Merge branch 'master' into bpang-runtest

15 files changed, 630 insertions, 138 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index c92db517cad..c61e3183409 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -91,8 +91,10 @@ use core::ops::{
     CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Receiver, Generator, GeneratorState
 };
 use core::ptr::{self, NonNull, Unique};
+use core::slice;
 use core::task::{Context, Poll};
 
+use crate::alloc::{self, Global, Alloc};
 use crate::vec::Vec;
 use crate::raw_vec::RawVec;
 use crate::str::from_boxed_utf8_unchecked;
@@ -121,6 +123,34 @@ impl<T> Box<T> {
         box x
     }
 
+    /// Constructs a new box with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut five = Box::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     five.as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
+        let layout = alloc::Layout::new::<mem::MaybeUninit<T>>();
+        let ptr = unsafe {
+            Global.alloc(layout)
+                .unwrap_or_else(|_| alloc::handle_alloc_error(layout))
+        };
+        Box(ptr.cast().into())
+    }
+
     /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
     /// `x` will be pinned in memory and unable to be moved.
     #[stable(feature = "pin", since = "1.33.0")]
@@ -130,6 +160,111 @@ impl<T> Box<T> {
     }
 }
 
+impl<T> Box<[T]> {
+    /// Constructs a new boxed slice with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     values[0].as_mut_ptr().write(1);
+    ///     values[1].as_mut_ptr().write(2);
+    ///     values[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
+        let layout = alloc::Layout::array::<mem::MaybeUninit<T>>(len).unwrap();
+        let ptr = unsafe { alloc::alloc(layout) };
+        let unique = Unique::new(ptr).unwrap_or_else(|| alloc::handle_alloc_error(layout));
+        let slice = unsafe { slice::from_raw_parts_mut(unique.cast().as_ptr(), len) };
+        Box(Unique::from(slice))
+    }
+}
+
+impl<T> Box<mem::MaybeUninit<T>> {
+    /// Converts to `Box<T>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut five = Box::<u32>::new_uninit();
+    ///
+    /// let five: Box<u32> = unsafe {
+    ///     // Deferred initialization:
+    ///     five.as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Box<T> {
+        Box(Box::into_unique(self).cast())
+    }
+}
+
+impl<T> Box<[mem::MaybeUninit<T>]> {
+    /// Converts to `Box<[T]>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the values
+    /// really are in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    ///
+    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     values[0].as_mut_ptr().write(1);
+    ///     values[1].as_mut_ptr().write(2);
+    ///     values[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Box<[T]> {
+        Box(Unique::new_unchecked(Box::into_raw(self) as _))
+    }
+}
+
 impl<T: ?Sized> Box<T> {
     /// Constructs a box from a raw pointer.
     ///
diff --git a/src/liballoc/collections/binary_heap.rs b/src/liballoc/collections/binary_heap.rs
index 9f531f5b83c..3d04f30e7bd 100644
--- a/src/liballoc/collections/binary_heap.rs
+++ b/src/liballoc/collections/binary_heap.rs
@@ -1163,6 +1163,9 @@ impl<T> FusedIterator for Drain<'_, T> {}
 
 #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
 impl<T: Ord> From<Vec<T>> for BinaryHeap<T> {
+    /// Converts a `Vec<T>` into a `BinaryHeap<T>`.
+    ///
+    /// This conversion happens in-place, and has `O(n)` time complexity.
     fn from(vec: Vec<T>) -> BinaryHeap<T> {
         let mut heap = BinaryHeap { data: vec };
         heap.rebuild();
diff --git a/src/liballoc/collections/btree/node.rs b/src/liballoc/collections/btree/node.rs
index e067096f0c7..0b5a271dbea 100644
--- a/src/liballoc/collections/btree/node.rs
+++ b/src/liballoc/collections/btree/node.rs
@@ -106,8 +106,8 @@ impl<K, V> LeafNode<K, V> {
         LeafNode {
             // As a general policy, we leave fields uninitialized if they can be, as this should
             // be both slightly faster and easier to track in Valgrind.
-            keys: uninit_array![_; CAPACITY],
-            vals: uninit_array![_; CAPACITY],
+            keys: [MaybeUninit::UNINIT; CAPACITY],
+            vals: [MaybeUninit::UNINIT; CAPACITY],
             parent: ptr::null(),
             parent_idx: MaybeUninit::uninit(),
             len: 0
@@ -159,7 +159,7 @@ impl<K, V> InternalNode<K, V> {
     unsafe fn new() -> Self {
         InternalNode {
             data: LeafNode::new(),
-            edges: uninit_array![_; 2*B],
+            edges: [MaybeUninit::UNINIT; 2*B]
         }
     }
 }
diff --git a/src/liballoc/collections/mod.rs b/src/liballoc/collections/mod.rs
index 5a33ddc14f0..f1f22fe48c5 100644
--- a/src/liballoc/collections/mod.rs
+++ b/src/liballoc/collections/mod.rs
@@ -41,32 +41,35 @@ pub use linked_list::LinkedList;
 #[doc(no_inline)]
 pub use vec_deque::VecDeque;
 
