Rollup merge of rust-lang#124013 - RalfJung:box-to-raw, r=oli-obk

Box::into_raw: make Miri understand that this is a box-to-raw cast

Turns out rust-lang#122647 went a bit too far in cleaning up `Box`... we still need a hack in `Box::into_raw`. The nicer fix would be to make Stacked Borrows not care about reference-to-raw-pointer casts, but it's unclear whether that will ever be possible without going to full Tree Borrows.

Fixes rust-lang/miri#3473.

Author: GuillaumeGomez

alloc/src/boxed.rs

@@ -1058,7 +1058,8 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     #[stable(feature = "box_raw", since = "1.4.0")]
     #[inline]
     pub fn into_raw(b: Self) -> *mut T {
-        Self::into_raw_with_allocator(b).0
+        // Make sure Miri realizes that we transition from a noalias pointer to a raw pointer here.
+        unsafe { addr_of_mut!(*&mut *Self::into_raw_with_allocator(b).0) }
     }
 
     /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
@@ -1112,7 +1113,10 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
         let mut b = mem::ManuallyDrop::new(b);
         // We carefully get the raw pointer out in a way that Miri's aliasing model understands what
-        // is happening: using the primitive "deref" of `Box`.
+        // is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
+        // want *no* aliasing requirements here!
+        // In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
+        // works around that.
         let ptr = addr_of_mut!(**b);
         let alloc = unsafe { ptr::read(&b.1) };
         (ptr, alloc)