Significantly expand onion message documentation

lightning/src/onion_message/messenger.rs

@@ -28,9 +28,56 @@ use sync::{Arc, Mutex};
 use prelude::*;
 
 /// A sender, receiver and forwarder of onion messages. In upcoming releases, this object will be
-/// used to retrieve invoices and fulfill invoice requests from [offers].
+/// used to retrieve invoices and fulfill invoice requests from [offers]. Currently, only sending
+/// and receiving empty onion messages is supported.
+///
+/// # Example
+///
+//  Needs to be `ignore` until the `onion_message` module is made public, otherwise this is a test
+//  failure.
+/// ```ignore
+/// # extern crate bitcoin;
+/// # use bitcoin::hashes::_export::_core::time::Duration;
+/// # use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
+/// # use lightning::chain::keysinterface::{InMemorySigner, KeysManager, KeysInterface};
+/// # use lightning::onion_message::{BlindedRoute, Destination, OnionMessenger};
+/// # use lightning::util::logger::{Logger, Record};
+/// # use std::sync::Arc;
+/// # struct FakeLogger {};
+/// # impl Logger for FakeLogger {
+/// #     fn log(&self, record: &Record) { unimplemented!() }
+/// # }
+/// # let seed = [42u8; 32];
+/// # let time = Duration::from_secs(123456);
+/// # let keys_manager = KeysManager::new(&seed, time.as_secs(), time.subsec_nanos());
+/// # let logger = Arc::new(FakeLogger {});
+/// # let node_secret = SecretKey::from_slice(&hex::decode("0101010101010101010101010101010101010101010101010101010101010101").unwrap()[..]).unwrap();
+/// # let secp_ctx = Secp256k1::new();
+/// # let hop_node_id1 = PublicKey::from_secret_key(&secp_ctx, &node_secret);
+/// # let (hop_node_id2, hop_node_id3, hop_node_id4) = (hop_node_id1, hop_node_id1,
+/// hop_node_id1);
+/// # let destination_node_id = hop_node_id1;
+/// #
+/// // Create the onion messenger. This must use the same `keys_manager` as is passed to your
+/// // ChannelManager.
+/// let onion_messenger = OnionMessenger::new(&keys_manager, logger);
+///
+/// // Send an empty onion message to a node id.
+/// let intermediate_hops = [hop_node_id1, hop_node_id2];
+/// onion_messenger.send_onion_message(&intermediate_hops, Destination::Node(destination_node_id));
+///
+/// // Create a blinded route to yourself, for someone to send an onion message to.
+/// # let your_node_id = hop_node_id1;
+/// let hops = [hop_node_id3, hop_node_id4, your_node_id];
+/// let blinded_route = BlindedRoute::new::<InMemorySigner, _, _>(&hops, &keys_manager, &secp_ctx).unwrap();
+///
+/// // Send an empty onion message to a blinded route.
+/// # let intermediate_hops = [hop_node_id1, hop_node_id2];
+/// onion_messenger.send_onion_message(&intermediate_hops, Destination::BlindedRoute(blinded_route));
+/// ```
 ///
 /// [offers]: <https://github.com/lightning/bolts/pull/798>
+/// [`OnionMessenger`]: crate::onion_message::OnionMessenger
 pub struct OnionMessenger<Signer: Sign, K: Deref, L: Deref>
 	where K::Target: KeysInterface<Signer = Signer>,
 	      L::Target: Logger,
@@ -79,6 +126,7 @@ impl<Signer: Sign, K: Deref, L: Deref> OnionMessenger<Signer, K, L>
 	}
 
 	/// Send an empty onion message to `destination`, routing it through `intermediate_nodes`.
+	/// See [`OnionMessenger`] for example usage.
 	pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination) -> Result<(), secp256k1::Error> {
 		let blinding_secret_bytes = self.keys_manager.get_secure_random_bytes();
 		let blinding_secret = SecretKey::from_slice(&blinding_secret_bytes[..]).expect("RNG is busted");

lightning/src/onion_message/mod.rs

@@ -8,6 +8,17 @@
 // licenses.
 
 //! Onion Messages: sending, receiving, forwarding, and ancillary utilities live here
+//!
+//! Onion messages are multi-purpose messages sent between peers over the lightning network. In the
+//! near future, they will be used to communicate invoices for [offers], unlocking use cases such as
+//! static invoices, refunds and proof of payer. Further, you will be able to accept payments
+//! without revealing your node id through the use of [blinded routes].
+//!
+//! LDK sends and receives onion messages via the [`OnionMessenger`]. See its documentation for more
+//! information on its usage.
+//!
+//! [offers]: <https://github.com/lightning/bolts/pull/798>
+//! [blinded routes]: crate::onion_message::BlindedRoute
 
 mod blinded_route;
 mod messenger;