[facturi]

That proposal doesn't mention how the union struct handles tearing under concurrent modification.

Tearing can cause memory safety issue. Variant A can have an integer field and variant B can have a reference field in the same offset. Tearing can cause it to treat an integer as reference.

[andygocke]

I’m the author of the original issue — I agree, we’ll have to ensure the struct layout is solved. I think the only thing that makes sense is to just waste a little space and store the fields side-by-side. In almost all cases where people would use a struct I think this is an acceptable tradeoff.

At the point where you have more than 5 cases, the GC overhead starts to get shrink in comparison to the calling convention and copying overhead anyway.

[neonsunset]

Will there be any layout optimizations performed by Roslyn for fields that can be aliased? E.g.: for a type union which has variants with 2 object fields and one ushort-sized at most each, have a base layout of (object, object, short)?

[facturi]

Tearing issue is mentioned in this issue https://github.com/dotnet/csharplang/issues/7016 but not in proposal

[jaredpar]

The design is not going to allow for struct layout to have memory safety issues when tearing is involved. That is a problem we are very well aware of and consider in designs.