feat: add deprecator

This utility adds deprecation comments to all exported types in a
given module. It's intended to be used by Boxo maintainers when moving
repos into Boxo.

For example, if your module is "github.com/ipfs/go-unixfs", then
checkout that repo and, in its root, run:

deprecator --path github.com/ipfs/boxo/ipld/unixfs

Then on the func "github.com/ipfs/go-unixfs/mod.NewDagModifier", the
deprecator will add the following comment:

// Deprecated: use github.com/ipfs/boxo/ipld/unixfs/mod.NewDagModifier

cmd/deprecator/go.mod

@@ -0,0 +1,15 @@
+module github.com/ipfs/boxo/cmd/deprecator
+
+go 1.20
+
+require github.com/dave/dst v0.27.2
+
+require (
+	github.com/cpuguy83/go-md2man/v2 v2.0.2 // indirect
+	github.com/russross/blackfriday/v2 v2.1.0 // indirect
+	github.com/urfave/cli/v2 v2.25.3 // indirect
+	github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673 // indirect
+	golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 // indirect
+	golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f // indirect
+	golang.org/x/tools v0.1.12 // indirect
+)

cmd/deprecator/go.sum

@@ -0,0 +1,18 @@
+github.com/cpuguy83/go-md2man/v2 v2.0.2 h1:p1EgwI/C7NhT0JmVkwCD2ZBK8j4aeHQX2pMHHBfMQ6w=
+github.com/cpuguy83/go-md2man/v2 v2.0.2/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
+github.com/dave/dst v0.27.2 h1:4Y5VFTkhGLC1oddtNwuxxe36pnyLxMFXT51FOzH8Ekc=
+github.com/dave/dst v0.27.2/go.mod h1:jHh6EOibnHgcUW3WjKHisiooEkYwqpHLBSX1iOBhEyc=
+github.com/dave/jennifer v1.5.0 h1:HmgPN93bVDpkQyYbqhCHj5QlgvUkvEOzMyEvKLgCRrg=
+github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
+github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
+github.com/sergi/go-diff v1.2.0 h1:XU+rvMAioB0UC3q1MFrIQy4Vo5/4VsRDQQXHsEya6xQ=
+github.com/urfave/cli/v2 v2.25.3 h1:VJkt6wvEBOoSjPFQvOkv6iWIrsJyCrKGtCtxXWwmGeY=
+github.com/urfave/cli/v2 v2.25.3/go.mod h1:GHupkWPMM0M/sj1a2b4wUrWBPzazNrIjouW6fmdJLxc=
+github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673 h1:bAn7/zixMGCfxrRTfdpNzjtPYqr8smhKouy9mxVdGPU=
+github.com/xrash/smetrics v0.0.0-20201216005158-039620a65673/go.mod h1:N3UwUGtsrSj3ccvlPHLoLsHnpR27oXr4ZE984MbSER8=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 h1:6zppjxzCulZykYSLyVDYbneBfbaBIQPYMevg0bEwv2s=
+golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f h1:v4INt8xihDGvnrfjMDVXGxw9wrfxYyCjk0KbXjhR55s=
+golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/tools v0.1.12 h1:VveCTK38A2rkS8ZqFY25HIDFscX5X9OoEhJd3quQmXU=
+golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=

