Box retags on drop causing Miri to report problem in custom allocator

[zakarumych]

I'm implementing custom allocator that stores allocation metadata in first bytes few of the allocated block and offsets pointer before returning it from Allocator::allocate.
It goes in reverse in Allocator::deallocate and reads metadata from before the passed pointer.

It works until I try to use Box::new_in or similar.
When Box is dropped, pointer passed to A::deallocate has different tag than one that returned from A::allocate.
And Miri emits error saying that tag does not encompass first bytes of the block and I can't read them.

Here's playground link

Same error happens if pointer from Allocator::allocate is converted to reference and then back to pointer.
That's, I guess, can be expected. But Box should not retag pointer on drop as I understand it.
Or this is something strictly prohibited by Rust.

[RalfJung]

This looks like a duplicate of rust-lang/miri#2104. This is a fundamental problem with Stacked Borrows (an instance of #256). Since recently we have the very experimental Tree Borrows mode in Miri that should be able to handle this code better. Could you try running this with '-Zmiri-tree-borrows'? (The retag happens much earlier than just on drop, it already happens when new_in returns, so I don't think this has much to do with dropping.)

[zakarumych]

Yes, that's the same issue.
With -Zmiri-tree-borrows Miri is happy.

[RalfJung]

Thanks for checking! Closing as a duplicate then.