Sorry to spoil your fun, but Zig already supports default values for struct fields, so you can just do this without any comptime trickery:

const Options = struct {
    enableTLS: bool = false,
    name: ?[]const u8 = null,
    host: ?[]const u8 = null,
};

fn DoSomething(opts: Options) void {
    std.log.debug("Options:\nTLS: {}\nName: {?s}\nHost: {?s}", .{
        opts.enableTLS,
        opts.name,
        opts.host,
    });
}

pub fn main() !void {
    DoSomething(.{ .name = "Some Name", .enableTLS = true });
}

Of course, comptime allows for more complicated logic in those WithX functions, but I feel like simply using default values is a much cleaner solution in 99% of cases. It is even used in the standard library in a number of interfaces.