diff -r 522ec3dc44b9 -r 33ded2d3f4fc rust/rhg/src/error.rs
--- a/rust/rhg/src/error.rs	Fri Sep 11 17:32:53 2020 +0200
+++ b/rust/rhg/src/error.rs	Tue Sep 15 16:51:11 2020 +0200
@@ -18,6 +18,8 @@
     StderrError,
     /// The command aborted
     Abort(Option<Vec<u8>>),
+    /// A mercurial capability as not been implemented.
+    Unimplemented,
 }
 
 impl CommandErrorKind {
@@ -28,6 +30,7 @@
             CommandErrorKind::StdoutError => exitcode::ABORT,
             CommandErrorKind::StderrError => exitcode::ABORT,
             CommandErrorKind::Abort(_) => exitcode::ABORT,
+            CommandErrorKind::Unimplemented => exitcode::UNIMPLEMENTED_COMMAND,
         }
     }