--- a/rust/rhg/src/commands/cat.rs	Mon Mar 01 13:51:35 2021 +0100
+++ b/rust/rhg/src/commands/cat.rs	Mon Mar 01 16:18:42 2021 +0100
@@ -60,6 +60,8 @@
             invocation.ui.write_stdout(&data)?;
             Ok(())
         }
-        None => Err(CommandError::Unimplemented.into()),
+        None => Err(CommandError::unsupported(
+            "`rhg cat` without `--rev` / `-r`",
+        )),
     }
 }