Add option to print help to stderr

src/build/app/mod.rs

@@ -658,6 +658,8 @@ impl<'help> App<'help> {
 
     /// Prints the short help message (`-h`) to [`io::stdout()`].
     ///
+    /// If [`AppSettings::PrintToStderr`] is set, this will print to [`io::stderr()`] instead of stdout.
+    ///
     /// See also [`App::print_long_help`].
     ///
     /// # Examples
@@ -668,18 +670,22 @@ impl<'help> App<'help> {
     /// app.print_help();
     /// ```
     /// [`io::stdout()`]: std::io::stdout()
+    /// [`io::stderr()`]: std::io::stderr()
     pub fn print_help(&mut self) -> io::Result<()> {
         self._build();
+        let use_stderr = self.settings.is_set(AppSettings::PrintToStderr);
         let color = self.get_color();
 
         let p = Parser::new(self);
-        let mut c = Colorizer::new(false, color);
+        let mut c = Colorizer::new(use_stderr, color);
         Help::new(HelpWriter::Buffer(&mut c), &p, false).write_help()?;
         c.print()
     }
 
     /// Prints the long help message (`--help`) to [`io::stdout()`].
     ///
+    /// If [`AppSettings::PrintToStderr`] is set, this will print to [`io::stderr()`] instead of stdout.
+    ///
     /// See also [`App::print_help`].
     ///
     /// # Examples
@@ -690,15 +696,17 @@ impl<'help> App<'help> {
     /// app.print_long_help();
     /// ```
     /// [`io::stdout()`]: std::io::stdout()
+    /// [`io::stderr()`]: std::io::stderr()
     /// [`BufWriter`]: std::io::BufWriter
     /// [`-h` (short)]: Arg::help()
     /// [`--help` (long)]: Arg::long_help()
     pub fn print_long_help(&mut self) -> io::Result<()> {
         self._build();
+        let use_stderr = self.settings.is_set(AppSettings::PrintToStderr);
         let color = self.get_color();
 
         let p = Parser::new(self);
-        let mut c = Colorizer::new(false, color);
+        let mut c = Colorizer::new(use_stderr, color);
         Help::new(HelpWriter::Buffer(&mut c), &p, true).write_help()?;
         c.print()
     }

src/build/app/settings.rs

@@ -950,6 +950,18 @@ pub enum AppSettings {
     /// ```
     NoAutoVersion,
 
+    /// Prints the autogenerated help and version messages to stderr instead of stdout.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use clap::{App, AppSettings};
+    /// App::new("myprog")
+    ///    .global_setting(AppSettings::PrintToStderr)
+    ///   .get_matches();
+    /// ```
+    PrintToStderr,
+
     /// Deprecated, replaced with [`AppSettings::AllowHyphenValues`]
     #[deprecated(
         since = "3.0.0",
@@ -1067,8 +1079,9 @@ bitflags! {
         const USE_LONG_FORMAT_FOR_HELP_SC    = 1 << 42;
         const INFER_LONG_ARGS                = 1 << 43;
         const IGNORE_ERRORS                  = 1 << 44;
+        const PRINT_TO_STDERR                = 1 << 45;
         #[cfg(feature = "unstable-multicall")]
-        const MULTICALL                      = 1 << 45;
+        const MULTICALL                      = 1 << 46;
         const NO_OP                          = 0;
     }
 }
@@ -1169,7 +1182,9 @@ impl_settings! { AppSettings, AppFlags,
     AllArgsOverrideSelf
         => Flags::ARGS_OVERRIDE_SELF,
     InferLongArgs
-        => Flags::INFER_LONG_ARGS
+        => Flags::INFER_LONG_ARGS,
+    PrintToStderr
+        => Flags::PRINT_TO_STDERR
 }
 
 /// Deprecated in [Issue #3087](https://github.com/clap-rs/clap/issues/3087), maybe [`clap::Parser`][crate::Parser] would fit your use case?
@@ -1229,6 +1244,7 @@ impl FromStr for AppSettings {
             "infersubcommands" => Ok(AppSettings::InferSubcommands),
             "allargsoverrideself" => Ok(AppSettings::AllArgsOverrideSelf),
             "inferlongargs" => Ok(AppSettings::InferLongArgs),
+            "printtostderr" => Ok(AppSettings::PrintToStderr),
             _ => Err(format!("unknown AppSetting: `{}`", s)),
         }
     }

src/parse/parser.rs

@@ -1600,7 +1600,10 @@ impl<'help, 'app> Parser<'help, 'app> {
         );
 
         use_long = use_long && self.use_long_help();
-        let mut c = Colorizer::new(false, self.color_help());
+        let mut c = Colorizer::new(
+            self.app.settings.is_set(AS::PrintToStderr),
+            self.color_help(),
+        );
 
         match Help::new(HelpWriter::Buffer(&mut c), self, use_long).write_help() {
             Err(e) => e.into(),
@@ -1616,7 +1619,10 @@ impl<'help, 'app> Parser<'help, 'app> {
         debug!("Parser::version_err");
 
         let msg = self.app._render_version(use_long);
-        let mut c = Colorizer::new(false, self.color_help());
+        let mut c = Colorizer::new(
+            self.app.settings.is_set(AS::PrintToStderr),
+            self.color_help(),
+        );
         c.none(msg);
         ClapError::new(
             c,