Do not allocate for ZST ThinBox attempt 2 (using const_allocate)

There's PR #123184
which avoids allocation for ZST ThinBox.

That PR has an issue with unsoundness with misuse of `MaybeUninit`
(see comments in that PR).

This PR is much simpler implementation which does not have this
problem, but it uses `const_allocate` feature.

library/alloc/src/boxed/thin.rs

@@ -4,10 +4,14 @@
 use crate::alloc::{self, Layout, LayoutError};
 use core::error::Error;
 use core::fmt::{self, Debug, Display, Formatter};
+#[cfg(not(no_global_oom_handling))]
+use core::intrinsics::const_allocate;
 use core::marker::PhantomData;
 #[cfg(not(no_global_oom_handling))]
 use core::marker::Unsize;
-use core::mem::{self, SizedTypeProperties};
+use core::mem;
+#[cfg(not(no_global_oom_handling))]
+use core::mem::SizedTypeProperties;
 use core::ops::{Deref, DerefMut};
 use core::ptr::Pointee;
 use core::ptr::{self, NonNull};
@@ -91,6 +95,58 @@ impl<T> ThinBox<T> {
 
 #[unstable(feature = "thin_box", issue = "92791")]
 impl<Dyn: ?Sized> ThinBox<Dyn> {
+    #[cfg(not(no_global_oom_handling))]
+    fn new_unsize_zst<T>(value: T) -> Self
+    where
+        T: Unsize<Dyn>,
+    {
+        assert!(mem::size_of::<T>() == 0);
+
+        const fn max(a: usize, b: usize) -> usize {
+            if a > b { a } else { b }
+        }
+
+        let alloc: &<Dyn as Pointee>::Metadata = const {
+            // ZST T pointer must be aligned to both the value and the metadata.
+            // Metadata must be at the address `&T - size_of::<Metadata>`.
+            // This is how we need to allocate the memory:
+            // [ padding | metadata | ZST T value ]
+
+            // FIXME: just call `WithHeader::alloc_layout` with size reset to 0.
+            // Currently that's blocked on `Layout::extend` not being `const fn`.
+
+            let alloc_align =
+                max(mem::align_of::<T>(), mem::align_of::<<Dyn as Pointee>::Metadata>());
+
+            let alloc_size =
+                max(mem::align_of::<T>(), mem::size_of::<<Dyn as Pointee>::Metadata>());
+
+            unsafe {
+                // SAFETY: align is power of two because it is the maximum of two alignments.
+                let alloc: *mut u8 = const_allocate(alloc_size, alloc_align);
+
+                let metadata_offset =
+                    alloc_size.checked_sub(mem::size_of::<<Dyn as Pointee>::Metadata>()).unwrap();
+                // SAFETY: adding offset within the allocation.
+                let metadata_ptr: *mut <Dyn as Pointee>::Metadata =
+                    alloc.add(metadata_offset).cast();
+                // SAFETY: `*metadata_ptr` is within the allocation.
+                metadata_ptr.write(ptr::metadata::<Dyn>(ptr::dangling::<T>() as *const Dyn));
+
+                // SAFETY: we have just written the metadata.
+                &*(metadata_ptr)
+            }
+        };
+
+        // SAFETY: `alloc` points to `<Dyn as Pointee>::Metadata`, so addition does not overflow.
+        let value_ptr =
+            unsafe { (alloc as *const <Dyn as Pointee>::Metadata).add(1) }.cast::<u8>().cast_mut();
+        let ptr = WithOpaqueHeader(NonNull::new(value_ptr).unwrap());
+        // Forget the value to avoid double drop.
+        mem::forget(value);
+        ThinBox::<Dyn> { ptr, _marker: PhantomData }
+    }
+
     /// Moves a type to the heap with its [`Metadata`] stored in the heap allocation instead of on
     /// the stack.
     ///
@@ -109,9 +165,13 @@ impl<Dyn: ?Sized> ThinBox<Dyn> {
     where
         T: Unsize<Dyn>,
     {
-        let meta = ptr::metadata(&value as &Dyn);
-        let ptr = WithOpaqueHeader::new(meta, value);
-        ThinBox { ptr, _marker: PhantomData }
+        if mem::size_of::<T>() == 0 {
+            Self::new_unsize_zst(value)
+        } else {
+            let meta = ptr::metadata(&value as &Dyn);
+            let ptr = WithOpaqueHeader::new(meta, value);
+            ThinBox { ptr, _marker: PhantomData }
+        }
     }
 }
 
@@ -300,20 +360,19 @@ impl<H> WithHeader<H> {
 
         impl<H> Drop for DropGuard<H> {
             fn drop(&mut self) {
+                // All ZST are allocated statically.
+                if self.value_layout.size() == 0 {
+                    return;
+                }
+
                 unsafe {
                     // SAFETY: Layout must have been computable if we're in drop
                     let (layout, value_offset) =
                         WithHeader::<H>::alloc_layout(self.value_layout).unwrap_unchecked();
 
-                    // Note: Don't deallocate if the layout size is zero, because the pointer
-                    // didn't come from the allocator.
-                    if layout.size() != 0 {
-                        alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
-                    } else {
-                        debug_assert!(
-                            value_offset == 0 && H::IS_ZST && self.value_layout.size() == 0
-                        );
-                    }
+                    // Since we only allocate for non-ZSTs, the layout size cannot be zero.
+                    debug_assert!(layout.size() != 0);
+                    alloc::dealloc(self.ptr.as_ptr().sub(value_offset), layout);
                 }
             }
         }

library/alloc/src/lib.rs

@@ -114,8 +114,10 @@
 #![feature(const_box)]
 #![feature(const_cow_is_borrowed)]
 #![feature(const_eval_select)]
+#![feature(const_heap)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_write)]
+#![feature(const_option)]
 #![feature(const_pin)]
 #![feature(const_refs_to_cell)]
 #![feature(const_size_of_val)]