-use crate::alloc::{AllocErr, LayoutErr};
+use crate::alloc::{Layout, LayoutErr};
 
-/// Augments `AllocErr` with a CapacityOverflow variant.
+/// The error type for `try_reserve` methods.
 #[derive(Clone, PartialEq, Eq, Debug)]
 #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-pub enum CollectionAllocErr {
+pub enum TryReserveError {
     /// Error due to the computed capacity exceeding the collection's maximum
     /// (usually `isize::MAX` bytes).
     CapacityOverflow,
-    /// Error due to the allocator (see the `AllocErr` type's docs).
-    AllocErr,
-}
 
-#[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-impl From<AllocErr> for CollectionAllocErr {
-    #[inline]
-    fn from(AllocErr: AllocErr) -> Self {
-        CollectionAllocErr::AllocErr
-    }
+    /// The memory allocator returned an error
+    AllocError {
+        /// The layout of allocation request that failed
+        layout: Layout,
+
+        #[doc(hidden)]
+        #[unstable(feature = "container_error_extra", issue = "0", reason = "\
+            Enable exposing the allocator’s custom error value \
+            if an associated type is added in the future: \
+            https://github.com/rust-lang/wg-allocators/issues/23")]
+        non_exhaustive: (),
+    },
 }
 
 #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-impl From<LayoutErr> for CollectionAllocErr {
+impl From<LayoutErr> for TryReserveError {
     #[inline]
     fn from(_: LayoutErr) -> Self {
-        CollectionAllocErr::CapacityOverflow
+        TryReserveError::CapacityOverflow
     }
 }
 
diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index 9240346ace9..7315963cc8b 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -18,7 +18,7 @@ use core::ptr::{self, NonNull};
 use core::slice;
 use core::hash::{Hash, Hasher};
 
-use crate::collections::CollectionAllocErr;
+use crate::collections::TryReserveError;
 use crate::raw_vec::RawVec;
 use crate::vec::Vec;
 
@@ -576,10 +576,10 @@ impl<T> VecDeque<T> {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     /// use std::collections::VecDeque;
     ///
-    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, CollectionAllocErr> {
+    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
     ///     let mut output = VecDeque::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -595,7 +595,7 @@ impl<T> VecDeque<T> {
     /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError>  {
         self.try_reserve(additional)
     }
 
@@ -614,10 +614,10 @@ impl<T> VecDeque<T> {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     /// use std::collections::VecDeque;
     ///
-    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, CollectionAllocErr> {
+    /// fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
     ///     let mut output = VecDeque::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -633,12 +633,12 @@ impl<T> VecDeque<T> {
     /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
         let old_cap = self.cap();
         let used_cap = self.len() + 1;
         let new_cap = used_cap.checked_add(additional)
             .and_then(|needed_cap| needed_cap.checked_next_power_of_two())
-            .ok_or(CollectionAllocErr::CapacityOverflow)?;
+            .ok_or(TryReserveError::CapacityOverflow)?;
 
         if new_cap > old_cap {
             self.buf.try_reserve_exact(used_cap, new_cap - used_cap)?;
@@ -1199,6 +1199,31 @@ impl<T> VecDeque<T> {
         }
     }
 
+    /// Removes the last element from the `VecDeque` and returns it, or `None` if
+    /// it is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// assert_eq!(buf.pop_back(), None);
+    /// buf.push_back(1);
+    /// buf.push_back(3);
+    /// assert_eq!(buf.pop_back(), Some(3));
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn pop_back(&mut self) -> Option<T> {
+        if self.is_empty() {
+            None
+        } else {
+            self.head = self.wrap_sub(self.head, 1);
+            let head = self.head;
+            unsafe { Some(self.buffer_read(head)) }
+        }
+    }
+
     /// Prepends an element to the `VecDeque`.
     ///
     /// # Examples
@@ -1243,38 +1268,13 @@ impl<T> VecDeque<T> {
         unsafe { self.buffer_write(head, value) }
     }
 
