setExtraField

    inline fn setExtraField(buffer: []u32, i: usize, comptime Field: type, value: anytype) usize {
        switch (@typeInfo(Field)) {
            .void => return 0,
            .int => |info| switch (info.bits) {
                32 => {
                    buffer[i] = value;
                    return 1;
                },
                64 => {
                    buffer[i..][0..2].* = @bitCast(value);
                    return 2;
                },
                else => comptime unreachable,
            },
            .@"enum" => {
                buffer[i] = @backingInt(value);
                return 1;
            },
            .@"struct" => |info| switch (info.layout) {
                .@"packed" => switch (info.backing_integer.?) {
                    u32 => {
                        buffer[i] = @bitCast(value);
                        return 1;
                    },
                    u64 => {
                        buffer[i..][0..2].* = @bitCast(value);
                        return 2;
                    },
                    else => comptime unreachable,
                },
                .auto => switch (Field) {
                    std.Target.Cpu.Feature.Set => {
                        const casted: []const u32 = @ptrCast(&value.ints);
                        @memcpy(buffer[i..][0..casted.len], casted);
                        return casted.len;
                    },
                    else => switch (Field.storage) {
                        .flag_optional, .enum_optional => {
                            return if (value.value) |v| setExtraField(buffer, i, Field.Value, v) else 0;
                        },
                        .flag_union => return switch (value.u) {
                            inline else => |x| setExtraField(buffer, i, @TypeOf(x), x),
                        },
                        .extended => @compileError("unimplemented"),
                        .flag_length_prefixed_list => {
                            const len: u32 = @intCast(value.slice.len);
                            if (len == 0) return 0; // Flag bit hides the length prefix.
                            buffer[i] = len;
                            const buf_len = len * @divExact(@sizeOf(Field.Elem), @sizeOf(u32));
                            @memcpy(buffer[i + 1 ..][0..buf_len], @as([]const u32, @ptrCast(value.slice)));
                            return 1 + buf_len;
                        },
                        .length_prefixed_list => {
                            const len: u32 = @intCast(value.slice.len);
                            buffer[i] = len;
                            const buf_len = len * @divExact(@sizeOf(Field.Elem), @sizeOf(u32));
                            @memcpy(buffer[i + 1 ..][0..buf_len], @as([]const u32, @ptrCast(value.slice)));
                            return 1 + buf_len;
                        },
                        .flag_list => {
                            const len: u32 = @intCast(value.slice.len);
                            @memcpy(buffer[i..][0..len], @as([]const u32, @ptrCast(value.slice)));
                            return len;
                        },
                        .multi_list => {
                            const len: u32 = @intCast(value.mal.len);
                            buffer[i] = len;
                            const field_names = @typeInfo(Field.Elem).@"struct".field_names;
                            inline for (0..field_names.len) |field_i| @memcpy(
                                buffer[i + 1 + field_i * len ..][0..len],
                                @as([]const u32, @ptrCast(value.mal.items(@fromBackingInt(@intCast(field_i))))),
                            );
                            return 1 + field_names.len * len;
                        },
                        .union_list => {
                            if (value.len == 0) return 0;
                            const Tag = @typeInfo(Field.Union).@"union".tag_type.?;
                            const slice_ptr: [*]const Field.Union = @ptrCast(@alignCast(value.data));
                            const slice = slice_ptr[0..value.len];
                            const meta_buffer = buffer[i..][0 .. (slice.len * @bitSizeOf(Field.Meta) + 31) / 32];
                            for (slice[0 .. slice.len - 1], 0..) |elem, elem_index| {
                                const union_tag: Tag = elem;
                                storeBits(u32, meta_buffer, elem_index * @bitSizeOf(Field.Meta), @as(Field.Meta, .{
                                    .tag = union_tag,
                                    .last = false,
                                }));
                            } else {
                                const elem_index = slice.len - 1;
                                const elem = slice[elem_index];
                                const union_tag: Tag = elem;
                                storeBits(u32, meta_buffer, elem_index * @bitSizeOf(Field.Meta), @as(Field.Meta, .{
                                    .tag = union_tag,
                                    .last = true,
                                }));
                            }
                            var total: usize = meta_buffer.len;
                            for (i + meta_buffer.len.., slice) |elem_index, src| switch (src) {
                                inline else => |x| total += setExtraField(buffer, elem_index, @TypeOf(x), x),
                            };
                            return total;
                        },
                    },
                },
                .@"extern" => {
                    const n = @divExact(@sizeOf(Field), @sizeOf(u32));
                    const ptr: *align(@alignOf(Field)) const [n]u32 = @ptrCast(&value);
                    buffer[i..][0..n].* = ptr.*;
                    return n;
                },
            },
            else => @compileError("bad field type: " ++ @typeName(Field)),
        }
    }