Allow unused tuple struct field in WithOpaqueHeader

Author: shepmaster

library/alloc/src/boxed/thin.rs

@@ -171,7 +171,7 @@ struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
 /// An opaque representation of `WithHeader<H>` to avoid the
 /// projection invariance of `<T as Pointee>::Metadata`.
 #[repr(transparent)]
-struct WithOpaqueHeader(NonNull<u8>);
+struct WithOpaqueHeader(#[allow(unused_tuple_struct_fields)] NonNull<u8>);
 
 impl WithOpaqueHeader {
     #[cfg(not(no_global_oom_handling))]