Rollup merge of #91884 - woppopo:const_box, r=oli-obk

Constify `Box<T, A>` methods

Tracking issue: none yet

Most of the methods bounded on `~const`. `intrinsics::const_eval_select` is used for handling an allocation error.

<details><summary>Constified API</summary>

```rust
impl<T, A: Allocator> Box<T, A> {
    pub const fn new_in(x: T, alloc: A) -> Self
    where
        A: ~const Allocator + ~const Drop;
    pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
    where
        T: ~const Drop,
        A: ~const Allocator + ~const Drop;
    pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
    where
        A: ~const Allocator + ~const Drop;
    pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
    where
        A: ~const Allocator + ~const Drop;
    pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
    where
        A: ~const Allocator + ~const Drop;
    pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
    where
        A: ~const Allocator + ~const Drop;
    pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
    where
        A: 'static,
        A: 'static + ~const Allocator + ~const Drop,
    pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A>;
    pub const fn into_inner(boxed: Self) -> T
    where
        Self: ~const Drop,
}

impl<T, A: Allocator> Box<MaybeUninit<T>, A> {
    pub const unsafe fn assume_init(self) -> Box<T, A>;
    pub const fn write(mut boxed: Self, value: T) -> Box<T, A>;
    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self;
    pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A);
    pub const fn into_unique(b: Self) -> (Unique<T>, A);
    pub const fn allocator(b: &Self) -> &A;
    pub const fn leak<'a>(b: Self) -> &'a mut T
    where
        A: 'a;
    pub const fn into_pin(boxed: Self) -> Pin<Self>
    where
        A: 'static;
}

unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A>;
impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
where
    A: 'static;
impl<T: ?Sized, A: Allocator> const Deref for Box<T, A>;
impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A>;
impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static;
```

</details>

<details><summary>Example</summary>

```rust
pub struct ConstAllocator;

unsafe impl const Allocator for ConstAllocator {
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        unsafe {
            let ptr = core::intrinsics::const_allocate(layout.size(), layout.align());
            Ok(NonNull::new_unchecked(ptr as *mut [u8; 0] as *mut [u8]))
        }
    }

    unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {
        /* do nothing */
    }

    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        self.allocate(layout)
    }

    unsafe fn grow(
        &self,
        _ptr: NonNull<u8>,
        _old_layout: Layout,
        _new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        unimplemented!()
    }

    unsafe fn grow_zeroed(
        &self,
        _ptr: NonNull<u8>,
        _old_layout: Layout,
        _new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        unimplemented!()
    }

    unsafe fn shrink(
        &self,
        _ptr: NonNull<u8>,
        _old_layout: Layout,
        _new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        unimplemented!()
    }

    fn by_ref(&self) -> &Self
    where
        Self: Sized,
    {
        self
    }
}

#[test]
fn const_box() {
    const VALUE: u32 = {
        let mut boxed = Box::new_in(1u32, ConstAllocator);
        assert!(*boxed == 1);

        *boxed = 42;
        assert!(*boxed == 42);

        *boxed
    };

    assert!(VALUE == 42);
}
```

</details>

5 files changed, 225 insertions, 32 deletions

diff --git a/library/alloc/src/alloc.rs b/library/alloc/src/alloc.rs
index 66ef92558d8..d075658f51a 100644
--- a/library/alloc/src/alloc.rs
+++ b/library/alloc/src/alloc.rs
@@ -323,17 +323,21 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 
 #[cfg_attr(not(test), lang = "box_free")]
 #[inline]
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
 // This signature has to be the same as `Box`, otherwise an ICE will happen.
 // When an additional parameter to `Box` is added (like `A: Allocator`), this has to be added here as
 // well.
 // For example if `Box` is changed to  `struct Box<T: ?Sized, A: Allocator>(Unique<T>, A)`,
 // this function has to be changed to `fn box_free<T: ?Sized, A: Allocator>(Unique<T>, A)` as well.
-pub(crate) unsafe fn box_free<T: ?Sized, A: Allocator>(ptr: Unique<T>, alloc: A) {
+pub(crate) const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Drop>(
+    ptr: Unique<T>,
+    alloc: A,
+) {
     unsafe {
         let size = size_of_val(ptr.as_ref());
         let align = min_align_of_val(ptr.as_ref());
         let layout = Layout::from_size_align_unchecked(size, align);
-        alloc.deallocate(ptr.cast().into(), layout)
+        alloc.deallocate(From::from(ptr.cast()), layout)
     }
 }
 
