+ /// The "payment secret". This authenticates the sender to the recipient, preventing a
+ /// number of deanonymization attacks during the routing process.
+ /// As nodes upgrade, the invoices you provide should likely migrate to setting the
+ /// payment_secret feature to required, at which point you should fail_backwards any HTLCs
+ /// which have a None here.
+ /// Until then, however, values of None should be ignored, and only incorrect Some values
+ /// should result in an HTLC fail_backwards.
+ /// Note that, in any case, this value must be passed as-is to any fail or claim calls as
+ /// the HTLC index includes this value.
+ payment_secret: Option<PaymentSecret>,