lintcmd: print reference to -explain check to stderr

Separate diagnostics from help output. The output of the plain
formatter is supposed to be machine readable, so don't emit text meant
strictly for humans.

For the stylish formatter we could've gone either way, because its
output is not meant to be processed by machines, but we copy the
behavior of the plain formatter for the sake of consistency.

lintcmd/cmd.go

@@ -632,9 +632,9 @@ func ProcessFlagSet(cs []*analysis.Analyzer, fs *flag.FlagSet) {
 	var f formatter
 	switch theFormatter {
 	case "text":
-		f = textFormatter{W: os.Stdout}
+		f = textFormatter{Diagnostics: os.Stdout, UI: os.Stderr}
 	case "stylish":
-		f = &stylishFormatter{W: os.Stdout}
+		f = &stylishFormatter{Diagnostics: os.Stdout, UI: os.Stderr}
 	case "json":
 		f = jsonFormatter{W: os.Stdout}
 	default:

lintcmd/format.go

@@ -48,18 +48,19 @@ type documentationMentioner interface {
 }
 
 type textFormatter struct {
-	W io.Writer
+	Diagnostics io.Writer
+	UI          io.Writer
 }
 
 func (o textFormatter) Format(p problem) {
-	fmt.Fprintf(o.W, "%s: %s\n", relativePositionString(p.Position), p.String())
+	fmt.Fprintf(o.Diagnostics, "%s: %s\n", relativePositionString(p.Position), p.String())
 	for _, r := range p.Related {
-		fmt.Fprintf(o.W, "\t%s: %s\n", relativePositionString(r.Position), r.Message)
+		fmt.Fprintf(o.Diagnostics, "\t%s: %s\n", relativePositionString(r.Position), r.Message)
 	}
 }
 
 func (o textFormatter) MentionCheckDocumentation(cmd string) {
-	fmt.Fprintf(o.W, "\nRun '%s -explain <check>' or visit https://staticcheck.io/docs/checks for documentation on checks.\n", cmd)
+	fmt.Fprintf(o.UI, "\nRun '%s -explain <check>' or visit https://staticcheck.io/docs/checks for documentation on checks.\n", cmd)
 }
 
 type jsonFormatter struct {
@@ -118,7 +119,8 @@ func (o jsonFormatter) Format(p problem) {
 }
 
 type stylishFormatter struct {
-	W io.Writer
+	Diagnostics io.Writer
+	UI          io.Writer
 
 	prevFile string
 	tw       *tabwriter.Writer
@@ -133,11 +135,11 @@ func (o *stylishFormatter) Format(p problem) {
 	if pos.Filename != o.prevFile {
 		if o.prevFile != "" {
 			o.tw.Flush()
-			fmt.Fprintln(o.W)
+			fmt.Fprintln(o.Diagnostics)
 		}
-		fmt.Fprintln(o.W, pos.Filename)
+		fmt.Fprintln(o.Diagnostics, pos.Filename)
 		o.prevFile = pos.Filename
-		o.tw = tabwriter.NewWriter(o.W, 0, 4, 2, ' ', 0)
+		o.tw = tabwriter.NewWriter(o.Diagnostics, 0, 4, 2, ' ', 0)
 	}
 	fmt.Fprintf(o.tw, "  (%d, %d)\t%s\t%s\n", pos.Line, pos.Column, p.Category, p.Message)
 	for _, r := range p.Related {
@@ -146,14 +148,14 @@ func (o *stylishFormatter) Format(p problem) {
 }
 
 func (o *stylishFormatter) MentionCheckDocumentation(cmd string) {
-	textFormatter{W: o.W}.MentionCheckDocumentation(cmd)
+	textFormatter{UI: o.UI}.MentionCheckDocumentation(cmd)
 }
 
 func (o *stylishFormatter) Stats(total, errors, warnings, ignored int) {
 	if o.tw != nil {
 		o.tw.Flush()
-		fmt.Fprintln(o.W)
+		fmt.Fprintln(o.Diagnostics)
 	}
-	fmt.Fprintf(o.W, " ✖ %d problems (%d errors, %d warnings, %d ignored)\n",
+	fmt.Fprintf(o.Diagnostics, " ✖ %d problems (%d errors, %d warnings, %d ignored)\n",
 		total, errors, warnings, ignored)
 }