@@ -361,13 +365,22 @@ extern "Rust" {
 /// [`set_alloc_error_hook`]: ../../std/alloc/fn.set_alloc_error_hook.html
 /// [`take_alloc_error_hook`]: ../../std/alloc/fn.take_alloc_error_hook.html
 #[stable(feature = "global_alloc", since = "1.28.0")]
+#[rustc_const_unstable(feature = "const_alloc_error", issue = "92523")]
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[rustc_allocator_nounwind]
 #[cold]
-pub fn handle_alloc_error(layout: Layout) -> ! {
-    unsafe {
-        __rust_alloc_error_handler(layout.size(), layout.align());
+pub const fn handle_alloc_error(layout: Layout) -> ! {
+    const fn ct_error(_: Layout) -> ! {
+        panic!("allocation failed");
     }
+
+    fn rt_error(layout: Layout) -> ! {
+        unsafe {
+            __rust_alloc_error_handler(layout.size(), layout.align());
+        }
+    }
+
+    unsafe { core::intrinsics::const_eval_select((layout,), ct_error, rt_error) }
 }
 
 // For alloc test `std::alloc::handle_alloc_error` can be used directly.
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index ab41f5646e5..aa7344ba405 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -346,9 +346,13 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline]
-    pub fn new_in(x: T, alloc: A) -> Self {
+    pub const fn new_in(x: T, alloc: A) -> Self
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let mut boxed = Self::new_uninit_in(alloc);
         unsafe {
             boxed.as_mut_ptr().write(x);
@@ -372,8 +376,13 @@ impl<T, A: Allocator> Box<T, A> {
     /// # Ok::<(), std::alloc::AllocError>(())
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError> {
+    pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
+    where
+        T: ~const Drop,
+        A: ~const Allocator + ~const Drop,
+    {
         let mut boxed = Self::try_new_uninit_in(alloc)?;
         unsafe {
             boxed.as_mut_ptr().write(x);
@@ -402,10 +411,14 @@ impl<T, A: Allocator> Box<T, A> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[cfg(not(no_global_oom_handling))]
     #[must_use]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
+    pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
         // That would make code size bigger.
@@ -439,7 +452,11 @@ impl<T, A: Allocator> Box<T, A> {
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate(layout)?.cast();
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
@@ -466,10 +483,14 @@ impl<T, A: Allocator> Box<T, A> {
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     #[must_use]
-    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
+    pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
         // That would make code size bigger.
@@ -503,7 +524,11 @@ impl<T, A: Allocator> Box<T, A> {
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
-    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
+    where
+        A: ~const Allocator + ~const Drop,
+    {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate_zeroed(layout)?.cast();
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
@@ -513,20 +538,22 @@ impl<T, A: Allocator> Box<T, A> {
     /// `x` will be pinned in memory and unable to be moved.
     #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline(always)]
-    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
+    pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
     where
-        A: 'static,
+        A: 'static + ~const Allocator + ~const Drop,
     {
-        Self::new_in(x, alloc).into()
+        Self::into_pin(Self::new_in(x, alloc))
     }
 
     /// Converts a `Box<T>` into a `Box<[T]>`
     ///
     /// This conversion does not allocate on the heap and happens in place.
     #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
-    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
         let (raw, alloc) = Box::into_raw_with_allocator(boxed);
         unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
     }
@@ -543,8 +570,12 @@ impl<T, A: Allocator> Box<T, A> {
     /// assert_eq!(Box::into_inner(c), 5);
     /// ```
     #[unstable(feature = "box_into_inner", issue = "80437")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn into_inner(boxed: Self) -> T {
+    pub const fn into_inner(boxed: Self) -> T
+    where
+        Self: ~const Drop,
+    {
         *boxed
     }
 }
@@ -758,8 +789,9 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub unsafe fn assume_init(self) -> Box<T, A> {
+    pub const unsafe fn assume_init(self) -> Box<T, A> {
         let (raw, alloc) = Box::into_raw_with_allocator(self);
         unsafe { Box::from_raw_in(raw as *mut T, alloc) }
     }
@@ -792,8 +824,9 @@ impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
     /// }
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
+    pub const fn write(mut boxed: Self, value: T) -> Box<T, A> {
         unsafe {
             (*boxed).write(value);
             boxed.assume_init()
@@ -938,8 +971,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// [memory layout]: self#memory-layout
     /// [`Layout`]: crate::Layout
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
+    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
         Box(unsafe { Unique::new_unchecked(raw) }, alloc)
     }
 
