Rollup merge of #62451 - SimonSapin:new_uninit, r=RalfJung

Add APIs for uninitialized Box, Rc, and Arc. (Plus get_mut_unchecked)

Assigning `MaybeUninit::<Foo>::uninit()` to a local variable is usually free, even when `size_of::<Foo>()` is large. However, passing it for example to `Arc::new` [causes at least one copy](https://youtu.be/F1AquroPfcI?t=4116) (from the stack to the newly allocated heap memory) even though there is no meaningful data. It is theoretically possible that a Sufficiently Advanced Compiler could optimize this copy away, but this is [reportedly unlikely to happen soon in LLVM](https://youtu.be/F1AquroPfcI?t=5431).

This PR proposes two sets of features:

* Constructors for containers (`Box`, `Rc`, `Arc`) of `MaybeUninit<T>` or `[MaybeUninit<T>]` that do not initialized the data, and unsafe conversions to the known-initialized types (without `MaybeUninit`). The constructors are guaranteed not to make unnecessary copies.

* On `Rc` and `Arc`, an unsafe `get_mut_unchecked` method that provides `&mut T` access without checking the reference count. `Arc::get_mut` involves multiple atomic operations whose cost can be non-trivial. `Rc::get_mut` is less costly, but we add `Rc::get_mut_unchecked` anyway for symmetry with `Arc`.

  These can be useful independently, but they will presumably be typical when the new constructors of `Rc` and `Arc` are used.

  An alternative with a safe API would be to introduce `UniqueRc` and `UniqueArc` types that have the same memory layout as `Rc` and `Arc` (and so zero-cost conversion to them) but are guaranteed to have only one reference. But introducing entire new types feels “heavier” than new constructors on existing types, and initialization of `MaybeUninit<T>` typically requires unsafe code anyway.

Summary of new APIs (all unstable in this PR):

```rust
impl<T> Box<T> { pub fn new_uninit() -> Box<MaybeUninit<T>> {…} }
impl<T> Box<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Box<T> {…} }
impl<T> Box<[T]> { pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]> {…} }
impl<T> Box<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Box<[T]> {…} }

impl<T> Rc<T> { pub fn new_uninit() -> Rc<MaybeUninit<T>> {…} }
impl<T> Rc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Rc<T> {…} }
impl<T> Rc<[T]> { pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]> {…} }
impl<T> Rc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Rc<[T]> {…} }

impl<T> Arc<T> { pub fn new_uninit() -> Arc<MaybeUninit<T>> {…} }
impl<T> Arc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Arc<T> {…} }
impl<T> Arc<[T]> { pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]> {…} }
impl<T> Arc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Arc<[T]> {…} }

impl<T: ?Sized> Rc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
impl<T: ?Sized> Arc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} }
```

3 files changed, 497 insertions, 10 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/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/sync.rs b/src/liballoc/sync.rs
index 7d3b2656a7b..341172136e2 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -311,6 +311,37 @@ 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 +392,118 @@ 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 +736,11 @@ 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 +768,7 @@ 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 +799,7 @@ 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 +1088,46 @@ 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.
     ///