Is the ManuallyDrop ABI guaranteed to be repr(transparent)?

Question

The nomicon says:

repr(transparent)

[...] This repr is only considered part of the public ABI of a type if either the single field is pub, or if its layout is documented in prose. Otherwise, the layout should not be relied upon by other crates.

ManuallyDrop<T> is repr(transparent) , but its single field is not pub . Its docs say:

[...] This wrapper is 0-cost.

ManuallyDrop<T> is subject to the same layout optimizations as T . [...]

Does this count as documenting its layout in the prose? Is it safe to assume that I can transmute or otherwise convert (eg pointer cast) from T to ManuallyDrop<T> ?

Answer 1

This was actually just clarified with today's release of Rust 1.61.0 . The documentation of ManuallyDrop now specifies (added text highlighted in bold):

ManuallyDrop<T> is guaranteed to have the same layout as T , and is subject to the same layout optimizations as T .