@@ -1035,8 +1069,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     ///
     /// [memory layout]: self#memory-layout
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
+    pub const fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
         let (leaked, alloc) = Box::into_unique(b);
         (leaked.as_ptr(), alloc)
     }
@@ -1046,9 +1081,10 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
         issue = "none",
         reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
     )]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
     #[doc(hidden)]
-    pub fn into_unique(b: Self) -> (Unique<T>, A) {
+    pub const fn into_unique(b: Self) -> (Unique<T>, A) {
         // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
         // raw pointer for the type system. Turning it directly into a raw pointer would not be
         // recognized as "releasing" the unique pointer to permit aliased raw accesses,
@@ -1064,8 +1100,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
     /// is so that there is no conflict with a method on the inner type.
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn allocator(b: &Self) -> &A {
+    pub const fn allocator(b: &Self) -> &A {
         &b.1
     }
 
@@ -1105,8 +1142,9 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// assert_eq!(*static_ref, [4, 2, 3]);
     /// ```
     #[stable(feature = "box_leak", since = "1.26.0")]
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
-    pub fn leak<'a>(b: Self) -> &'a mut T
+    pub const fn leak<'a>(b: Self) -> &'a mut T
     where
         A: 'a,
     {
@@ -1119,7 +1157,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     ///
     /// This is also available via [`From`].
     #[unstable(feature = "box_into_pin", issue = "62370")]
-    pub fn into_pin(boxed: Self) -> Pin<Self>
+    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    pub const fn into_pin(boxed: Self) -> Pin<Self>
     where
         A: 'static,
     {
@@ -1131,7 +1170,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A> {
     fn drop(&mut self) {
         // FIXME: Do nothing, drop is currently performed by compiler.
     }
@@ -1341,7 +1381,8 @@ impl<T> From<T> for Box<T> {
 }
 
 #[stable(feature = "pin", since = "1.33.0")]
-impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Pin<Box<T, A>>
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const From<Box<T, A>> for Pin<Box<T, A>>
 where
     A: 'static,
 {
@@ -1720,7 +1761,8 @@ impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const Deref for Box<T, A> {
     type Target = T;
 
     fn deref(&self) -> &T {
@@ -1729,7 +1771,8 @@ impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const DerefMut for Box<T, A> {
     fn deref_mut(&mut self) -> &mut T {
         &mut **self
     }
@@ -1908,7 +1951,8 @@ impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
  *  could have a method to project a Pin<T> from it.
  */
 #[stable(feature = "pin", since = "1.33.0")]
-impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+impl<T: ?Sized, A: Allocator> const Unpin for Box<T, A> where A: 'static {}
 
 #[unstable(feature = "generator_trait", issue = "43122")]
 impl<G: ?Sized + Generator<R> + Unpin, R, A: Allocator> Generator<R> for Box<G, A>
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 600862c4224..7e663fab16a 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -96,9 +96,20 @@
 #![feature(array_windows)]
 #![feature(async_stream)]
 #![feature(coerce_unsized)]
+#![cfg_attr(not(no_global_oom_handling), feature(const_alloc_error))]
+#![feature(const_box)]
 #![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
 #![feature(const_cow_is_borrowed)]
+#![feature(const_convert)]
+#![feature(const_size_of_val)]
+#![feature(const_align_of_val)]
+#![feature(const_ptr_read)]
+#![feature(const_maybe_uninit_write)]
+#![feature(const_maybe_uninit_as_mut_ptr)]
+#![feature(const_refs_to_cell)]
 #![feature(core_intrinsics)]
+#![feature(const_eval_select)]
+#![feature(const_pin)]
 #![feature(dispatch_from_dyn)]
 #![feature(exact_size_is_empty)]
 #![feature(extend_one)]
@@ -134,8 +145,13 @@
 #![feature(box_syntax)]
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
+#![feature(const_deref)]
 #![feature(const_fn_trait_bound)]
+#![feature(const_mut_refs)]
+#![feature(const_ptr_write)]
+#![feature(const_precise_live_drops)]
 #![feature(const_trait_impl)]
+#![feature(const_try)]
 #![cfg_attr(bootstrap, feature(destructuring_assignment))]
 #![feature(dropck_eyepatch)]
 #![feature(exclusive_range_pattern)]
diff --git a/library/alloc/tests/boxed.rs b/library/alloc/tests/boxed.rs
index bfe66b2687e..0d7acfed8c6 100644
--- a/library/alloc/tests/boxed.rs
+++ b/library/alloc/tests/boxed.rs
@@ -1,6 +1,7 @@
-use std::cell::Cell;
-use std::mem::MaybeUninit;
-use std::ptr::NonNull;
+use core::alloc::{AllocError, Allocator, Layout};
+use core::cell::Cell;
+use core::mem::MaybeUninit;
+use core::ptr::NonNull;
 