cmd/deprecator/main.go

@@ -0,0 +1,180 @@
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io"
+	"io/fs"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"github.com/dave/dst"
+	"github.com/dave/dst/decorator"
+	"github.com/urfave/cli/v2"
+)
+
+func main() {
+	app := &cli.App{
+		Name:  "migrate",
+		Usage: "Adds deprecation comments to all exported types in the module, with pointers to a new location. You should run this in your module root.",
+		Flags: []cli.Flag{
+			&cli.StringFlag{
+				Name:     "path",
+				Usage:    "the new package path that the deprecated message should point users to",
+				Required: true,
+			},
+		},
+		Action: func(ctx *cli.Context) error {
+			newPkgPath := ctx.String("path")
+			wd, err := os.Getwd()
+			if err != nil {
+				return fmt.Errorf("getting working dir: %w", err)
+			}
+			fileToPackage, err := buildFileToPackage(wd)
+			if err != nil {
+				panic(err)
+			}
+
+			modPath, err := getModulePath(wd)
+			if err != nil {
+				panic(err)
+			}
+			fmt.Printf("mod path: %s\n", modPath)
+
+			fset := token.NewFileSet()
+			return filepath.Walk(wd, func(path string, info fs.FileInfo, err error) error {
+				if err != nil {
+					return err
+				}
+				if info.IsDir() || filepath.Ext(path) != ".go" {
+					return nil
+				}
+
+				commentStr := func(name string) string {
+					oldPkg := fileToPackage[path]
+					newPkg := strings.Replace(oldPkg, modPath, newPkgPath, 1)
+					newSym := newPkg + "." + name
+					comment := fmt.Sprintf("// Deprecated: use %s", newSym)
+					return comment
+				}
+
+				file, err := decorator.ParseFile(fset, path, nil, parser.ParseComments)
+				if err != nil {
+					panic(err)
+				}
+
+				if _, ok := fileToPackage[path]; !ok {
+					// this happens in the case of e.g. test files, which we want to skip
+					return nil
+				}
+
+				dst.Inspect(file, func(n dst.Node) bool {
+					switch x := n.(type) {
+					case *dst.GenDecl:
+						if x.Tok == token.CONST || x.Tok == token.VAR || x.Tok == token.TYPE {
+							for _, spec := range x.Specs {
+								switch s := spec.(type) {
+								case *dst.ValueSpec:
+									// if parenthesized, put a comment above each exported type in the group
+									if x.Lparen {
+										for _, name := range s.Names {
+											if !name.IsExported() {
+												continue
+											}
+											name.Decs.Start.Append(commentStr(name.Name))
+										}
+									} else {
+										name := s.Names[0]
+										if !name.IsExported() {
+											continue
+										}
+										x.Decs.Start.Append(commentStr(name.Name))
+									}
+								case *dst.TypeSpec:
+									name := s.Name
+									if !name.IsExported() {
+										continue
+									}
+									x.Decs.Start.Append(commentStr(name.Name))
+								}
+							}
+						}
+					case *dst.FuncDecl:
+						// don't add notices to methods
+						if x.Name.IsExported() && x.Recv == nil {
+							x.Decs.Start.Append(commentStr(x.Name.Name))
+						}
+					}
+					return true
+				})
+				outFile, err := os.Create(path)
+				if err != nil {
+					panic(err)
+				}
+				defer outFile.Close()
+				err = decorator.Fprint(outFile, file)
+				if err != nil {
+					panic(err)
+				}
+
+				return nil
+			})
+		},
+	}
+	err := app.Run(os.Args)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+type pkg struct {
+	Dir        string
+	ImportPath string
+	GoFiles    []string
+}
+
+func getModulePath(dir string) (string, error) {
+	cmd := exec.Command("go", "list", "-m")
+	cmd.Dir = dir
+	stdout := &bytes.Buffer{}
+	cmd.Stdout = stdout
+	err := cmd.Run()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(stdout.String()), nil
+}
+
+func buildFileToPackage(dir string) (map[string]string, error) {
+	cmd := exec.Command("go", "list", "-json", "./...")
+	cmd.Dir = dir
+	stdout := &bytes.Buffer{}
+	stderr := &bytes.Buffer{}
+	cmd.Stdout = stdout
+	cmd.Stderr = stderr
+	err := cmd.Run()
+	if err != nil {
+		return nil, err
+	}
+	dec := json.NewDecoder(stdout)
+	fileToPackage := map[string]string{}
+	for {
+		var p pkg
+		err := dec.Decode(&p)
+		if err == io.EOF {
+			return fileToPackage, nil
+		}
+		if err != nil {
+			return nil, err
+		}
+		for _, f := range p.GoFiles {
+			fileToPackage[filepath.Join(p.Dir, f)] = p.ImportPath
+		}
+	}
+}