-    /// Removes the last element from the `VecDeque` and returns it, or `None` if
-    /// it is empty.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use std::collections::VecDeque;
-    ///
-    /// let mut buf = VecDeque::new();
-    /// assert_eq!(buf.pop_back(), None);
-    /// buf.push_back(1);
-    /// buf.push_back(3);
-    /// assert_eq!(buf.pop_back(), Some(3));
-    /// ```
-    #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn pop_back(&mut self) -> Option<T> {
-        if self.is_empty() {
-            None
-        } else {
-            self.head = self.wrap_sub(self.head, 1);
-            let head = self.head;
-            unsafe { Some(self.buffer_read(head)) }
-        }
-    }
-
     #[inline]
     fn is_contiguous(&self) -> bool {
         self.tail <= self.head
     }
 
-    /// Removes an element from anywhere in the `VecDeque` and returns it, replacing it with the
-    /// last element.
+    /// Removes an element from anywhere in the `VecDeque` and returns it,
+    /// replacing it with the first element.
     ///
     /// This does not preserve ordering, but is O(1).
     ///
@@ -1288,28 +1288,28 @@ impl<T> VecDeque<T> {
     /// use std::collections::VecDeque;
     ///
     /// let mut buf = VecDeque::new();
-    /// assert_eq!(buf.swap_remove_back(0), None);
+    /// assert_eq!(buf.swap_remove_front(0), None);
     /// buf.push_back(1);
     /// buf.push_back(2);
     /// buf.push_back(3);
     /// assert_eq!(buf, [1, 2, 3]);
     ///
-    /// assert_eq!(buf.swap_remove_back(0), Some(1));
-    /// assert_eq!(buf, [3, 2]);
+    /// assert_eq!(buf.swap_remove_front(2), Some(3));
+    /// assert_eq!(buf, [2, 1]);
     /// ```
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
-    pub fn swap_remove_back(&mut self, index: usize) -> Option<T> {
+    pub fn swap_remove_front(&mut self, index: usize) -> Option<T> {
         let length = self.len();
-        if length > 0 && index < length - 1 {
-            self.swap(index, length - 1);
+        if length > 0 && index < length && index != 0 {
+            self.swap(index, 0);
         } else if index >= length {
             return None;
         }
-        self.pop_back()
+        self.pop_front()
     }
 
-    /// Removes an element from anywhere in the `VecDeque` and returns it,
-    /// replacing it with the first element.
+    /// Removes an element from anywhere in the `VecDeque` and returns it, replacing it with the
+    /// last element.
     ///
     /// This does not preserve ordering, but is O(1).
     ///
@@ -1323,24 +1323,24 @@ impl<T> VecDeque<T> {
     /// use std::collections::VecDeque;
     ///
     /// let mut buf = VecDeque::new();
-    /// assert_eq!(buf.swap_remove_front(0), None);
+    /// assert_eq!(buf.swap_remove_back(0), None);
     /// buf.push_back(1);
     /// buf.push_back(2);
     /// buf.push_back(3);
     /// assert_eq!(buf, [1, 2, 3]);
     ///
-    /// assert_eq!(buf.swap_remove_front(2), Some(3));
-    /// assert_eq!(buf, [2, 1]);
+    /// assert_eq!(buf.swap_remove_back(0), Some(1));
+    /// assert_eq!(buf, [3, 2]);
     /// ```
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
-    pub fn swap_remove_front(&mut self, index: usize) -> Option<T> {
+    pub fn swap_remove_back(&mut self, index: usize) -> Option<T> {
         let length = self.len();
-        if length > 0 && index < length && index != 0 {
-            self.swap(index, 0);
+        if length > 0 && index < length - 1 {
+            self.swap(index, length - 1);
         } else if index >= length {
             return None;
         }
-        self.pop_front()
+        self.pop_back()
     }
 
     /// Inserts an element at `index` within the `VecDeque`, shifting all elements with indices
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index a1936b36ac6..4a48945adc3 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -69,7 +69,7 @@
 #![warn(missing_debug_implementations)]
 #![deny(intra_doc_link_resolution_failure)] // rustdoc is run without -D warnings
 #![allow(explicit_outlives_requirements)]
-#![cfg_attr(not(bootstrap), allow(incomplete_features))]
+#![allow(incomplete_features)]
 
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(test))]
@@ -84,9 +84,10 @@
 #![feature(coerce_unsized)]
 #![feature(const_generic_impls_guard)]
 #![feature(const_generics)]
-#![cfg_attr(not(bootstrap), feature(const_in_array_repeat_expressions))]
+#![feature(const_in_array_repeat_expressions)]
 #![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
+#![feature(container_error_extra)]
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]
 #![feature(fmt_internals)]
@@ -118,7 +119,7 @@
 #![feature(rustc_const_unstable)]
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
-#![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_array)]
+#![feature(maybe_uninit_extra, maybe_uninit_slice)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
 #![feature(mem_take)]
diff --git a/src/liballoc/macros.rs b/src/liballoc/macros.rs
index 250c419c531..0b5e186d4c7 100644
--- a/src/liballoc/macros.rs
+++ b/src/liballoc/macros.rs
@@ -98,5 +98,5 @@ macro_rules! vec {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! format {
-    ($($arg:tt)*) => ($crate::fmt::format(format_args!($($arg)*)))
+    ($($arg:tt)*) => ($crate::fmt::format(::core::format_args!($($arg)*)))
 }
diff --git a/src/liballoc/raw_vec.rs b/src/liballoc/raw_vec.rs
index 0abab45e920..bc8a38f6b3a 100644
--- a/src/liballoc/raw_vec.rs
+++ b/src/liballoc/raw_vec.rs
@@ -7,8 +7,8 @@ use core::ops::Drop;
 use core::ptr::{self, NonNull, Unique};
 use core::slice;
 
