Only set the exit code if --exit-code is set

This is important for usage with git log, which terminates on non-zero
exit codes.

CHANGELOG.md

@@ -17,9 +17,10 @@ interleaved output from different files.
 
 ### Command Line Interface
 
-Difftastic now sets the exit code if it finds changes. See [usage in
-the manual](https://difftastic.wilfred.me.uk/usage.html) for the full
-list of exit codes used.
+Difftastic now sets the exit code if it finds changes and
+`--exit-code` is set. See [usage in the
+manual](https://difftastic.wilfred.me.uk/usage.html) for the full list
+of exit codes used.
 
 ## 0.38 (released 14th November 2022)
 

manual/src/usage.md

@@ -56,11 +56,12 @@ where you are not invoking the `difft` binary directly.
 
 ## Exit Codes
 
-0: Difftastic found no syntactic changes (in text files) or no byte
-changes (in binary files).
+2: Difftastic was given invalid arguments. This includes invalid usage
+(e.g. the wrong number of arguments) as well as paths that difftastic
+cannot read (e.g. non-existent paths or insufficient permissions).
 
-1: Difftastic found syntactic changes (in text files) or byte changes
+1: When called with `--exit-code`, difftastic will return an exit code
+of 1 when it finds any syntactic changes (in text files) or byte changes
 (in binary files).
 
-2: Difftastic was given invalid arguments, such as file paths that it
-couldn't read, or the wrong number of arguments.
+0: All other cases.

src/main.rs

@@ -161,6 +161,7 @@ fn main() {
             byte_limit,
             display_options,
             missing_as_empty,
+            set_exit_code,
             language_override,
             lhs_path,
             rhs_path,
@@ -239,7 +240,7 @@ fn main() {
                 }
             }
 
-            let exit_code = if encountered_changes.load(Ordering::Relaxed) {
+            let exit_code = if set_exit_code && encountered_changes.load(Ordering::Relaxed) {
                 EXIT_FOUND_CHANGES
             } else {
                 EXIT_SUCCESS

src/options.rs

@@ -152,6 +152,11 @@ fn app() -> clap::Command<'static> {
                 .default_value("on")
                 .help("Enable or disable syntax highlighting.")
         )
+        .arg(
+            Arg::new("exit-code").long("exit-code")
+                .env("DFT_EXIT_CODE")
+                .help("Set the exit code to 1 if there are syntactic changes in any text files, or byte changes in any binary files.")
+        )
         .arg(
             Arg::new("skip-unchanged").long("skip-unchanged")
                 .help("Don't display anything if a file is unchanged.")
@@ -259,6 +264,7 @@ pub enum Mode {
         byte_limit: usize,
         display_options: DisplayOptions,
         missing_as_empty: bool,
+        set_exit_code: bool,
         language_override: Option<guess_language::Language>,
         /// The path where we can read the LHS file. This is often a
         /// temporary file generated by source control.
@@ -451,6 +457,8 @@ pub fn parse_args() -> Mode {
     // file on all platforms?
     let missing_as_empty = matches.is_present("missing-as-empty");
 
+    let set_exit_code = matches.is_present("exit-code");
+
     let display_options = DisplayOptions {
         background_color,
         use_color,
@@ -468,6 +476,7 @@ pub fn parse_args() -> Mode {
         byte_limit,
         display_options,
         missing_as_empty,
+        set_exit_code,
         language_override,
         lhs_path,
         rhs_path,