Improve display for tor-dircache errors.

These errors no longer use Debug to dump the `Option<SourceInfo>`, but
instead produce reasonable text.  Also, I've fixed the SourceInfo
Display implementation so that it now says that it got the error
"from $source via $circuit" rather than the other way around.

crates/tor-dirclient/src/err.rs

@@ -18,7 +18,7 @@ pub enum Error {
     CircMgr(#[from] tor_circmgr::Error),
 
     /// An error that has occurred after we have contacted a directory cache and made a circuit to it.
-    #[error("Error fetching directory information from {source:?}")]
+    #[error("Error fetching directory information{}", FromSource(.source))]
     RequestFailed {
         /// The source that gave us this error.
         source: Option<SourceInfo>,
@@ -29,6 +29,19 @@ pub enum Error {
     },
 }
 
+/// Helper type to display an optional source of directory information.
+struct FromSource<'a>(&'a Option<SourceInfo>);
+
+impl std::fmt::Display for FromSource<'_> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        if let Some(si) = self.0 {
+            write!(f, " from {}", si)
+        } else {
+            Ok(())
+        }
+    }
+}
+
 /// An error originating from the tor-dirclient crate.
 #[derive(Error, Debug, Clone)]
 #[non_exhaustive]

crates/tor-dirclient/src/response.rs

@@ -24,7 +24,7 @@ pub struct DirResponse {
 /// We use this to remember when a request has failed, so we can
 /// abandon the circuit.
 #[derive(Debug, Clone, derive_more::Display)]
-#[display(fmt = "{} via {}", circuit, cache_id)]
+#[display(fmt = "{} via {}", cache_id, circuit)]
 pub struct SourceInfo {
     /// Unique identifier for the circuit we're using
     circuit: UniqId,