-use crate::alloc::{Alloc, Layout, Global, handle_alloc_error};
-use crate::collections::CollectionAllocErr::{self, *};
+use crate::alloc::{Alloc, Layout, Global, AllocErr, handle_alloc_error};
+use crate::collections::TryReserveError::{self, *};
 use crate::boxed::Box;
 
 #[cfg(test)]
@@ -385,7 +385,7 @@ impl<T, A: Alloc> RawVec<T, A> {
 
     /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
     pub fn try_reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize)
-           -> Result<(), CollectionAllocErr> {
+           -> Result<(), TryReserveError> {
 
         self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
     }
@@ -413,7 +413,7 @@ impl<T, A: Alloc> RawVec<T, A> {
     pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
         match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
             Err(CapacityOverflow) => capacity_overflow(),
-            Err(AllocErr) => unreachable!(),
+            Err(AllocError { .. }) => unreachable!(),
             Ok(()) => { /* yay */ }
          }
      }
@@ -422,7 +422,7 @@ impl<T, A: Alloc> RawVec<T, A> {
     /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
     /// Returns `(new_capacity, new_alloc_size)`.
     fn amortized_new_size(&self, used_capacity: usize, needed_extra_capacity: usize)
-        -> Result<usize, CollectionAllocErr> {
+        -> Result<usize, TryReserveError> {
 
         // Nothing we can really do about these checks :(
         let required_cap = used_capacity.checked_add(needed_extra_capacity)
@@ -435,7 +435,7 @@ impl<T, A: Alloc> RawVec<T, A> {
 
     /// The same as `reserve`, but returns on errors instead of panicking or aborting.
     pub fn try_reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize)
-        -> Result<(), CollectionAllocErr> {
+        -> Result<(), TryReserveError> {
         self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
     }
 
@@ -494,7 +494,7 @@ impl<T, A: Alloc> RawVec<T, A> {
     pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
         match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
             Err(CapacityOverflow) => capacity_overflow(),
-            Err(AllocErr) => unreachable!(),
+            Err(AllocError { .. }) => unreachable!(),
             Ok(()) => { /* yay */ }
         }
     }
@@ -640,10 +640,8 @@ impl<T, A: Alloc> RawVec<T, A> {
         needed_extra_capacity: usize,
         fallibility: Fallibility,
         strategy: ReserveStrategy,
-    ) -> Result<(), CollectionAllocErr> {
+    ) -> Result<(), TryReserveError> {
         unsafe {
-            use crate::alloc::AllocErr;
-
             // NOTE: we don't early branch on ZSTs here because we want this
             // to actually catch "asking for more than usize::MAX" in that case.
             // If we make it past the first branch then we are guaranteed to
@@ -672,12 +670,16 @@ impl<T, A: Alloc> RawVec<T, A> {
                 None => self.a.alloc(new_layout),
             };
 
-            match (&res, fallibility) {
+            let ptr = match (res, fallibility) {
                 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
-                _ => {}
-            }
+                (Err(AllocErr), Fallible) => return Err(TryReserveError::AllocError {
+                    layout: new_layout,
+                    non_exhaustive: (),
+                }),
+                (Ok(ptr), _) => ptr,
+            };
 
-            self.ptr = res?.cast().into();
+            self.ptr = ptr.cast().into();
             self.cap = new_cap;
 
             Ok(())
@@ -737,7 +739,7 @@ unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
 // all 4GB in user-space. e.g., PAE or x32
 
 #[inline]
-fn alloc_guard(alloc_size: usize) -> Result<(), CollectionAllocErr> {
+fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
     if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
         Err(CapacityOverflow)
     } else {
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 0c406a92029..2b222caf13f 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -327,6 +327,37 @@ impl<T> Rc<T> {
         }))
     }
 
