Force tag to the same position in KStringInner #77
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tag() relies on KStringInner having a tag field at the same offset in every union variant, and every union variant is repr(C) so that this holds (provided that `B` has the same size, anyway -- since it's a sealed trait, that is guaranteed for any constructed instance, though it's a bit subtle). However, the union _itself_ does not have a guaranteed layout unless one uses repr(C). It's a bit silly, but technically every single variant could be in a different location, causing the tags not to line up within the union even though they do within the structs individually. See e.g. https://rust-lang.github.io/unsafe-code-guidelines/layout/unions.html The fixes would be: 1. use a struct with a tag field first, and then a union field second. (Or even an enum, as is done with the non-`unsafe` implementation). 2. use a repr(C) union instead of a repr(Rust) union. 3. Maybe add some assertions so that even with repr(Rust), the layout is as expected? I opted to do (2) as a quick fix to send out.
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tag()relies onKStringInnerhaving a tag field at the same offset in every union variant, and every union variant isrepr(C)so that this holds (provided thatBhas the same size, anyway -- since it's a sealed trait, that is guaranteed for any constructed instance, though it's a bit subtle).However, the union itself does not have a guaranteed layout unless one uses
repr(C). It's a bit silly, but technically every single variant could be in a different location, causing the tags not to line up within the union even though they do within the structs individually. See e.g. https://rust-lang.github.io/unsafe-code-guidelines/layout/unions.htmlThe fixes would be:
unsafeimplementation).I opted to do (2) as a quick fix to send out.
(Did not add tests, because I believe this has no change in behavior -- the layout is presumably the same now as it was before, only now it is guaranteed.)