Tool to generate go file with "usage" constang holding "main" package docs

update_usage.go

//+build generate

package main

import (
	"bytes"
	"fmt"
	"go/doc"
	"go/parser"
	"go/token"
	"io/ioutil"
	"os"
)

func main() {
	if err := run(); err != nil {
		os.Stderr.WriteString(err.Error() + "\n")
		os.Exit(1)
	}
}

func run() error {
	fset := token.NewFileSet()
	m, err := parser.ParseDir(fset, ".", nil, parser.ParseComments)
	if err != nil {
		return err
	}
	p, ok := m["main"]
	if !ok {
		return fmt.Errorf("cannot find main package")
	}
	docBuf := new(bytes.Buffer)
	for _, f := range p.Files {
		if f.Doc == nil {
			continue
		}
		doc.ToText(docBuf, f.Doc.Text(), "", "\t", 80)
	}
	if docBuf.Len() == 0 {
		return fmt.Errorf("could not extract any docs")
	}
	buf := new(bytes.Buffer)
	fmt.Fprintf(buf, templateFormat, docBuf.String())
	return ioutil.WriteFile("usage_generated.go", buf.Bytes(), 0666)
}

const templateFormat = `// go run update_usage.go
// Code generated by the command above; DO NOT EDIT.

package main

const usage = %#v
`

For standalone tool see https://godoc.org/github.com/artyom/usagegen