+    /// Constructs a new `Rc` with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let mut five = Rc::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit() -> Rc<mem::MaybeUninit<T>> {
+        unsafe {
+            Rc::from_ptr(Rc::allocate_for_layout(
+                Layout::new::<T>(),
+                |mem| mem as *mut RcBox<mem::MaybeUninit<T>>,
+            ))
+        }
+    }
+
     /// Constructs a new `Pin<Rc<T>>`. If `T` does not implement `Unpin`, then
     /// `value` will be pinned in memory and unable to be moved.
     #[stable(feature = "pin", since = "1.33.0")]
@@ -377,6 +408,118 @@ impl<T> Rc<T> {
     }
 }
 
+impl<T> Rc<[T]> {
+    /// Constructs a new reference-counted slice with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
+    ///     Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
+    ///     Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit_slice(len: usize) -> Rc<[mem::MaybeUninit<T>]> {
+        unsafe {
+            Rc::from_ptr(Rc::allocate_for_slice(len))
+        }
+    }
+}
+
+impl<T> Rc<mem::MaybeUninit<T>> {
+    /// Converts to `Rc<T>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let mut five = Rc::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     Rc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Rc<T> {
+        Rc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
+    }
+}
+
+impl<T> Rc<[mem::MaybeUninit<T>]> {
+    /// Converts to `Rc<[T]>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     Rc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
+    ///     Rc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
+    ///     Rc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Rc<[T]> {
+        Rc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
+    }
+}
+
 impl<T: ?Sized> Rc<T> {
     /// Consumes the `Rc`, returning the wrapped pointer.
     ///
@@ -560,13 +703,46 @@ impl<T: ?Sized> Rc<T> {
     pub fn get_mut(this: &mut Self) -> Option<&mut T> {
         if Rc::is_unique(this) {
             unsafe {
-                Some(&mut this.ptr.as_mut().value)
+                Some(Rc::get_mut_unchecked(this))
             }
         } else {
             None
         }
     }
 
+    /// Returns a mutable reference to the inner value,
+    /// without any check.
+    ///
+    /// See also [`get_mut`], which is safe and does appropriate checks.
+    ///
+    /// [`get_mut`]: struct.Rc.html#method.get_mut
+    ///
+    /// # Safety
+    ///
+    /// Any other `Rc` or [`Weak`] pointers to the same value must not be dereferenced
+    /// for the duration of the returned borrow.
+    /// This is trivially the case if no such pointers exist,
+    /// for example immediately after `Rc::new`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let mut x = Rc::new(String::new());
+    /// unsafe {
+    ///     Rc::get_mut_unchecked(&mut x).push_str("foo")
+    /// }
+    /// assert_eq!(*x, "foo");
+    /// ```
+    #[inline]
+    #[unstable(feature = "get_mut_unchecked", issue = "63292")]
+    pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
+        &mut this.ptr.as_mut().value
+    }
+
     #[inline]
     #[stable(feature = "ptr_eq", since = "1.17.0")]
     /// Returns `true` if the two `Rc`s point to the same value (not
@@ -704,11 +880,11 @@ impl Rc<dyn Any> {
 
 impl<T: ?Sized> Rc<T> {
     /// Allocates an `RcBox<T>` with sufficient space for
-    /// an unsized value where the value has the layout provided.
+    /// a possibly-unsized value where the value has the layout provided.
     ///
     /// The function `mem_to_rcbox` is called with the data pointer
     /// and must return back a (potentially fat)-pointer for the `RcBox<T>`.
-    unsafe fn allocate_for_unsized(
+    unsafe fn allocate_for_layout(
         value_layout: Layout,
         mem_to_rcbox: impl FnOnce(*mut u8) -> *mut RcBox<T>
     ) -> *mut RcBox<T> {
@@ -737,7 +913,7 @@ impl<T: ?Sized> Rc<T> {
     /// Allocates an `RcBox<T>` with sufficient space for an unsized value
     unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
         // Allocate for the `RcBox<T>` using the given value.
-        Self::allocate_for_unsized(
+        Self::allocate_for_layout(
             Layout::for_value(&*ptr),
             |mem| set_data_ptr(ptr as *mut T, mem) as *mut RcBox<T>,
         )
@@ -768,7 +944,7 @@ impl<T: ?Sized> Rc<T> {
 impl<T> Rc<[T]> {
     /// Allocates an `RcBox<[T]>` with the given length.
     unsafe fn allocate_for_slice(len: usize) -> *mut RcBox<[T]> {
-        Self::allocate_for_unsized(
+        Self::allocate_for_layout(
             Layout::array::<T>(len).unwrap(),
             |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut RcBox<[T]>,
         )
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index eca726cd410..b65f191836e 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -56,7 +56,7 @@ use core::ptr;
 use core::str::{pattern::Pattern, lossy};
 
 use crate::borrow::{Cow, ToOwned};
-use crate::collections::CollectionAllocErr;
+use crate::collections::TryReserveError;
 use crate::boxed::Box;
 use crate::str::{self, from_boxed_utf8_unchecked, FromStr, Utf8Error, Chars};
 use crate::vec::Vec;
@@ -937,9 +937,9 @@ impl String {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     ///
-    /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
+    /// fn process_data(data: &str) -> Result<String, TryReserveError> {
     ///     let mut output = String::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -953,7 +953,7 @@ impl String {
     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
         self.vec.try_reserve(additional)
     }
 
