Add log_iter utility macro

This is a useful primitive to have that formats the contents of the
iterator as a comma-separated list.

lightning/src/util/logger.rs

@@ -169,6 +169,29 @@ impl<'a> core::fmt::Display for DebugBytes<'a> {
 	}
 }
 
+/// Wrapper for logging `Iterator`s.
+///
+/// This is not exported to bindings users as fmt can't be used in C
+#[doc(hidden)]
+pub struct DebugIter<T: fmt::Display, I: core::iter::Iterator<Item = T> + Clone>(pub core::cell::RefCell<I>);
+impl<T: fmt::Display, I: core::iter::Iterator<Item = T> + Clone> fmt::Display for DebugIter<T, I> {
+	fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+		use core::ops::DerefMut;
+		write!(f, "[")?;
+		let iter_ref = self.0.clone();
+		let mut iter = iter_ref.borrow_mut();
+		for item in iter.deref_mut() {
+			write!(f, "{}", item)?;
+			break;
+		}
+		for item in iter.deref_mut() {
+			write!(f, ", {}", item)?;
+		}
+		write!(f, "]")?;
+		Ok(())
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use crate::util::logger::{Logger, Level};

lightning/src/util/macro_logger.rs

@@ -17,6 +17,12 @@ use crate::routing::router::Route;
 use crate::ln::chan_utils::HTLCClaim;
 use crate::util::logger::DebugBytes;
 
+macro_rules! log_iter {
+	($obj: expr) => {
+		$crate::util::logger::DebugIter(core::cell::RefCell::new($obj))
+	}
+}
+
 /// Logs a pubkey in hex format.
 #[macro_export]
 macro_rules! log_pubkey {