diff --git a/alloc/src/boxed.rs b/alloc/src/boxed.rs index a924feaf15f2a..f265b7b6f26ec 100644 --- a/alloc/src/boxed.rs +++ b/alloc/src/boxed.rs @@ -1059,6 +1059,59 @@ impl Box { pub unsafe fn from_raw(raw: *mut T) -> Self { unsafe { Self::from_raw_in(raw, Global) } } + + /// Constructs a box from a `NonNull` pointer. + /// + /// After calling this function, the `NonNull` pointer is owned by + /// the resulting `Box`. Specifically, the `Box` destructor will call + /// the destructor of `T` and free the allocated memory. For this + /// to be safe, the memory must have been allocated in accordance + /// with the [memory layout] used by `Box` . + /// + /// # Safety + /// + /// This function is unsafe because improper use may lead to + /// memory problems. For example, a double-free may occur if the + /// function is called twice on the same `NonNull` pointer. + /// + /// The safety conditions are described in the [memory layout] section. + /// + /// # Examples + /// + /// Recreate a `Box` which was previously converted to a `NonNull` + /// pointer using [`Box::into_non_null`]: + /// ``` + /// #![feature(box_vec_non_null)] + /// + /// let x = Box::new(5); + /// let non_null = Box::into_non_null(x); + /// let x = unsafe { Box::from_non_null(non_null) }; + /// ``` + /// Manually create a `Box` from scratch by using the global allocator: + /// ``` + /// #![feature(box_vec_non_null)] + /// + /// use std::alloc::{alloc, Layout}; + /// use std::ptr::NonNull; + /// + /// unsafe { + /// let non_null = NonNull::new(alloc(Layout::new::()).cast::()) + /// .expect("allocation failed"); + /// // In general .write is required to avoid attempting to destruct + /// // the (uninitialized) previous contents of `non_null`. + /// non_null.write(5); + /// let x = Box::from_non_null(non_null); + /// } + /// ``` + /// + /// [memory layout]: self#memory-layout + /// [`Layout`]: crate::Layout + #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "none")] + #[inline] + #[must_use = "call `drop(Box::from_non_null(ptr))` if you intend to drop the `Box`"] + pub unsafe fn from_non_null(ptr: NonNull) -> Self { + unsafe { Self::from_raw(ptr.as_ptr()) } + } } impl Box { @@ -1116,6 +1169,61 @@ impl Box { Box(unsafe { Unique::new_unchecked(raw) }, alloc) } + /// Constructs a box from a `NonNull` pointer in the given allocator. + /// + /// After calling this function, the `NonNull` pointer is owned by + /// the resulting `Box`. Specifically, the `Box` destructor will call + /// the destructor of `T` and free the allocated memory. For this + /// to be safe, the memory must have been allocated in accordance + /// with the [memory layout] used by `Box` . + /// + /// # Safety + /// + /// This function is unsafe because improper use may lead to + /// memory problems. For example, a double-free may occur if the + /// function is called twice on the same raw pointer. + /// + /// + /// # Examples + /// + /// Recreate a `Box` which was previously converted to a `NonNull` pointer + /// using [`Box::into_non_null_with_allocator`]: + /// ``` + /// #![feature(allocator_api, box_vec_non_null)] + /// + /// use std::alloc::System; + /// + /// let x = Box::new_in(5, System); + /// let (non_null, alloc) = Box::into_non_null_with_allocator(x); + /// let x = unsafe { Box::from_non_null_in(non_null, alloc) }; + /// ``` + /// Manually create a `Box` from scratch by using the system allocator: + /// ``` + /// #![feature(allocator_api, box_vec_non_null, slice_ptr_get)] + /// + /// use std::alloc::{Allocator, Layout, System}; + /// + /// unsafe { + /// let non_null = System.allocate(Layout::new::())?.cast::(); + /// // In general .write is required to avoid attempting to destruct + /// // the (uninitialized) previous contents of `non_null`. + /// non_null.write(5); + /// let x = Box::from_non_null_in(non_null, System); + /// } + /// # Ok::<(), std::alloc::AllocError>(()) + /// ``` + /// + /// [memory layout]: self#memory-layout + /// [`Layout`]: crate::Layout + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "none")] + #[rustc_const_unstable(feature = "const_box", issue = "92521")] + #[inline] + pub const unsafe fn from_non_null_in(raw: NonNull, alloc: A) -> Self { + // SAFETY: guaranteed by the caller. + unsafe { Box::from_raw_in(raw.as_ptr(), alloc) } + } + /// Consumes the `Box`, returning a wrapped raw pointer. /// /// The pointer will be properly aligned and non-null. @@ -1171,6 +1279,66 @@ impl Box { unsafe { addr_of_mut!