From dd2c6c318bfe3191e936f4500d60b5089db1ef60 Mon Sep 17 00:00:00 2001
From: Manish Goregaokar <manishsmail@gmail.com>
Date: Thu, 31 Dec 2020 06:40:17 +0000
Subject: [PATCH] Add fallible box APIs (`Box::try_new_*`)

---
 library/alloc/src/boxed.rs | 74 +++++++++++++++++++++++++++++++++++++-
 library/alloc/src/rc.rs    | 22 ++++++++++++
 2 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index bc56c7a3c4f..eb00d1a8b47 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -241,6 +241,77 @@ impl<T> Box<T> {
     pub fn pin(x: T) -> Pin<Box<T>> {
         (box x).into()
     }
+
+    /// Allocates memory on the heap then places `x` into it,
+    /// returning an error if the allocation fails
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api)]
+    ///
+    /// let five = Box::try_new(5)?;
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    #[inline]
+    pub fn try_new(x: T) -> Result<Self, AllocError> {
+        Self::try_new_in(x, Global)
+    }
+
+    /// Constructs a new box with uninitialized contents on the heap,
+    /// returning an error if the allocation fails
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    ///
+    /// let mut five = Box::<u32>::try_new_uninit()?;
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     five.as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    // #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
+        Box::try_new_uninit_in(Global)
+    }
+
+    /// Constructs a new `Box` with uninitialized contents, with the memory
+    /// being filled with `0` bytes on the heap
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
+    /// of this method.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    ///
+    /// let zero = Box::<u32>::try_new_zeroed()?;
+    /// let zero = unsafe { zero.assume_init() };
+    ///
+    /// assert_eq!(*zero, 0);
+    /// # Ok::<(), std::alloc::AllocError>(())
+    /// ```
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
+    #[unstable(feature = "allocator_api", issue = "32838")]
+    // #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
+        Box::try_new_zeroed_in(Global)
+    }
 }
 
 impl<T, A: Allocator> Box<T, A> {
@@ -380,7 +451,8 @@ impl<T, A: Allocator> Box<T, A> {
     }
 
     /// Constructs a new `Box` with uninitialized contents, with the memory
-    /// being filled with `0` bytes in the provided allocator.
+    /// being filled with `0` bytes in the provided allocator,
+    /// returning an error if the allocation fails,
     ///
     /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
     /// of this method.
diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
index a96be57143d..739ffd31f60 100644
--- a/library/alloc/src/rc.rs
+++ b/library/alloc/src/rc.rs
@@ -346,6 +346,28 @@ impl<T> Rc<T> {
         )
     }
 
+    /// Constructs a new `Rc<T>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(allocator_api, new_uninit)]
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn try_new(value: T) -> Result<Rc<T>, AllocError> {
+        // There is an implicit weak pointer owned by all the strong
+        // pointers, which ensures that the weak destructor never frees
+        // the allocation while the strong destructor is running, even
+        // if the weak pointer is stored inside the strong one.
+        Ok(Self::from_inner(
+            Box::leak(Box::try_new(RcBox { strong: Cell::new(1), weak: Cell::new(1), value })?)
+                .into(),
+        ))
+    }
+
     /// Constructs a new `Rc<T>` using a weak reference to itself. Attempting
     /// to upgrade the weak reference before this function returns will result
     /// in a `None` value. However, the weak reference may be cloned freely and