit or other
/// invalid bit patterns. Its drop glue will execute during Drop::drop rather than
/// during the drop glue for KindaSortaDangling, which means that we have to be careful about
/// not touching the values as initialized during `drop` after that, but that's a short period of time.
dangle: MaybeUninit<T>,
}
impl<T: 'static> KindaSortaDangling<T> {
#[inline]
pub(crate) const fn new(dangle: T) -> Self {
KindaSortaDangling {
dangle: MaybeUninit::new(dangle),
}
}
#[inline]
pub(crate) fn into_inner(self) -> T {
// Self has a destructor, we want to avoid having it be called
let manual = ManuallyDrop::new(self);
// Safety:
// We can call assume_init_read() due to the library invariant on this type,
// however since it is a read() we must be careful about data duplication.
// The only code using `self` after this is the drop glue, which we have disabled via
// the ManuallyDrop.
unsafe { manual.dangle.assume_init_read() }
}
}
impl<T: 'static> Deref for KindaSortaDangling<T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
// Safety: Safe due to the safety invariant on `dangle`;
// we can always assume initialized
unsafe { self.dangle.assume_init_ref() }
}
}
impl<T: 'static> DerefMut for KindaSortaDangling<T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
// Safety: Safe due to the safety invariant on `dangle`;
// we can always assume initialized
unsafe { self.dangle.assume_init_mut() }
}
}
impl<T: 'static> Drop for KindaSortaDangling<T> {
#[inline]
fn drop(&mut self) {
unsafe {
// Safety: We are reading and dropping a valid initialized T.
//
// As `drop_in_place()` is a `read()`-like duplication operation we must be careful that the original value isn't
// used afterwards. It won't be because this is drop and the only
// code that will run after this is `self`'s drop glue, and that drop glue is empty
// because MaybeUninit has no drop.
//
// We use `drop_in_place()` instead of `let _ = ... .assume_init_read()` to avoid creating a move
// of the inner `T` (without `KindaSortaDangling` protection!) type into a local -- we don't want to
// assert any of `T`'s memory-related validity properties here.
self.dangle.as_mut_ptr().drop_in_place();
}
}
}
