Restructure code to let linker perform deadcode elimination step

Cobra, in its default configuration, will execute a template to generate
help, usage and version outputs. Text/template execution calls MethodByName
and MethodByName disables dead code elimination in the Go linker, therefore
all programs that make use of cobra will be linked with dead code
elimination disabled, even if they end up replacing the default usage, help
and version formatters with a custom function and no actual text/template
evaluations are ever made at runtime.

Dead code elimination in the linker helps reduce disk space and memory
utilization of programs. For example, for the simple example program used by
TestDeadcodeElimination 40% of the final executable size is dead code. For a
more realistic example, 12% of the size of Delve's executable is deadcode.

This PR changes Cobra so that, in its default configuration, it does not
automatically inhibit deadcode elimination by:

1. changing Cobra's default behavior to emit output for usage and help using
   simple Go functions instead of template execution
2. quarantining all calls to template execution into SetUsageTemplate,
   SetHelpTemplate and SetVersionTemplate so that the linker can statically
   determine if they are reachable

cobra.go

@@ -176,12 +176,16 @@ func rpad(s string, padding int) string {
 	return fmt.Sprintf(formattedString, s)
 }
 
-// tmpl executes the given template text on data, writing the result to w.
-func tmpl(w io.Writer, text string, data interface{}) error {
-	t := template.New("top")
-	t.Funcs(templateFuncs)
-	template.Must(t.Parse(text))
-	return t.Execute(w, data)
+func tmpl(text string) *tmplFunc {
+	return &tmplFunc{
+		tmpl: text,
+		fn: func(w io.Writer, data interface{}) error {
+			t := template.New("top")
+			t.Funcs(templateFuncs)
+			template.Must(t.Parse(text))
+			return t.Execute(w, data)
+		},
+	}
 }
 
 // ld compares two strings and returns the levenshtein distance between them.

cobra_test.go

@@ -15,6 +15,10 @@
 package cobra
 
 import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
 	"testing"
 	"text/template"
 )
@@ -222,3 +226,57 @@ func TestRpad(t *testing.T) {
 		})
 	}
 }
+
+func TestDeadcodeElimination(t *testing.T) {
+	// check that a simple program using cobra in its default configuration is
+	// linked with deadcode elimination enabled.
+	const (
+		dirname  = "test_deadcode"
+		progname = "test_deadcode_elimination"
+	)
+	os.Mkdir(dirname, 0770)
+	filename := filepath.Join(dirname, progname+".go")
+	err := os.WriteFile(filename, []byte(`package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+var rootCmd = &cobra.Command{
+	Version: "1.0",
+	Use:     "example_program",
+	Short:   "example_program - test fixture to check that deadcode elimination is allowed",
+	Run: func(cmd *cobra.Command, args []string) {
+		fmt.Println("hello world")
+	},
+	Aliases: []string{"alias1", "alias2"},
+	Example: "stringer --help",
+}
+
+func main() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Whoops. There was an error while executing your CLI '%s'", err)
+		os.Exit(1)
+	}
+}
+`), 0660)
+	if err != nil {
+		t.Fatalf("could not write test program: %v", err)
+	}
+	defer os.RemoveAll(dirname)
+	buf, err := exec.Command("go", "build", filename).CombinedOutput()
+	if err != nil {
+		t.Fatalf("could not compile test program: %s", string(buf))
+	}
+	defer os.Remove(progname)
+	buf, err = exec.Command("go", "tool", "nm", progname).CombinedOutput()
+	if err != nil {
+		t.Fatalf("could not run go tool nm: %v", err)
+	}
+	if strings.Contains(string(buf), "MethodByName") {
+		t.Error("compiled programs contains MethodByName symbol")
+	}
+}