forbid unused bits in packed unions

[andrewrk]

closes #19754

Upgrade Guide

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

⬇️

const U = packed union(u16) {
    x: packed struct(u16) {
        data: u8,
        padding: u8 = 0,
    },
    y: u16,
};

[xdBronch]

dont the (planned?) semantics of packed structs mean any undefined bits cause the entire thing to be undefined? probably shouldnt recommend doing that in an upgrade guide

[andrewrk]

Good call, thank you.

[andrewrk]

/home/ci/actions-runner1/_work/zig/zig/test/behavior/field_parent_ptr.zig:1755:1: error: packed store

should I disable the test for stage2_aarch64 for now?

[Justus2308]

Weren't explicit packed union backing ints part of #19754? That's still a compile error after this patch

[andrewrk]

Sounds like #24715 to me

[Justus2308]

I thought that's just about making the tag mandatory, I was referring to this comment
This snippet:

const U = packed union(u16) {
    x: packed struct(u16) {
        data: u8,
        padding: u8 = 0,
    },
    y: u16,
};

comptime {
    _ = U;
}

still doesn't compile with:

check.zig:1:24: error: packed union does not support enum tag type
const U = packed union(u16) {
                       ^~~

[andrewrk]

Sorry, you're right. There is not a proposal open which is limited to supporting backing integer type for packed unions, although several proposals imply its existence. I'll add that right now.

[andrewrk]

#25350