refactor/rename: attempt to fix windows build...

...by using buildutil.ContainingPackage instead of guessImportPath.
The former should be more portable.

(I meant to do this earlier, so this is also a nice cleanup.)

LGTM=gri
R=gri, sameer
CC=golang-codereviews
https://golang.org/cl/148050043

refactor/rename/spec.go

@@ -115,12 +115,12 @@ func parseFromFlag(ctxt *build.Context, fromFlag string) (*spec, error) {
 		if !buildutil.FileExists(ctxt, spec.filename) {
 			return nil, fmt.Errorf("no such file: %s", spec.filename)
 		}
-		// Guess the default package.
+
-		var err error
+		bp, err := buildutil.ContainingPackage(ctxt, wd, spec.filename)
-		if spec.pkg, err = guessImportPath(spec.filename, ctxt); err != nil {
+		if err != nil {
-			return nil, fmt.Errorf("-from: couldn't guess package from filename: %s: %s",
+			return nil, err
-				spec.filename, err)
 		}
+		spec.pkg = bp.ImportPath
 
 	} else if a, b := splitAtLastDot(main); b == "" {
 		// importpath e.g. "encoding/json"
@@ -194,12 +194,12 @@ func parseOffsetFlag(ctxt *build.Context, offsetFlag string) (*spec, error) {
 	if !buildutil.FileExists(ctxt, spec.filename) {
 		return nil, fmt.Errorf("no such file: %s", spec.filename)
 	}
-	// Guess the default package.
+
-	var err error
+	bp, err := buildutil.ContainingPackage(ctxt, wd, spec.filename)
-	if spec.pkg, err = guessImportPath(spec.filename, ctxt); err != nil {
+	if err != nil {
-		return nil, fmt.Errorf("couldn't guess package from filename: %s: %s",
+		return nil, err
-			spec.filename, err)
 	}
+	spec.pkg = bp.ImportPath
 
 	for _, r := range parts[1] {
 		if !isDigit(r) {

refactor/rename/util.go

@@ -5,8 +5,6 @@
 package rename
 
 import (
-	"fmt"
-	"go/build"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -79,61 +77,6 @@ func isDigit(ch rune) bool {
 
 // -- Plundered from code.google.com/p/go.tools/oracle -----------------
 
-// guessImportPath finds the package containing filename, and returns
-// its import path relative to it.
-func guessImportPath(filename string, ctxt *build.Context) (importPath string, err error) {
-	// TODO(adonovan): move this to package "buildutil"; factor in common with oracle.
-	// bp, err := buildutil.ContainingPackage(ctxt, wd, filename)
-	// if err != nil {
-	// 	return
-	// }
-	// return bp.ImportPath, nil
-
-	absFile, err := filepath.Abs(filename)
-	if err != nil {
-		err = fmt.Errorf("can't form absolute path of %s", filename)
-		return
-	}
-	absFileDir := segments(filepath.Dir(absFile))
-
-	// Find the innermost directory in $GOPATH that encloses filename.
-	minD := 1024
-	for _, gopathDir := range ctxt.SrcDirs() {
-		// We can assume $GOPATH and $GOROOT dirs are absolute,
-		// thus gopathDir too, and that it exists.
-		d := prefixLen(segments(gopathDir), absFileDir)
-		// If there are multiple matches,
-		// prefer the innermost enclosing directory
-		// (smallest d).
-		if d >= 0 && d < minD {
-			minD = d
-			importPath = strings.Join(absFileDir[len(absFileDir)-minD:], string(os.PathSeparator))
-		}
-	}
-	if importPath == "" {
-		err = fmt.Errorf("can't find package for file %s", filename)
-	}
-	return
-}
-
-func segments(path string) []string {
-	return strings.Split(path, string(os.PathSeparator))
-}
-
-// prefixLen returns the length of the remainder of y if x is a prefix
-// of y, a negative number otherwise.
-func prefixLen(x, y []string) int {
-	d := len(y) - len(x)
-	if d >= 0 {
-		for i := range x {
-			if y[i] != x[i] {
-				return -1 // not a prefix
-			}
-		}
-	}
-	return d
-}
-
 // sameFile returns true if x and y have the same basename and denote
 // the same file.
 //