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>

1 files changed, 18 insertions, 5 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.