Wrong compile error when using packed struct with fields not matching backing int type in extern function

[mlugg]

Zig Version

0.11.0-dev.3+0bbb00035

Steps to Reproduce and Observed Behavior

const Foo = packed struct(u32) {
    x: u1,
};

fn bar(_: Foo) callconv(.C) void {}

pub fn main() void {
    bar(.{ .x = 0 });
}

This causes a compile error:

[mlugg@vega test]$ zig build-exe test.zig
test.zig:5:8: error: parameter of type 'test.Foo' not allowed in function with calling convention 'C'
fn bar(_: Foo) callconv(.C) void {}
       ^~~~~~
test.zig:5:8: note: only extern structs and ABI sized packed structs are extern compatible
test.zig:1:20: note: struct declared here
const Foo = packed struct(u32) {
            ~~~~~~~^~~~~~
referenced by:
    main: test.zig:8:5
    callMain: /home/mlugg/.opt/zig/lib/std/start.zig:596:17
    remaining reference traces hidden; use '-freference-trace' to see all reference traces

Expected Behavior

The compilation should succeed, which happens if the struct fields fill the backing type (e.g. by adding a y: u31).

I'm not entirely confident that this is a bug, but if it's an intentional decision it seems like an odd one to me; while it has some undefined bits, the in-memory layout is still well-defined and the backing type is passable over C ABI.

[ifreund]

This is a compile error regardless of whether one tries to use it on the C ABI, and this is 100% intentional:

const Foo = packed struct(u32) {
    x: u1,
};

test {
    _ = @sizeOf(Foo);
}

/tmp/foo.zig:1:27: error: backing integer type 'u32' has bit size 32 but the struct fields have a total bit size of 1
const Foo = packed struct(u32) {
                          ^~~

The bug here is that your code example should trigger the same error message as mine.

[mlugg]

Huh, I somehow totally missed that. But yeah, okay, makes sense, just a bad error message