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Comment by milspec | Hacker News Reader

Rust should have untagged unions. Obviously, ones that allow pointer abuse would be restricted to code marked unsafe.

Note that a union of two pointer-containing structs is safe as long as the pointers line up, having the same type and offset in each struct.

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They've been proposed:

https://github.com/rust-lang/rfcs/pull/724

If you really need them, you can implement them in a library by declaring a suitably sized byte array and having methods that return casted pointers to it. This would be illegal in C due to strict aliasing, but IIRC Rust does not have a strict aliasing rule (because almost all pointers being the equivalent of 'restrict' drastically reduces the benefit).

There are issues with size_of not being a constant expression and such (at least in stable), but those are definitely going to be fixed.

Do you have a compelling use case? I can't imagine a use for untagged unions in Rust that isn't subsumed by other features in the language.

The most common use case would be interop with C libraries that use untagged unions. This is a significant hassle when writing C bindings in Rust today.

I can think of many, mostly revolving around cases where the tag is not stored inline with the structure in question. Where space efficiency is important this is quite common. C interoperability is obviously another important use case (the current solution of passing around [u8] arrays is pretty atrocious).

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