Print out stdlib packages not used within the stdlib.

results.txt

$ go run unused.go
container/ring
debug/gosym
debug/pe
encoding/ascii85
encoding/csv
expvar
go/build
hash/crc64
hash/fnv
index/suffixarray
log/syslog
net/http/cookiejar
net/http/fcgi
net/http/pprof
net/mail
net/rpc/jsonrpc
net/smtp
os/user
runtime/cgo
runtime/race
text/scanner

unused.go

// Print a list of stdlib packages not used in the stdlib.
package main

import (
	"bytes"
	"fmt"
	"go/build"
	"log"
	"os/exec"
	"strings"
)

func main() {
	// Use 'go list std' to get a list of stdlib packages.
	// Maybe there's a better way.
	out, err := exec.Command("go", "list", "std").Output()
	if err != nil {
		log.Fatal(err)
	}
	var pkgs []string
	for _, name := range bytes.Fields(out) {
		pkg := string(name)
		if !strings.HasPrefix(pkg, "cmd/") {
			pkgs = append(pkgs, string(name))
		}
	}

	used := make(map[string]bool)
	for _, name := range pkgs {
		pkg, err := build.Import(name, "", 0)
		if err != nil {
			log.Fatal(err)
		}
		imports := append(pkg.Imports, pkg.TestImports...)
		imports = append(imports, pkg.XTestImports...)
		for _, imp := range imports {
			used[imp] = true
		}
	}

	for _, name := range pkgs {
		if !used[name] {
			fmt.Println(name)
		}
	}
}