From 979a2bd6a0d5659c6784a74906e7fc00936f53af Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Fri, 9 Jun 2023 15:50:25 -0400
Subject: [PATCH] linkspec: Implement HasRelayIds::display_relay_ids().

---
 crates/tor-linkspec/semver.md     |  1 +
 crates/tor-linkspec/src/traits.rs | 55 +++++++++++++++++++++++++++----
 2 files changed, 50 insertions(+), 6 deletions(-)
 create mode 100644 crates/tor-linkspec/semver.md

diff --git a/crates/tor-linkspec/semver.md b/crates/tor-linkspec/semver.md
new file mode 100644
index 000000000..a5f4d7102
--- /dev/null
+++ b/crates/tor-linkspec/semver.md
@@ -0,0 +1 @@
+ADDED: display_relay_ids() on HasRelayIds.
diff --git a/crates/tor-linkspec/src/traits.rs b/crates/tor-linkspec/src/traits.rs
index 953ce8f45..fcdf310b9 100644
--- a/crates/tor-linkspec/src/traits.rs
+++ b/crates/tor-linkspec/src/traits.rs
@@ -119,6 +119,12 @@ pub trait HasRelayIds {
         }
         std::cmp::Ordering::Equal
     }
+
+    /// Return a reference to this object suitable for formatting its
+    /// [`HasRelayIds`] members.
+    fn display_relay_ids(&self) -> DisplayRelayIds<'_, Self> {
+        DisplayRelayIds { inner: self }
+    }
 }
 
 impl<T: HasRelayIdsLegacy> HasRelayIds for T {
@@ -130,6 +136,47 @@ impl<T: HasRelayIdsLegacy> HasRelayIds for T {
     }
 }
 
+/// A helper type used to format the [`RelayId`](crate::RelayId)s in a
+/// [`HasRelayIds`].
+#[derive(Clone)]
+pub struct DisplayRelayIds<'a, T: HasRelayIds + ?Sized> {
+    /// The HasRelayIds that we're displaying.
+    inner: &'a T,
+}
+// Redactable must implement Debug.
+impl<'a, T: HasRelayIds + ?Sized> std::fmt::Debug for DisplayRelayIds<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("DisplayRelayIds").finish_non_exhaustive()
+    }
+}
+
+impl<'a, T: HasRelayIds + ?Sized> DisplayRelayIds<'a, T> {
+    /// Helper: output `self` in a possibly redacted way.
+    fn fmt_impl(&self, f: &mut fmt::Formatter<'_>, redact: bool) -> fmt::Result {
+        let mut iter = self.inner.identities();
+        if let Some(ident) = iter.next() {
+            write!(f, "{}", ident.maybe_redacted(redact))?;
+        }
+        if redact {
+            return Ok(());
+        }
+        for ident in iter {
+            write!(f, " {}", ident.maybe_redacted(redact))?;
+        }
+        Ok(())
+    }
+}
+impl<'a, T: HasRelayIds + ?Sized> std::fmt::Display for DisplayRelayIds<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.fmt_impl(f, false)
+    }
+}
+impl<'a, T: HasRelayIds + ?Sized> Redactable for DisplayRelayIds<'a, T> {
+    fn display_redacted(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.fmt_impl(f, true)
+    }
+}
+
 /// An iterator over all of the relay identities held by a [`HasRelayIds`]
 #[derive(Clone)]
 pub struct RelayIdIter<'a, T: HasRelayIds + ?Sized> {
@@ -280,12 +327,8 @@ impl<'a, T: ChanTarget> DisplayChanTarget<'a, T> {
             }
         }
 
-        for ident in self.inner.identities() {
-            write!(f, " {}", ident.maybe_redacted(redact))?;
-            if redact {
-                break;
-            }
-        }
+        write!(f, " ")?;
+        self.inner.display_relay_ids().fmt_impl(f, redact)?;
 
         write!(f, "]")
     }