The previous behavior of Miri was a false negative. rustc applies noalias to newtype wrappers around &mut, so Miri has to retag &mut when passed by value to a function even if it is in a wrapper struct, such as bumpalo's Box. mem::forget is a common aliasing footgun, because for a unique-owning wrapper like Box, leaking an RAII handle re-asserts the uniqueness of the handle as it is sent to the void. Ouch.
Miri now reports UB with some uses of bumpalo's Box where it did not before: https://github.com/rust-lang/miri/issues/2704
The previous behavior of Miri was a false negative. rustc applies noalias to newtype wrappers around &mut, so Miri has to retag &mut when passed by value to a function even if it is in a wrapper struct, such as bumpalo's Box. mem::forget is a common aliasing footgun, because for a unique-owning wrapper like Box, leaking an RAII handle re-asserts the uniqueness of the handle as it is sent to the void. Ouch.
ManuallyDrop solves this problem.
Closes https://github.com/fitzgen/bumpalo/issues/187