Avoid Box invalidation to be compatible with tag-raw-pointers #1166
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It looks like the only outstanding issue aliasing issue in this repo (with raw pointer tagging) was this one move of a Box after saving a pointer to the allocation that the Box guards. This is UB according to the Stacked Borrows with raw pointer tagging in combination with the way rustc applies noalias to Box. As is often the case, the resolution here is to convert the Box down to a raw pointer before a pointer into the allocation is created.
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It looks like the only outstanding issue aliasing issue in this repo
(with raw pointer tagging) was this one move of a Box after saving a
pointer to the allocation that the Box guards. This is UB according to
the Stacked Borrows with raw pointer tagging in combination with the way
rustc applies noalias to Box. As is often the case, the resolution here
is to convert the Box down to a raw pointer before a pointer into the
allocation is created.
It is unclear to me if the spec will change and make this code valid without this PR. In rust-lang/unsafe-code-guidelines#326 I've advocated for removing this special property from
Boxaltogether. But by the same token, it's entirely possible that this code is UB today because it does aliased access through anoaliaspointer. I don't know if the access toentries.len()is problematic aliasing with thegroup_uppointer.