@@ -975,9 +975,9 @@ impl String {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     ///
-    /// fn process_data(data: &str) -> Result<String, CollectionAllocErr> {
+    /// fn process_data(data: &str) -> Result<String, TryReserveError> {
     ///     let mut output = String::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -991,7 +991,7 @@ impl String {
     /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError>  {
         self.vec.try_reserve_exact(additional)
     }
 
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 7d3b2656a7b..9ffc1673e5a 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -107,10 +107,6 @@ const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 /// // a, b, and foo are all Arcs that point to the same memory location
 /// ```
 ///
-/// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
-/// the meaning of the code. In the example above, this syntax makes it easier to see that
-/// this code is creating a new reference rather than copying the whole content of foo.
-///
 /// ## `Deref` behavior
 ///
 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
@@ -311,6 +307,37 @@ impl<T> Arc<T> {
         Self::from_inner(Box::into_raw_non_null(x))
     }
 
+    /// Constructs a new `Arc` with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let mut five = Arc::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
+        unsafe {
+            Arc::from_ptr(Arc::allocate_for_layout(
+                Layout::new::<T>(),
+                |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
+            ))
+        }
+    }
+
     /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
     /// `data` will be pinned in memory and unable to be moved.
     #[stable(feature = "pin", since = "1.33.0")]
@@ -361,6 +388,118 @@ impl<T> Arc<T> {
     }
 }
 
+impl<T> Arc<[T]> {
+    /// Constructs a new reference-counted slice with uninitialized contents.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
+    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
+    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
+        unsafe {
+            Arc::from_ptr(Arc::allocate_for_slice(len))
+        }
+    }
+}
+
+impl<T> Arc<mem::MaybeUninit<T>> {
+    /// Converts to `Arc<T>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let mut five = Arc::<u32>::new_uninit();
+    ///
+    /// let five = unsafe {
+    ///     // Deferred initialization:
+    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
+    ///
+    ///     five.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*five, 5)
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Arc<T> {
+        Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
+    }
+}
+
+impl<T> Arc<[mem::MaybeUninit<T>]> {
+    /// Converts to `Arc<[T]>`.
+    ///
+    /// # Safety
+    ///
+    /// As with [`MaybeUninit::assume_init`],
+    /// it is up to the caller to guarantee that the value
+    /// really is in an initialized state.
+    /// Calling this when the content is not yet fully initialized
+    /// causes immediate undefined behavior.
+    ///
+    /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(new_uninit)]
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
+    ///
+    /// let values = unsafe {
+    ///     // Deferred initialization:
+    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
+    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
+    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
+    ///
+    ///     values.assume_init()
+    /// };
+    ///
+    /// assert_eq!(*values, [1, 2, 3])
+    /// ```
+    #[unstable(feature = "new_uninit", issue = "63291")]
+    #[inline]
+    pub unsafe fn assume_init(self) -> Arc<[T]> {
+        Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
+    }
+}
+
 impl<T: ?Sized> Arc<T> {
     /// Consumes the `Arc`, returning the wrapped pointer.
     ///
@@ -593,11 +732,11 @@ impl<T: ?Sized> Arc<T> {
 
 impl<T: ?Sized> Arc<T> {
     /// Allocates an `ArcInner<T>` with sufficient space for
-    /// an unsized value where the value has the layout provided.
+    /// a possibly-unsized value where the value has the layout provided.
     ///
     /// The function `mem_to_arcinner` is called with the data pointer
     /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
-    unsafe fn allocate_for_unsized(
+    unsafe fn allocate_for_layout(
         value_layout: Layout,
         mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>
     ) -> *mut ArcInner<T> {
@@ -625,7 +764,7 @@ impl<T: ?Sized> Arc<T> {
     /// Allocates an `ArcInner<T>` with sufficient space for an unsized value.
     unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
         // Allocate for the `ArcInner<T>` using the given value.
-        Self::allocate_for_unsized(
+        Self::allocate_for_layout(
             Layout::for_value(&*ptr),
             |mem| set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>,
         )
@@ -656,7 +795,7 @@ impl<T: ?Sized> Arc<T> {
 impl<T> Arc<[T]> {
     /// Allocates an `ArcInner<[T]>` with the given length.
     unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
-        Self::allocate_for_unsized(
+        Self::allocate_for_layout(
             Layout::array::<T>(len).unwrap(),
             |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
         )
@@ -945,13 +1084,46 @@ impl<T: ?Sized> Arc<T> {
             // the Arc itself to be `mut`, so we're returning the only possible
             // reference to the inner data.
             unsafe {
-                Some(&mut this.ptr.as_mut().data)
+                Some(Arc::get_mut_unchecked(this))
             }
         } else {
             None
         }
     }
 