 #[test]
 fn uninitialized_zero_size_box() {
@@ -57,3 +58,110 @@ fn box_deref_lval() {
     x.set(1000);
     assert_eq!(x.get(), 1000);
 }
+
+pub struct ConstAllocator;
+
+unsafe impl const Allocator for ConstAllocator {
+    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        match layout.size() {
+            0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
+            _ => unsafe {
+                let ptr = core::intrinsics::const_allocate(layout.size(), layout.align());
+                Ok(NonNull::new_unchecked(ptr as *mut [u8; 0] as *mut [u8]))
+            },
+        }
+    }
+
+    unsafe fn deallocate(&self, _ptr: NonNull<u8>, layout: Layout) {
+        match layout.size() {
+            0 => { /* do nothing */ }
+            _ => { /* do nothing too */ }
+        }
+    }
+
+    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        let ptr = self.allocate(layout)?;
+        if layout.size() > 0 {
+            unsafe {
+                ptr.as_mut_ptr().write_bytes(0, layout.size());
+            }
+        }
+        Ok(ptr)
+    }
+
+    unsafe fn grow(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        debug_assert!(
+            new_layout.size() >= old_layout.size(),
+            "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
+        );
+
+        let new_ptr = self.allocate(new_layout)?;
+        if new_layout.size() > 0 {
+            new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), old_layout.size());
+            self.deallocate(ptr, old_layout);
+        }
+        Ok(new_ptr)
+    }
+
+    unsafe fn grow_zeroed(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        let new_ptr = self.grow(ptr, old_layout, new_layout)?;
+        if new_layout.size() > 0 {
+            let old_size = old_layout.size();
+            let new_size = new_layout.size();
+            let raw_ptr = new_ptr.as_mut_ptr();
+            raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
+        }
+        Ok(new_ptr)
+    }
+
+    unsafe fn shrink(
+        &self,
+        ptr: NonNull<u8>,
+        old_layout: Layout,
+        new_layout: Layout,
+    ) -> Result<NonNull<[u8]>, AllocError> {
+        debug_assert!(
+            new_layout.size() <= old_layout.size(),
+            "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
+        );
+
+        let new_ptr = self.allocate(new_layout)?;
+        if new_layout.size() > 0 {
+            new_ptr.as_mut_ptr().copy_from_nonoverlapping(ptr.as_ptr(), new_layout.size());
+            self.deallocate(ptr, old_layout);
+        }
+        Ok(new_ptr)
+    }
+
+    fn by_ref(&self) -> &Self
+    where
+        Self: Sized,
+    {
+        self
+    }
+}
+
+#[test]
+fn const_box() {
+    const VALUE: u32 = {
+        let mut boxed = Box::new_in(1u32, ConstAllocator);
+        assert!(*boxed == 1);
+
+        *boxed = 42;
+        assert!(*boxed == 42);
+
+        *boxed
+    };
+
+    assert!(VALUE == 42);
+}
diff --git a/library/alloc/tests/lib.rs b/library/alloc/tests/lib.rs
index 68e48348b07..eec24a5c3f7 100644
--- a/library/alloc/tests/lib.rs
+++ b/library/alloc/tests/lib.rs
@@ -1,8 +1,19 @@
 #![feature(allocator_api)]
+#![feature(alloc_layout_extra)]
 #![feature(assert_matches)]
 #![feature(box_syntax)]
 #![feature(cow_is_borrowed)]
+#![feature(const_box)]
+#![feature(const_convert)]
 #![feature(const_cow_is_borrowed)]
+#![feature(const_heap)]
+#![feature(const_intrinsic_copy)]
+#![feature(const_mut_refs)]
+#![feature(const_nonnull_slice_from_raw_parts)]
+#![feature(const_ptr_offset)]
+#![feature(const_ptr_write)]
+#![feature(const_try)]
+#![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(new_uninit)]
@@ -26,6 +37,7 @@
 #![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 #![feature(const_str_from_utf8)]
+#![feature(nonnull_slice_from_raw_parts)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};