rhg: print error message when argument parsing fails

Differential Revision: https://phab.mercurial-scm.org/D8956

rust/rhg/src/main.rs

@@ -21,7 +21,8 b' fn main() {'
             SubCommand::with_name("files").about(commands::files::HELP_TEXT),
         );
 
-    let matches = app.clone().get_matches_safe().unwrap_or_else(|_| {
+    let matches = app.clone().get_matches_safe().unwrap_or_else(|err| {
+        let _ = ui::Ui::new().writeln_stderr_str(&err.message);
         std::process::exit(exitcode::UNIMPLEMENTED_COMMAND)
     });
 

rust/rhg/src/ui.rs

@@ -47,6 +47,11 b' impl Ui {'
 
         stderr.flush().or_else(handle_stderr_error)
     }
+
+    /// Write string line to stderr
+    pub fn writeln_stderr_str(&self, s: &str) -> Result<(), UiError> {
+        self.write_stderr(&format!("{}\n", s).as_bytes())
+    }
 }
 
 /// A buffered stdout writer for faster batch printing operations.

tests/test-rhg.t

@@ -12,6 +12,12 b' Define an rhg function that will only ru'
 
 Unimplemented command
   $ rhg unimplemented-command
+  error: Found argument 'unimplemented-command' which wasn't expected, or isn't valid in this context
+  
+  USAGE:
+      rhg <SUBCOMMAND>
+  
+  For more information try --help
   [252]
 
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