+    /// Returns a mutable reference to the inner value,
+    /// without any check.
+    ///
+    /// See also [`get_mut`], which is safe and does appropriate checks.
+    ///
+    /// [`get_mut`]: struct.Arc.html#method.get_mut
+    ///
+    /// # Safety
+    ///
+    /// Any other `Arc` or [`Weak`] pointers to the same value must not be dereferenced
+    /// for the duration of the returned borrow.
+    /// This is trivially the case if no such pointers exist,
+    /// for example immediately after `Arc::new`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(get_mut_unchecked)]
+    ///
+    /// use std::sync::Arc;
+    ///
+    /// let mut x = Arc::new(String::new());
+    /// unsafe {
+    ///     Arc::get_mut_unchecked(&mut x).push_str("foo")
+    /// }
+    /// assert_eq!(*x, "foo");
+    /// ```
+    #[inline]
+    #[unstable(feature = "get_mut_unchecked", issue = "63292")]
+    pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
+        &mut this.ptr.as_mut().data
+    }
+
     /// Determine whether this is the unique reference (including weak refs) to
     /// the underlying data.
     ///
diff --git a/src/liballoc/tests/string.rs b/src/liballoc/tests/string.rs
index 765210e5aa6..55edf56345b 100644
--- a/src/liballoc/tests/string.rs
+++ b/src/liballoc/tests/string.rs
@@ -1,5 +1,5 @@
 use std::borrow::Cow;
-use std::collections::CollectionAllocErr::*;
+use std::collections::TryReserveError::*;
 use std::mem::size_of;
 use std::{usize, isize};
 
@@ -566,11 +566,11 @@ fn test_try_reserve() {
             } else { panic!("usize::MAX should trigger an overflow!") }
         } else {
             // Check isize::MAX + 1 is an OOM
-            if let Err(AllocErr) = empty_string.try_reserve(MAX_CAP + 1) {
+            if let Err(AllocError { .. }) = empty_string.try_reserve(MAX_CAP + 1) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
 
             // Check usize::MAX is an OOM
-            if let Err(AllocErr) = empty_string.try_reserve(MAX_USIZE) {
+            if let Err(AllocError { .. }) = empty_string.try_reserve(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an OOM!") }
         }
     }
@@ -590,7 +590,7 @@ fn test_try_reserve() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         // Should always overflow in the add-to-len
@@ -629,10 +629,10 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = empty_string.try_reserve_exact(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an overflow!") }
         } else {
-            if let Err(AllocErr) = empty_string.try_reserve_exact(MAX_CAP + 1) {
+            if let Err(AllocError { .. }) = empty_string.try_reserve_exact(MAX_CAP + 1) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
 
-            if let Err(AllocErr) = empty_string.try_reserve_exact(MAX_USIZE) {
+            if let Err(AllocError { .. }) = empty_string.try_reserve_exact(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an OOM!") }
         }
     }
@@ -651,7 +651,7 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_USIZE) {
diff --git a/src/liballoc/tests/vec.rs b/src/liballoc/tests/vec.rs
index 6e8ffe18522..29a22aa0315 100644
--- a/src/liballoc/tests/vec.rs
+++ b/src/liballoc/tests/vec.rs
@@ -2,7 +2,7 @@ use std::borrow::Cow;
 use std::mem::size_of;
 use std::{usize, isize};
 use std::vec::{Drain, IntoIter};
-use std::collections::CollectionAllocErr::*;
+use std::collections::TryReserveError::*;
 
 struct DropCounter<'a> {
     count: &'a mut u32,
@@ -1121,11 +1121,11 @@ fn test_try_reserve() {
             } else { panic!("usize::MAX should trigger an overflow!") }
         } else {
             // Check isize::MAX + 1 is an OOM
-            if let Err(AllocErr) = empty_bytes.try_reserve(MAX_CAP + 1) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve(MAX_CAP + 1) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
 
