Print an error instead of panic on invalid write-mode

Author: kamalmarhubi

src/bin/rustfmt.rs

@@ -110,17 +110,18 @@ fn match_cli_path_or_file(config_path: Option<PathBuf>,
     resolve_config(input_file)
 }
 
-fn update_config(config: &mut Config, matches: &Matches) {
+fn update_config(config: &mut Config, matches: &Matches) -> Result<(), String> {
     config.verbose = matches.opt_present("verbose");
     config.skip_children = matches.opt_present("skip-children");
 
-    let write_mode = matches.opt_str("write-mode").map(|ref s| {
-        WriteMode::from_str(s).expect(&format!("Invalid write-mode: {}", s))
-    });
-
-    if let Some(write_mode) = write_mode {
-        config.write_mode = write_mode;
+    if let Some(write_mode) = matches.opt_str("write-mode") {
+        match WriteMode::from_str(&write_mode) {
+            Ok(write_mode) => config.write_mode = write_mode,
+            Err(_) => return Err(format!("Invalid write-mode: {}", write_mode)),
+        }
     }
+
+    Ok(())
 }
 
 fn execute() -> i32 {
@@ -210,7 +211,10 @@ fn execute() -> i32 {
                     config = config_tmp;
                 }
 
-                update_config(&mut config, &matches);
+                if let Err(e) = update_config(&mut config, &matches) {
+                    print_usage(&opts, &e);
+                    return 1;
+                }
                 run(&file, &config);
             }
             0