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Disable field reordering for repr(int). #56887

Merged
merged 2 commits into from
Dec 22, 2018
Merged

Disable field reordering for repr(int). #56887

merged 2 commits into from
Dec 22, 2018

Commits on Dec 16, 2018

  1. Disable field reordering for repr(int).

    @emilio
    emilio committed Dec 16, 2018
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  2. Add 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.
    @petertodd @emilio
    petertodd authored and emilio committed Dec 16, 2018
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