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Comment by vlovich123 | Hacker News Reader

I think the better approach would be to standardize some kind of feature that lets you annotate variables as "not noalias" so that Rust knows to elide the noalias for those values. That's a much more generic way to solve the ecosystem problem without changing Box semantics or introducing one off types. That being said, there's already a solution in the ecosystem which is to not use aliased pointers after giving the pointer to Box & moving it.

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For the record, Rust does have a "not noalias" builtin magic type in the standard library: UnsafeCell.

https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell....

No, UnsafeCell doesn't change any rules around non-aliasing for &mut/Box. (E.g., if you move a Box<UnsafeCell<T>>, it will still invalidate any pointers to the data.) All it does is change the rules around immutability for & references. &T is "not noalias" regardless of UnsafeCell, but it's immutable in its absence.

Meanwhile, there is a magic "not noalias" mechanism currently recognized by Miri, in the form of !Unpin types. But this is considered a temporary hack to keep it from complaining about pinned futures that reference their own fields, in the absence of an actual language feature. Also, it only applies to accessing values through unique references, not to moving or writing to them by their binding.

&T is noalias - while there may be aliases none can write, which is the important thing - without UnsafeCell, and &UnsafeCell<T> loses noalias for that reason.

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