Add --backup flag

src/bin/main.rs

@@ -91,6 +91,7 @@ fn make_opts() -> Options {
     let mut opts = Options::new();
 
     // Sorted in alphabetical order.
+    opts.optflag("", "backup", "Backup any modified files.");
     opts.optflag(
         "",
         "check",

src/config/options.rs

@@ -368,7 +368,11 @@ pub fn from_matches(matches: &Matches) -> FmtResult<CliOptions> {
                 return Err(format_err!("Invalid to set write-mode and `--check`"));
             }
             if let Ok(write_mode) = WriteMode::from_str(write_mode) {
-                options.write_mode = Some(write_mode);
+                if write_mode == WriteMode::Overwrite && matches.opt_present("backup") {
+                    options.write_mode = Some(WriteMode::Replace);
+                } else {
+                    options.write_mode = Some(write_mode);
+                }
             } else {
                 return Err(format_err!(
                     "Invalid write-mode: {}, expected one of {}",

src/lib.rs

@@ -68,7 +68,7 @@
 
 pub type FmtResult<T> = std::result::Result<T, failure::Error>;
 
-pub const WRITE_MODE_LIST: &str = "[replace|overwrite|display|plain|diff|coverage|checkstyle]";
+pub const WRITE_MODE_LIST: &str = "[overwrite|display|plain|diff|coverage|checkstyle]";
 
 #[macro_use]
 mod utils;