Use stderr for various errors and warnings.

Adjusts several error and warning report cases to output using eprintln!
instead of println! so that messages are sent to stderr.

src/bin/cargo-fmt.rs

@@ -55,14 +55,14 @@ fn execute() -> i32 {
         .take_while(|a| a != "--")
         .find(|a| !a.starts_with('-'))
     {
-        print_usage(&opts, &format!("Invalid argument: `{}`.", arg));
+        print_usage_to_stderr(&opts, &format!("Invalid argument: `{}`.", arg));
         return failure;
     }
 
     let matches = match opts.parse(env::args().skip(1).take_while(|a| a != "--")) {
         Ok(m) => m,
         Err(e) => {
-            print_usage(&opts, &e.to_string());
+            print_usage_to_stderr(&opts, &e.to_string());
             return failure;
         }
     };
@@ -72,21 +72,21 @@ fn execute() -> i32 {
         (false, true) => Verbosity::Quiet,
         (true, false) => Verbosity::Verbose,
         (true, true) => {
-            print_usage(&opts, "quiet mode and verbose mode are not compatible");
+            print_usage_to_stderr(&opts, "quiet mode and verbose mode are not compatible");
             return failure;
         }
     };
 
     if matches.opt_present("h") {
-        print_usage(&opts, "");
+        print_usage_to_stdout(&opts, "");
         return success;
     }
 
     let workspace_hitlist = WorkspaceHitlist::from_matches(&matches);
 
     match format_crate(verbosity, &workspace_hitlist) {
         Err(e) => {
-            print_usage(&opts, &e.to_string());
+            print_usage_to_stderr(&opts, &e.to_string());
             failure
         }
         Ok(status) => if status.success() {
@@ -97,13 +97,23 @@ fn execute() -> i32 {
     }
 }
 
-fn print_usage(opts: &Options, reason: &str) {
-    let msg = format!("{}\nusage: cargo fmt [options]", reason);
-    println!(
-        "{}\nThis utility formats all bin and lib files of the current crate using rustfmt. \
-         Arguments after `--` are passed to rustfmt.",
-        opts.usage(&msg)
-    );
+macro_rules! print_usage {
+    ($print:ident, $opts:ident, $reason:expr) => ({
+        let msg = format!("{}\nusage: cargo fmt [options]", $reason);
+        $print!(
+            "{}\nThis utility formats all bin and lib files of the current crate using rustfmt. \
+             Arguments after `--` are passed to rustfmt.",
+            $opts.usage(&msg)
+        );
+    })
+}
+
+fn print_usage_to_stdout(opts: &Options, reason: &str) {
+    print_usage!(println, opts, reason);
+}
+
+fn print_usage_to_stderr(opts: &Options, reason: &str) {
+    print_usage!(eprintln, opts, reason);
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]

src/bin/rustfmt.rs

@@ -161,7 +161,7 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
 
     match determine_operation(&matches)? {
         Operation::Help => {
-            print_usage(opts, "");
+            print_usage_to_stdout(opts, "");
             Summary::print_exit_codes();
             Ok(Summary::default())
         }
@@ -196,7 +196,7 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
                 config.set().file_lines(file_lines.parse()?);
                 for f in config.file_lines().files() {
                     if f != "stdin" {
-                        println!("Warning: Extra file listed in file_lines option '{}'", f);
+                        eprintln!("Warning: Extra file listed in file_lines option '{}'", f);
                     }
                 }
             }
@@ -212,7 +212,7 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
 
             for f in options.file_lines.files() {
                 if !files.contains(&PathBuf::from(f)) {
-                    println!("Warning: Extra file listed in file_lines option '{}'", f);
+                    eprintln!("Warning: Extra file listed in file_lines option '{}'", f);
                 }
             }
 
@@ -231,10 +231,10 @@ fn execute(opts: &Options) -> FmtResult<Summary> {
             let mut error_summary = Summary::default();
             for file in files {
                 if !file.exists() {
-                    println!("Error: file `{}` does not exist", file.to_str().unwrap());
+                    eprintln!("Error: file `{}` does not exist", file.to_str().unwrap());
                     error_summary.add_operational_error();
                 } else if file.is_dir() {
-                    println!("Error: `{}` is a directory", file.to_str().unwrap());
+                    eprintln!("Error: `{}` is a directory", file.to_str().unwrap());
                     error_summary.add_operational_error();
                 } else {
                     // Check the file directory if the config-path could not be read or not provided
@@ -293,7 +293,7 @@ fn main() {
             }
         }
         Err(e) => {
-            print_usage(&opts, &e.to_string());
+            print_usage_to_stderr(&opts, &e.to_string());
             1
         }
     };
@@ -307,13 +307,23 @@ fn main() {
     std::process::exit(exit_code);
 }
 
-fn print_usage(opts: &Options, reason: &str) {
-    let reason = format!(
-        "{}\n\nusage: {} [options] <file>...",
-        reason,
-        env::args_os().next().unwrap().to_string_lossy()
-    );
-    println!("{}", opts.usage(&reason));
+macro_rules! print_usage {
+    ($print:ident, $opts:ident, $reason:expr) => ({
+        let msg = format!(
+            "{}\n\nusage: {} [options] <file>...",
+            $reason,
+            env::args_os().next().unwrap().to_string_lossy()
+        );
+        $print!("{}", $opts.usage(&msg));
+    })
+}
+
+fn print_usage_to_stdout(opts: &Options, reason: &str) {
+    print_usage!(println, opts, reason);
+}
+
+fn print_usage_to_stderr(opts: &Options, reason: &str) {
+    print_usage!(eprintln, opts, reason);
 }
 
 fn print_version() {

src/visitor.rs

@@ -601,7 +601,7 @@ impl<'a> FmtVisitor<'a> {
         match self.codemap.span_to_snippet(span) {
             Ok(s) => s,
             Err(_) => {
-                println!(
+                eprintln!(
                     "Couldn't make snippet for span {:?}->{:?}",
                     self.codemap.lookup_char_pos(span.lo()),
                     self.codemap.lookup_char_pos(span.hi())