Add MaybeUninit::drop.

ManuallyDrop's documentation tells the user to use MaybeUninit instead
when handling uninitialized data. However, the main functionality of
ManuallyDrop (drop) was not available directly on MaybeUninit. Adding it
makes it easier to switch from one to the other.

diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs
index b64abf68c5e4a6184d0af45d889e7f1cba0891f2..e826ec0a5e3eee78c53668203f6293e7c3be8801 100644 (file)
--- a/library/core/src/mem/maybe_uninit.rs
+++ b/library/core/src/mem/maybe_uninit.rs
@@ -2,6 +2,7 @@
 use crate::fmt;
 use crate::intrinsics;
 use crate::mem::ManuallyDrop;
+use crate::ptr;
 
 /// A wrapper type to construct uninitialized instances of `T`.
 ///
@@ -573,6 +574,28 @@ pub unsafe fn read(&self) -> T {
         }
     }
 
+    /// Drops the contained value in place.
+    ///
+    /// If you have ownership of the `MaybeUninit`, it is preferable to use
+    /// [`assume_init`] instead, which prevents duplicating the content.
+    ///
+    /// # Safety
+    ///
+    /// Calling this when the content is not yet fully initialized causes undefined
+    /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
+    /// is in an initialized state.
+    ///
+    /// This function runs the destructor of the contained value in place.
+    /// Afterwards, the memory is considered uninitialized again, but remains unmodified.
+    ///
+    /// [`assume_init`]: MaybeUninit::assume_init
+    #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
+    pub unsafe fn drop(&mut self) {
+        // SAFETY: the caller must guarantee that `self` is initialized.
+        // 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