std.mem.byteSwapAllFields not working with packed structs.

[AndrewKraevskii]

Zig Version

0.16.0-dev.43+99b2b6151

Steps to Reproduce and Observed Behavior

const std = @import("std");

const A = packed struct(u8) {
    a: u4,
    b: u4,
};

test {
    var foo: A = undefined;
    std.mem.byteSwapAllFields(A, &foo);
}

/home/andrewkraevskii/.zvm/master/lib/std/mem.zig:2159:57: error: @byteSwap requires the number of bits to be evenly divisible by 8, but u4 has 4 bits
                        @field(ptr, f.name) = @byteSwap(@field(ptr, f.name));
                                                        ^~~~~~~~~~~~~~~~~~~
referenced by:
    test_0: main.zig:10:30

Provided example by itself is not really useful but I encountered this issue while porting to writergate. std.Io.Reader.readSliceEndianAlloc uses this function internally and it prevents me from been able to read Slices of packed structs.

Mentioned code looked something like these:

const std = @import("std");
const A = packed struct(u32) {
    block_metadata: u4,
    block_id: u12,
    y_coordinate: u8,
    /// relative to chunk
    z_coordinate: u4,
    /// relative to chunk
    x_coordinate: u4,
};
test {
    var in: [0x100]u8 = undefined;
    var fixed: std.Io.Reader = .fixed(&in);

    std.testing.allocator.free(try fixed.readSliceEndianAlloc(std.testing.allocator, A, 100, .big));
}

Expected Behavior

I expect this code to compile successfully. packed struct is just a number so handling of endians shouldn't go through fields one by one and instead flip entire packed struct.

[SquareMan]

I just hit this bug myself. Interestingly it does seem to work if the packed struct is a field of the struct you pass byteSwapAllFields. In that case it treats the packed struct as its backing integer and byteswaps that.