Rollup merge of #76640 - fusion-engineering-forks:synconcecell-drop, r=matklad
Simplify SyncOnceCell's `take` and `drop`. Prevents copies by using `assume_init_read` and `assume_init_drop`.
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f9b9467866
@ -7,7 +7,7 @@ use crate::{
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cell::{Cell, UnsafeCell},
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fmt,
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marker::PhantomData,
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mem::{self, MaybeUninit},
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mem::MaybeUninit,
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ops::{Deref, Drop},
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panic::{RefUnwindSafe, UnwindSafe},
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sync::Once,
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@ -316,13 +316,7 @@ impl<T> SyncOnceCell<T> {
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn into_inner(mut self) -> Option<T> {
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// SAFETY: Safe because we immediately free `self` without dropping
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let inner = unsafe { self.take_inner() };
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// Don't drop this `SyncOnceCell`. We just moved out one of the fields, but didn't set
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// the state to uninitialized.
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mem::forget(self);
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inner
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self.take()
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}
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/// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
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@ -348,22 +342,12 @@ impl<T> SyncOnceCell<T> {
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn take(&mut self) -> Option<T> {
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mem::take(self).into_inner()
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}
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/// Takes the wrapped value out of a `SyncOnceCell`.
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/// Afterwards the cell is no longer initialized.
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///
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/// Safety: The cell must now be free'd WITHOUT dropping. No other usages of the cell
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/// are valid. Only used by `into_inner` and `drop`.
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unsafe fn take_inner(&mut self) -> Option<T> {
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// The mutable reference guarantees there are no other threads that can observe us
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// taking out the wrapped value.
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// Right after this function `self` is supposed to be freed, so it makes little sense
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// to atomically set the state to uninitialized.
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if self.is_initialized() {
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let value = mem::replace(&mut self.value, UnsafeCell::new(MaybeUninit::uninit()));
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Some(value.into_inner().assume_init())
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self.once = Once::new();
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// SAFETY: `self.value` is initialized and contains a valid `T`.
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// `self.once` is reset, so `is_initialized()` will be false again
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// which prevents the value from being read twice.
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unsafe { Some((&mut *self.value.get()).assume_init_read()) }
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} else {
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None
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}
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@ -416,9 +400,12 @@ impl<T> SyncOnceCell<T> {
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unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
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fn drop(&mut self) {
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// SAFETY: The cell is being dropped, so it can't be accessed again.
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// We also don't touch the `T`, which validates our usage of #[may_dangle].
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unsafe { self.take_inner() };
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if self.is_initialized() {
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// Safety: The cell is initialized and being dropped, so it can't
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// be accessed again. We also don't touch the `T` other than
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// dropping it, which validates our usage of #[may_dangle].
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unsafe { (&mut *self.value.get()).assume_init_drop() };
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}
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}
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}
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@ -191,6 +191,7 @@ struct WaiterQueue<'a> {
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impl Once {
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/// Creates a new `Once` value.
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#[inline]
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#[stable(feature = "once_new", since = "1.2.0")]
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#[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]
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pub const fn new() -> Once {
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