diff --git a/library/core/src/array/iter.rs b/library/core/src/array/iter.rs index 2e8b6419eea..cafb002c01a 100644 --- a/library/core/src/array/iter.rs +++ b/library/core/src/array/iter.rs @@ -103,7 +103,7 @@ impl Iterator for IntoIter { // dead now (i.e. do not touch). As `idx` was the start of the // alive-zone, the alive zone is now `data[alive]` again, restoring // all invariants. - unsafe { self.data.get_unchecked(idx).read() } + unsafe { self.data.get_unchecked(idx).assume_init_read() } }) } @@ -136,7 +136,7 @@ impl DoubleEndedIterator for IntoIter { // dead now (i.e. do not touch). As `idx` was the end of the // alive-zone, the alive zone is now `data[alive]` again, restoring // all invariants. - unsafe { self.data.get_unchecked(idx).read() } + unsafe { self.data.get_unchecked(idx).assume_init_read() } }) } } diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs index 3a7489aa279..b0ebaa6a12b 100644 --- a/library/core/src/mem/maybe_uninit.rs +++ b/library/core/src/mem/maybe_uninit.rs @@ -2,6 +2,7 @@ use crate::any::type_name; use crate::fmt; use crate::intrinsics; use crate::mem::ManuallyDrop; +use crate::ptr; /// A wrapper type to construct uninitialized instances of `T`. /// @@ -471,6 +472,8 @@ impl MaybeUninit { /// *immediate* undefined behavior, but will cause undefined behavior with most /// safe operations (including dropping it). /// + /// [`Vec`]: ../../std/vec/struct.Vec.html + /// /// # Examples /// /// Correct usage of this method: @@ -519,8 +522,8 @@ impl MaybeUninit { /// this initialization invariant. /// /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit`. When using - /// multiple copies of the data (by calling `read` multiple times, or first - /// calling `read` and then [`assume_init`]), it is your responsibility + /// multiple copies of the data (by calling `assume_init_read` multiple times, or first + /// calling `assume_init_read` and then [`assume_init`]), it is your responsibility /// to ensure that that data may indeed be duplicated. /// /// [inv]: #initialization-invariant @@ -536,16 +539,16 @@ impl MaybeUninit { /// /// let mut x = MaybeUninit::::uninit(); /// x.write(13); - /// let x1 = unsafe { x.read() }; + /// let x1 = unsafe { x.assume_init_read() }; /// // `u32` is `Copy`, so we may read multiple times. - /// let x2 = unsafe { x.read() }; + /// let x2 = unsafe { x.assume_init_read() }; /// assert_eq!(x1, x2); /// /// let mut x = MaybeUninit::>>::uninit(); /// x.write(None); - /// let x1 = unsafe { x.read() }; + /// let x1 = unsafe { x.assume_init_read() }; /// // Duplicating a `None` value is okay, so we may read multiple times. - /// let x2 = unsafe { x.read() }; + /// let x2 = unsafe { x.assume_init_read() }; /// assert_eq!(x1, x2); /// ``` /// @@ -557,14 +560,14 @@ impl MaybeUninit { /// /// let mut x = MaybeUninit::>>::uninit(); /// x.write(Some(vec![0,1,2])); - /// let x1 = unsafe { x.read() }; - /// let x2 = unsafe { x.read() }; + /// let x1 = unsafe { x.assume_init_read() }; + /// let x2 = unsafe { x.assume_init_read() }; /// // We now created two copies of the same vector, leading to a double-free ⚠️ when /// // they both get dropped! /// ``` #[unstable(feature = "maybe_uninit_extra", issue = "63567")] #[inline(always)] - pub unsafe fn read(&self) -> T { + pub unsafe fn assume_init_read(&self) -> T { // SAFETY: the caller must guarantee that `self` is initialized. // Reading from `self.as_ptr()` is safe since `self` should be initialized. unsafe { @@ -573,6 +576,34 @@ impl MaybeUninit { } } + /// Drops the contained value in place. + /// + /// If you have ownership of the `MaybeUninit`, you can use [`assume_init`] instead. + /// + /// # Safety + /// + /// It is up to the caller to guarantee that the `MaybeUninit` really is + /// in an initialized state. Calling this when the content is not yet fully + /// initialized causes undefined behavior. + /// + /// On top of that, all additional invariants of the type `T` must be + /// satisfied, as the `Drop` implementation of `T` (or its members) may + /// rely on this. For example, a `1`-initialized [`Vec`] is considered + /// initialized (under the current implementation; this does not constitute + /// a stable guarantee) because the only requirement the compiler knows + /// about it is that the data pointer must be non-null. Dropping such a + /// `Vec` however will cause undefined behaviour. + /// + /// [`assume_init`]: MaybeUninit::assume_init + /// [`Vec`]: ../../std/vec/struct.Vec.html + #[unstable(feature = "maybe_uninit_extra", issue = "63567")] + pub unsafe fn assume_init_drop(&mut self) { + // SAFETY: the caller must guarantee that `self` is initialized and + // satisfies all invariants of `T`. + // Dropping the value in place is safe if that is the case. + unsafe { ptr::drop_in_place(self.as_mut_ptr()) } + } + /// Gets a shared reference to the contained value. /// /// This can be useful when we want to access a `MaybeUninit` that has been