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Add (failing) test to check order of repr(int) enum fields
RFC rust-lang#2195 specifies that a repr(int) enum such as: #[repr(u8)] enum MyEnum { B { x: u8, y: i16, z: u8 }, } has a layout that is equivalent to: #[repr(C)] enum MyEnumVariantB { tag: u8, x: u8, y: i16, z: u8 }, However this isn't actually implemented, with the actual layout being roughly equivalent to: union MyEnumPayload { B { x: u8, y: i16, z: u8 }, } #[repr(packed)] struct MyEnum { tag: u8, payload: MyEnumPayload, } Thus the variant payload is *not* subject to repr(C) ordering rules, and gets re-ordered as `{ x: u8, z: u8, z: i16 }` The existing tests added in pull-req rust-lang#45688 fail to catch this as the repr(C) ordering just happens to match the current Rust ordering in this case; adding a third field reveals the problem.
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