Add --check flag.

cc #1976

src/bin/main.rs

@@ -91,6 +91,12 @@ fn make_opts() -> Options {
     let mut opts = Options::new();
 
     // Sorted in alphabetical order.
+    opts.optflag(
+        "",
+        "check",
+        "Run in 'check' mode. Exits with 0 if input if formatted correctly. Exits \
+         with 1 and prints a diff if formatting is required.",
+    );
     opts.optopt(
         "",
         "color",
@@ -308,7 +314,8 @@ fn print_usage_to_stdout(opts: &Options, reason: &str) {
 }
 
 fn print_help_file_lines() {
-    println!("If you want to restrict reformatting to specific sets of lines, you can
+    println!(
+        "If you want to restrict reformatting to specific sets of lines, you can
 use the `--file-lines` option. Its argument is a JSON array of objects
 with `file` and `range` properties, where `file` is a file name, and
 `range` is an array representing a range of lines like `[7,13]`. Ranges
@@ -325,7 +332,8 @@ rustfmt --file-lines '[
 
 would format lines `7-13` and `21-29` of `src/lib.rs`, and lines `10-11`,
 and `15` of `src/foo.rs`. No other files would be formatted, even if they
-are included as out of line modules from `src/lib.rs`.");
+are included as out of line modules from `src/lib.rs`."
+    );
 }
 
 fn print_version() {

src/config/options.rs

@@ -332,6 +332,7 @@ pub struct CliOptions {
     verbose: bool,
     pub(super) config_path: Option<PathBuf>,
     write_mode: Option<WriteMode>,
+    check: bool,
     color: Option<Color>,
     file_lines: FileLines, // Default is all lines in all files.
     unstable_features: bool,
@@ -361,7 +362,11 @@ impl CliOptions {
 
         options.config_path = matches.opt_str("config-path").map(PathBuf::from);
 
+        options.check = matches.opt_present("check");
         if let Some(ref write_mode) = matches.opt_str("write-mode") {
+            if options.check {
+                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);
             } else {
@@ -410,7 +415,9 @@ impl CliOptions {
         if let Some(error_on_unformatted) = self.error_on_unformatted {
             config.set().error_on_unformatted(error_on_unformatted);
         }
-        if let Some(write_mode) = self.write_mode {
+        if self.check {
+            config.set().write_mode(WriteMode::Check);
+        } else if let Some(write_mode) = self.write_mode {
             config.set().write_mode(write_mode);
         }
         if let Some(color) = self.color {

src/lib.rs

@@ -68,8 +68,7 @@ pub use config::{file_lines, load_config, Config, Verbosity, WriteMode};
 
 pub type FmtResult<T> = std::result::Result<T, failure::Error>;
 
-pub const WRITE_MODE_LIST: &str =
-    "[replace|overwrite|display|plain|diff|coverage|checkstyle|check]";
+pub const WRITE_MODE_LIST: &str = "[replace|overwrite|display|plain|diff|coverage|checkstyle]";
 
 #[macro_use]
 mod utils;