(*&mut *Self::into_raw_with_allocator(b).0) } } + /// Consumes the `Box`, returning a wrapped `NonNull` pointer. + /// + /// The pointer will be properly aligned. + /// + /// After calling this function, the caller is responsible for the + /// memory previously managed by the `Box`. In particular, the + /// caller should properly destroy `T` and release the memory, taking + /// into account the [memory layout] used by `Box`. The easiest way to + /// do this is to convert the `NonNull` pointer back into a `Box` with the + /// [`Box::from_non_null`] function, allowing the `Box` destructor to + /// perform the cleanup. + /// + /// Note: this is an associated function, which means that you have + /// to call it as `Box::into_non_null(b)` instead of `b.into_non_null()`. + /// This is so that there is no conflict with a method on the inner type. + /// + /// # Examples + /// Converting the `NonNull` pointer back into a `Box` with [`Box::from_non_null`] + /// for automatic cleanup: + /// ``` + /// #![feature(box_vec_non_null)] + /// + /// let x = Box::new(String::from("Hello")); + /// let non_null = Box::into_non_null(x); + /// let x = unsafe { Box::from_non_null(non_null) }; + /// ``` + /// Manual cleanup by explicitly running the destructor and deallocating + /// the memory: + /// ``` + /// #![feature(box_vec_non_null)] + /// + /// use std::alloc::{dealloc, Layout}; + /// + /// let x = Box::new(String::from("Hello")); + /// let non_null = Box::into_non_null(x); + /// unsafe { + /// non_null.drop_in_place(); + /// dealloc(non_null.as_ptr().cast::(), Layout::new::()); + /// } + /// ``` + /// Note: This is equivalent to the following: + /// ``` + /// #![feature(box_vec_non_null)] + /// + /// let x = Box::new(String::from("Hello")); + /// let non_null = Box::into_non_null(x); + /// unsafe { + /// drop(Box::from_non_null(non_null)); + /// } + /// ``` + /// + /// [memory layout]: self#memory-layout + #[must_use = "losing the pointer will leak memory"] + #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "none")] + #[inline] + pub fn into_non_null(b: Self) -> NonNull { + // SAFETY: `Box` is guaranteed to be non-null. + unsafe { NonNull::new_unchecked(Self::into_raw(b)) } + } + /// Consumes the `Box`, returning a wrapped raw pointer and the allocator. /// /// The pointer will be properly aligned and non-null. @@ -1232,6 +1400,61 @@ impl Box { (ptr, alloc) } + /// Consumes the `Box`, returning a wrapped `NonNull` pointer and the allocator. + /// + /// The pointer will be properly aligned. + /// + /// After calling this function, the caller is responsible for the + /// memory previously managed by the `Box`. In particular, the + /// caller should properly destroy `T` and release the memory, taking + /// into account the [memory layout] used by `Box`. The easiest way to + /// do this is to convert the `NonNull` pointer back into a `Box` with the + /// [`Box::from_non_null_in`] function, allowing the `Box` destructor to + /// perform the cleanup. + /// + /// Note: this is an associated function, which means that you have + /// to call it as `Box::into_non_null_with_allocator(b)` instead of + /// `b.into_non_null_with_allocator()`. This is so that there is no + /// conflict with a method on the inner type. + /// + /// # Examples + /// Converting the `NonNull` pointer back into a `Box` with + /// [`Box::from_non_null_in`] for automatic cleanup: + /// ``` + /// #![feature(allocator_api, box_vec_non_null)] + /// + /// use std::alloc::System; + /// + /// let x = Box::new_in(String::from("Hello"), System); + /// let (non_null, alloc) = Box::into_non_null_with_allocator(x); + /// let x = unsafe { Box::from_non_null_in(non_null, alloc) }; + /// ``` + /// Manual cleanup by explicitly running the destructor and deallocating + /// the memory: + /// ``` + /// #![feature(allocator_api, box_vec_non_null)] + /// + /// use std::alloc::{Allocator, Layout, System}; + /// + /// let x = Box::new_in(String::from("Hello"), System); + /// let (non_null, alloc) = Box::into_non_null_with_allocator(x); + /// unsafe { + /// non_null.drop_in_place(); + /// alloc.deallocate(non_null.cast::(), Layout::new::()); + /// } + /// ``` + /// + /// [memory layout]: self#memory-layout + #[must_use = "losing the pointer will leak memory"] + #[unstable(feature = "allocator_api", issue = "32838")] + // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "none")] + #[inline] + pub fn into_non_null_with_allocator(b: Self) -> (NonNull, A) { + let (ptr, alloc) = Box::into_raw_with_allocator(b); + // SAFETY: `Box` is guaranteed to be non-null. + unsafe { (NonNull::new_unchecked(ptr), alloc) } + } + #[unstable( feature = "ptr_internals", issue = "none",