Use Infallible for the unconstructable default custom message type

When we landed custom messages, we used the empty tuple for the
custom message type for `IgnoringMessageHandler`. This was fine,
except that we also implemented `Writeable` to panic when writing
a `()`. Later, we added support for anchor output construction in
CommitmentTransaction, signified by setting a field to `Some(())`,
which is serialized as-is.

This causes us to panic when writing a `CommitmentTransaction`
with `opt_anchors` set. Note that we never set it inside of LDK,
but downstream users may.

Instead, we implement `Writeable` to write nothing for `()` and use
`core::convert::Infallible` for the default custom message type as
it is, appropriately, unconstructable.

This also makes it easier to implement various things in bindings,
as we can always assume `Infallible`-conversion logic is
unreachable.

Author: TheBlueMatt

lightning/src/ln/peer_handler.rs

@@ -37,6 +37,7 @@ use sync::{Arc, Mutex};
 use core::sync::atomic::{AtomicUsize, Ordering};
 use core::{cmp, hash, fmt, mem};
 use core::ops::Deref;
+use core::convert::Infallible;
 #[cfg(feature = "std")] use std::error;
 
 use bitcoin::hashes::sha256::Hash as Sha256;
@@ -47,7 +48,7 @@ use bitcoin::hashes::{HashEngine, Hash};
 pub trait CustomMessageHandler: wire::CustomMessageReader {
 	/// Called with the message type that was received and the buffer to be read.
 	/// Can return a `MessageHandlingError` if the message could not be handled.
-	fn handle_custom_message(&self, msg: Self::CustomMessage, sender_node_id: &PublicKey) -> Result<(), LightningError>;
+	fn handle_custom_message(&self, msg: Self::CustomMessage, sender_node_id: &PublicKey) -> Result<Infallible, LightningError>;
 
 	/// Gets the list of pending messages which were generated by the custom message
 	/// handler, clearing the list in the process. The first tuple element must
@@ -80,28 +81,28 @@ impl Deref for IgnoringMessageHandler {
 	fn deref(&self) -> &Self { self }
 }
 
-impl wire::Type for () {
+// Implement Type for Infallible, note that it cannot be constructed, and thus you can never call a
+// method that takes self for it.
+impl wire::Type for Infallible {
 	fn type_id(&self) -> u16 {
-		// We should never call this for `DummyCustomType`
 		unreachable!();
 	}
 }
-
-impl Writeable for () {
+impl Writeable for Infallible {
 	fn write<W: Writer>(&self, _: &mut W) -> Result<(), io::Error> {
 		unreachable!();
 	}
 }
 
 impl wire::CustomMessageReader for IgnoringMessageHandler {
-	type CustomMessage = ();
+	type CustomMessage = Infallible;
 	fn read<R: io::Read>(&self, _message_type: u16, _buffer: &mut R) -> Result<Option<Self::CustomMessage>, msgs::DecodeError> {
 		Ok(None)
 	}
 }
 
 impl CustomMessageHandler for IgnoringMessageHandler {
-	fn handle_custom_message(&self, _msg: Self::CustomMessage, _sender_node_id: &PublicKey) -> Result<(), LightningError> {
+	fn handle_custom_message(&self, _msg: Self::CustomMessage, _sender_node_id: &PublicKey) -> Result<Infallible, LightningError> {
 		// Since we always return `None` in the read the handle method should never be called.
 		unreachable!();
 	}

lightning/src/util/ser.rs

@@ -879,6 +879,11 @@ impl<A: Writeable, B: Writeable, C: Writeable> Writeable for (A, B, C) {
 	}
 }
 
+impl Writeable for () {
+	fn write<W: Writer>(&self, _: &mut W) -> Result<(), io::Error> {
+		Ok(())
+	}
+}
 impl Readable for () {
 	fn read<R: Read>(_r: &mut R) -> Result<Self, DecodeError> {
 		Ok(())
@@ -892,7 +897,6 @@ impl Writeable for String {
 		w.write_all(self.as_bytes())
 	}
 }
-
 impl Readable for String {
 	#[inline]
 	fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {