If there was a &write_only T type, would it be contravariant with respect to T? The reason why I think it would be is that it seems like it would always be fine to write a &'long T to a location of type &'short T is required.
Yes I believe it could be contravariant w. r. t. T. But it would have to be truly write-only, so no destructor calls for T either; in terms of API design, this could be achieved for example by only allowing T: Copy; or giving it some (very clearly documented) leaking properties.
Maybe &mut MaybeUninit<T> may be something like this? It is safe to write to it, but unsafe to read.
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