             // Check usize::MAX is an OOM
-            if let Err(AllocErr) = empty_bytes.try_reserve(MAX_USIZE) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an OOM!") }
         }
     }
@@ -1145,7 +1145,7 @@ fn test_try_reserve() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         // Should always overflow in the add-to-len
@@ -1168,7 +1168,7 @@ fn test_try_reserve() {
             if let Err(CapacityOverflow) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
+            if let Err(AllocError { .. }) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         // Should fail in the mul-by-size
@@ -1209,10 +1209,10 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = empty_bytes.try_reserve_exact(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an overflow!") }
         } else {
-            if let Err(AllocErr) = empty_bytes.try_reserve_exact(MAX_CAP + 1) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve_exact(MAX_CAP + 1) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
 
-            if let Err(AllocErr) = empty_bytes.try_reserve_exact(MAX_USIZE) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve_exact(MAX_USIZE) {
             } else { panic!("usize::MAX should trigger an OOM!") }
         }
     }
@@ -1231,7 +1231,7 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_USIZE) {
@@ -1252,7 +1252,7 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
+            if let Err(AllocError { .. }) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         if let Err(CapacityOverflow) = ten_u32s.try_reserve_exact(MAX_USIZE - 20) {
diff --git a/src/liballoc/tests/vec_deque.rs b/src/liballoc/tests/vec_deque.rs
index 1bbcca97b3c..d49b553fc02 100644
--- a/src/liballoc/tests/vec_deque.rs
+++ b/src/liballoc/tests/vec_deque.rs
@@ -1,6 +1,6 @@
 use std::fmt::Debug;
 use std::collections::{VecDeque, vec_deque::Drain};
-use std::collections::CollectionAllocErr::*;
+use std::collections::TryReserveError::*;
 use std::mem::size_of;
 use std::{usize, isize};
 
@@ -1168,7 +1168,7 @@ fn test_try_reserve() {
             // VecDeque starts with capacity 7, always adds 1 to the capacity
             // and also rounds the number to next power of 2 so this is the
             // furthest we can go without triggering CapacityOverflow
-            if let Err(AllocErr) = empty_bytes.try_reserve(MAX_CAP) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve(MAX_CAP) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
     }
@@ -1188,7 +1188,7 @@ fn test_try_reserve() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         // Should always overflow in the add-to-len
@@ -1211,7 +1211,7 @@ fn test_try_reserve() {
             if let Err(CapacityOverflow) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
+            if let Err(AllocError { .. }) = ten_u32s.try_reserve(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         // Should fail in the mul-by-size
@@ -1256,7 +1256,7 @@ fn test_try_reserve_exact() {
             // VecDeque starts with capacity 7, always adds 1 to the capacity
             // and also rounds the number to next power of 2 so this is the
             // furthest we can go without triggering CapacityOverflow
-            if let Err(AllocErr) = empty_bytes.try_reserve_exact(MAX_CAP) {
+            if let Err(AllocError { .. }) = empty_bytes.try_reserve_exact(MAX_CAP) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
     }
@@ -1275,7 +1275,7 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
+            if let Err(AllocError { .. }) = ten_bytes.try_reserve_exact(MAX_CAP - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         if let Err(CapacityOverflow) = ten_bytes.try_reserve_exact(MAX_USIZE) {
@@ -1296,7 +1296,7 @@ fn test_try_reserve_exact() {
             if let Err(CapacityOverflow) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an overflow!"); }
         } else {
-            if let Err(AllocErr) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
+            if let Err(AllocError { .. }) = ten_u32s.try_reserve_exact(MAX_CAP/4 - 9) {
             } else { panic!("isize::MAX + 1 should trigger an OOM!") }
         }
         if let Err(CapacityOverflow) = ten_u32s.try_reserve_exact(MAX_USIZE - 20) {
diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index dac04e4e624..d2798955c46 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -70,7 +70,7 @@ use core::ptr::{self, NonNull};
 use core::slice::{self, SliceIndex};
 
 use crate::borrow::{ToOwned, Cow};
-use crate::collections::CollectionAllocErr;
+use crate::collections::TryReserveError;
 use crate::boxed::Box;
 use crate::raw_vec::RawVec;
 
@@ -498,9 +498,9 @@ impl<T> Vec<T> {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     ///
-    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, CollectionAllocErr> {
+    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
     ///     let mut output = Vec::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -516,7 +516,7 @@ impl<T> Vec<T> {
     /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
         self.buf.try_reserve(self.len, additional)
     }
 
@@ -538,9 +538,9 @@ impl<T> Vec<T> {
     ///
     /// ```
     /// #![feature(try_reserve)]
-    /// use std::collections::CollectionAllocErr;
+    /// use std::collections::TryReserveError;
     ///
-    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, CollectionAllocErr> {
+    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
     ///     let mut output = Vec::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
@@ -556,7 +556,7 @@ impl<T> Vec<T> {
     /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
     /// ```
     #[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
-    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr>  {
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError>  {
         self.buf.try_reserve_exact(self.len, additional)
     }