Regression: bitcast initialized empty packed struct should be zero but is non-zero

[mitchellh]

Zig Version

0.12.0-dev.3674+a0de07760

Steps to Reproduce and Observed Behavior

const Cell = packed struct(u64) {
    content: packed union {
        a: u21,
        b: u22,
    } = .{ .a = 0 },
    _padding: u42 = 0,
};

test {
    const cell: Cell = .{};
    const cell_int: u64 = @bitCast(cell);
    try @import("std").testing.expectEqual(@as(u64, 0), cell_int);
}

Expected Behavior

I expect the test to pass but it fails. It used to pass with 0.12.0-dev.3405+31791ae15

I can understand why it's failing: I'm setting the smaller field of the packed union so the extra bits are probably undefined? Should I be setting the largest field always?

I'm willing to accept this is my problem and not yours but wanted to raise it in case you believe this is a regression.

[mlugg]

Yes, I think it's accurate to say that initializing a packed union with a small field leaves the remaining bits undefined, so that code snippet just has a bug. That said, I'll wait for @andrewrk to confirm the intended behavior before closing.

[andrewrk]

Thanks for the report.

This is indeed working as designed given how unions work in zig: only the active field has a defined value, and the bitcast therefore accesses undefined memory, making cell_int have undefined bits, or be fully undefined, depending on the resolution of #19634.

Based on the recent guidance that I wrote on struct field initialization, I would recommend this instead:

const Cell = packed struct(u64) {
    content: packed union {
        a: u21,
        b: u22,
    },
    _padding: u42,

    const init: Cell = @bitCast(@as(u64, 0));
};
test {
    const cell = Cell.init;
    const cell_int: u64 = @bitCast(cell);
    try @import("std").testing.expectEqual(@as(u64, 0), cell_int);
}

This code snippet should be considered when evaluating #19634, but I don't think it needs to remain open as a bug or a proposal.

[mitchellh]

Thanks, that's exactly what I ended up doing.