7 ms·

Yeah, that's a nice trick. I wonder how general it is, though.

• SamReidHughes 11y ago
  If one alternative is larger than the others and it has an enum tag or pointer inside of it, such that you can squeeze the other alternatives before/after that tag word, you're good to go. If you had multiple alternatives of maximum size, they'd need some word in them, at the same offset/size, such that they don't have conflicting representations. An enum tag could overlap with a non-null pointer, and two enum tags with user-declared tag values (such that they do not overlap) could work too. Or if you're crazy you could discard having each type's representation be computed solely as a function of what the type is made of, choosing their representations based on how well they pack into other types. (Or worse yet, if you've got a borrow checker and your types are memcpyable, you could pack things and rejigger bytes however you wanted and then unfurl them into a temporary whenever something uses the interior value, unless I'm overlooking something. So if you had Either<Either<u32, u32>, Either<u32, u32>>, you could represent that with a single tag whose value is 0, 1, 2, or 3, and when if the tag's 2 or 3 and you make a reference to the interior value, either fix the tag in-place and fix it back when you're done (if you're the sole owner of the Either<Either...>), or copy the value out and let the borrower borrow that (and copy it back in when it's done, if it was a mutable borrow).)