fix: move Box reexport to ngx::allocator

Protect allocator_api2::alloc::Global reexport with "alloc".
Reimplement allocator_api2::unsize_box, as the original macro is not
reexportable.

Author: bavshin-f5

src/allocator.rs

@@ -11,10 +11,10 @@ use ::core::alloc::Layout;
 use ::core::mem;
 use ::core::ptr::{self, NonNull};
 
-pub use allocator_api2::alloc::{AllocError, Allocator, Global};
+pub use allocator_api2::alloc::{AllocError, Allocator};
 
 #[cfg(feature = "alloc")]
-pub use allocator_api2::boxed;
+pub use allocator_api2::{alloc::Global, boxed::Box};
 
 /// Explicitly duplicate an object using the specified Allocator.
 pub trait TryCloneIn: Sized {
@@ -81,3 +81,24 @@ mod impls {
         }
     }
 }
+
+/// Allows turning a [`Box<T: Sized, A>`][Box] into a [`Box<U: ?Sized, A>`][Box] where `T` can be
+/// unsizing-coerced into a `U`.
+///
+/// See [allocator_api2::unsize_box] for an explanation why this macro is necessary.
+#[cfg(feature = "alloc")]
+#[doc(inline)]
+pub use crate::__unsize_box as unsize_box;
+
+// We have to reimplement this macro because the original implementation is not reexportable.
+// Macro definitions float to the top of the crate, thus we also mark it as hidden and reexport
+// again from the right namespace.
+#[cfg(feature = "alloc")]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __unsize_box {
+    ( $boxed:expr $(,)? ) => {{
+        let (ptr, alloc) = $crate::allocator::Box::into_raw_with_allocator($boxed);
+        unsafe { $crate::allocator::Box::from_raw_in(ptr as *mut _, alloc) }
+    }};
+}