Proposal: don't allow unused bits in packed unions

[jacobly0]

Currently it is valid to do:

const U = packed union {
    x: u8,
    y: u16,
};

However, there is not plainly one possible way of mapping this representation to bits, a feature of other packed types that seems desirable. For example, enum (u5) { ... } plainly represents 5 bits in an obvious manner and is allowed in packed contexts, but ?u8 has two reasonable ways of mapping to 9 bits and is not allowed in packed contexts.

My proposal for resolving this ambiguity is to require all fields of a packed union to have the same @bitSizeOf as a required backing integer type. The above type is still representable as:

const U = packed union(u16) {
    x: packed struct(u16) {
        used: u8,
        unused: u8 = undefined,
    },
    y: u16,
};

But this type now has the benefit of being explicit about the order of the unused bits and therefore only has one reasonable bitwise representation. Also, depending on how #19634 is resolved, this explicitness could be very important as having an undefined field in a packed container may become much less useful in the future, and it may be necessary to initialize to 0 instead for many use cases. To that end, this proposal encourages that explicit thought is put into how the padding bits should be defined.

Related:

• Improve packed semantics #19660

[Rexicon226]

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.

[jacobly0]

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!

[expikr]

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

[mochalins]

#19395 seems relevant here