structops - how to make argument optional based on boolean flag

Question

I have program that accepts 2 boolean flags -d for decode and -e for encode. However, if -e is specified, additional message string needs to be provided (message to encode). This string should not be present if -d is specified. How can I accomplish this using structops?

Here is what I have tried:

#[derive(Debug, StructOpt)]
#[structopt(name="encode_test", about="encodes messages")]
struct Opt {
    #[structopt(short="e", long="encode")]
    encode: bool,

    #[structopt(requires("encode"))]
    message: String,

    #[structopt(short="d", long="decode")]
    decode: bool,
}

running the program using cargo run -- -e "test message" works fine, but cargo run -- -d gives me the error that <message> and --encode arguments were not provided.

How can I require <message> only if -e is present?

I have also tried the following:

#[derive(Debug, StructOpt)]
#[structopt(name="encode_test", about="encodes messages")]
struct Opt {
    #[structopt(short="d", long="decode", required_unless("encode"))]
    decode: bool,

    // I've tried using conflicts_with("decode") here as well, same panic
    #[structopt(short="e", long="encode", required_unless("decode"))]
    encode: String, //does not panic if I change type here from string to bool
}

Which again, works for -e message but panics when specifying only -d parameter