std.mem.bytesAsValue/asBytes for padded packed structs

[motiejus]

Currently std.mem.asBytes/bytesAsValue cannot be used for packed structs that "may be" padded. I.e. if 8*@sizeOf(T) != @bitSizeOf(T). In particular, this fails:

const std = @import("std");
const testing = std.testing;

test "bytesAsValue and asBytes work with non-word-aligned structs" {
    const Padded = packed struct { a: u12, b: u12 };
    const padded = Padded{ .a = 42, .b = 1 };
    const bytes = std.mem.asBytes(&padded);
    try testing.expectEqual(bytes.len, 3);
    const padded2 = std.mem.bytesAsValue(Padded, bytes[0..3]);
    try testing.expectEqual(padded, padded2.*);
}

With:

./code/zig-linux-x86_64-0.10.0-dev.865+8841a71aa/lib/std/mem.zig:2698:9: error: expected *[4]u8, passed *const [3]u8
        @compileError(std.fmt.bufPrint(&buf, "expected *[{}]u8, passed " ++ @typeName(B), .{size}) catch unreachable);
        ^
./code/zig-linux-x86_64-0.10.0-dev.865+8841a71aa/lib/std/mem.zig:2706:77: note: called from here
pub fn bytesAsValue(comptime T: type, bytes: anytype) BytesAsValueReturnType(T, @TypeOf(bytes)) {
                                                                            ^
./code/zig-linux-x86_64-0.10.0-dev.865+8841a71aa/lib/std/mem.zig:2706:77: note: called from here
pub fn bytesAsValue(comptime T: type, bytes: anytype) BytesAsValueReturnType(T, @TypeOf(bytes)) {
                                                                            ^
./test.zig:4:68: note: called from here
test "bytesAsValue and asBytes work with non-word-aligned structs" {

I've attempted to fix this in #10941 for bytesAsValue, but asBytes is more nuanced. Before spending much more time on this, I'd like to receive a "consensus" that this (bytesAsValue/asBytes) is something that ought to work for byte-aligned packed structs. If yes, how it should behave for non-byte aligned or padded packed structs?

[ominitay]

This is impossible. Regular structs don't have a fixed memory layout.

[motiejus]

This is impossible. Regular structs don't have a fixed memory layout.

Github failed me, you saw the stub issue before it was submitted. :)

[ominitay]

Ahhh, apologies :)