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use crate::ptr; use crate::ops::{Deref, DerefMut}; /// A wrapper to inhibit compiler from automatically calling `T`’s destructor. /// /// This wrapper is 0-cost. /// /// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`. /// As a consequence, it has *no effect* on the assumptions that the compiler makes /// about all values being initialized at their type. In particular, initializing /// a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined behavior. /// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead. /// /// # Examples /// /// This wrapper helps with explicitly documenting the drop order dependencies between fields of /// the type: /// /// ```rust /// use std::mem::ManuallyDrop; /// struct Peach; /// struct Banana; /// struct Melon; /// struct FruitBox { /// // Immediately clear there’s something non-trivial going on with these fields. /// peach: ManuallyDrop<Peach>, /// melon: Melon, // Field that’s independent of the other two. /// banana: ManuallyDrop<Banana>, /// } /// /// impl Drop for FruitBox { /// fn drop(&mut self) { /// unsafe { /// // Explicit ordering in which field destructors are run specified in the intuitive /// // location – the destructor of the structure containing the fields. /// // Moreover, one can now reorder fields within the struct however much they want. /// ManuallyDrop::drop(&mut self.peach); /// ManuallyDrop::drop(&mut self.banana); /// } /// // After destructor for `FruitBox` runs (this function), the destructor for Melon gets /// // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`. /// } /// } /// ``` /// /// [`mem::zeroed`]: fn.zeroed.html /// [`MaybeUninit<T>`]: union.MaybeUninit.html #[stable(feature = "manually_drop", since = "1.20.0")] #[lang = "manually_drop"] #[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)] #[repr(transparent)] pub struct ManuallyDrop<T: ?Sized> { value: T, } impl<T> ManuallyDrop<T> { /// Wrap a value to be manually dropped. /// /// # Examples /// /// ```rust /// use std::mem::ManuallyDrop; /// ManuallyDrop::new(Box::new(())); /// ``` #[stable(feature = "manually_drop", since = "1.20.0")] #[inline(always)] pub const fn new(value: T) -> ManuallyDrop<T> { ManuallyDrop { value } } /// Extracts the value from the `ManuallyDrop` container. /// /// This allows the value to be dropped again. /// /// # Examples /// /// ```rust /// use std::mem::ManuallyDrop; /// let x = ManuallyDrop::new(Box::new(())); /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`. /// ``` #[stable(feature = "manually_drop", since = "1.20.0")] #[inline(always)] pub const fn into_inner(slot: ManuallyDrop<T>) -> T { slot.value } /// Takes the contained value out. /// /// This method is primarily intended for moving out values in drop. /// Instead of using [`ManuallyDrop::drop`] to manually drop the value, /// you can use this method to take the value and use it however desired. /// `Drop` will be invoked on the returned value following normal end-of-scope rules. /// /// If you have ownership of the container, you can use [`ManuallyDrop::into_inner`] instead. /// /// # Safety /// /// This function semantically moves out the contained value without preventing further usage. /// It is up to the user of this method to ensure that this container is not used again. /// /// [`ManuallyDrop::drop`]: #method.drop /// [`ManuallyDrop::into_inner`]: #method.into_inner #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"] #[unstable(feature = "manually_drop_take", issue = "55422")] #[inline] pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T { ManuallyDrop::into_inner(ptr::read(slot)) } } impl<T: ?Sized> ManuallyDrop<T> { /// Manually drops the contained value. /// /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead. /// /// # Safety /// /// This function runs the destructor of the contained value and thus the wrapped value /// now represents uninitialized data. It is up to the user of this method to ensure the /// uninitialized data is not actually used. /// In particular, this function can only be called called at most once /// for a given instance of `ManuallyDrop<T>`. /// /// [`ManuallyDrop::into_inner`]: #method.into_inner #[stable(feature = "manually_drop", since = "1.20.0")] #[inline] pub unsafe fn drop(slot: &mut ManuallyDrop<T>) { ptr::drop_in_place(&mut slot.value) } } #[stable(feature = "manually_drop", since = "1.20.0")] impl<T: ?Sized> Deref for ManuallyDrop<T> { type Target = T; #[inline(always)] fn deref(&self) -> &T { &self.value } } #[stable(feature = "manually_drop", since = "1.20.0")] impl<T: ?Sized> DerefMut for ManuallyDrop<T> { #[inline(always)] fn deref_mut(&mut self) -> &mut T { &mut self.value } }