Proposal: don't allow unused bits in packed unions about zig HOT 4 OPEN

and have packed struct(u8) and packed union(u8)

Actually, thinking about it some more, this should definitely be a requirement for many different reasons. The most obvious is that an 8-bit packed union should be allowed in callconv(.C) function parameters, but the C abi of u8 and i8 differ, so the user needs to specify which one to use!

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Can multiple unused fields use _ as their field name provided that they have a default initialization value?

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A small change I propose is to instead of requiring them to have the same @bitSizeOf, which could lead to some weird behaviour and error messages like in cases such as:

const U = packed union {
    a: u7,
    b: u8, 
    c: u8,
    d: u9,
}

What would the error here be? Either

1. it would resolve to being u8 as the majority, and u7 and u9 are wrong
2. it'll coerce to the largest size which might not be desired

Instead I think it makes more sense to unify the current semantics, and have packed struct(u8) and packed union(u8), and the "same @bitSizeOf" must match the provided backing int.

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#19395 seems relevant here

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