go.tools/imports: fix fileset mismatch bug

Fixes golang/go#6884

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/43890043

imports/fix.go

@@ -45,7 +45,7 @@ func importGroup(importPath string) int {
 	return 0
 }
 
-func fixImports(f *ast.File) (added []string, err error) {
+func fixImports(fset *token.FileSet, f *ast.File) (added []string, err error) {
 	// refs are a set of possible package references currently unsatisified by imports.
 	// first key: either base package (e.g. "fmt") or renamed package
 	// second key: referenced package symbol (e.g. "Println")
@@ -196,8 +196,6 @@ func loadPkgIndex() {
 	wg.Wait()
 }
 
-var fset = token.NewFileSet()
-
 func loadPkg(wg *sync.WaitGroup, root, pkgrelpath string) {
 	importpath := filepath.ToSlash(pkgrelpath)
 	shortName := importPathToName(importpath)
@@ -250,6 +248,7 @@ func loadExportsGoPath(dir string) map[string]bool {
 		fmt.Fprintf(os.Stderr, "could not import %q: %v", dir, err)
 		return nil
 	}
+	fset := token.NewFileSet()
 	for _, file := range buildPkg.GoFiles {
 		f, err := parser.ParseFile(fset, filepath.Join(dir, file), nil, 0)
 		if err != nil {

imports/fix_test.go

@@ -447,6 +447,27 @@ func f() {
 	_ = fmt.Printf
 	_ = snappy.Foo
 }
+`,
+	},
+
+	// golang.org/issue/6884
+	{
+		name: "issue 6884",
+		in: `package main
+
+// A comment
+func main() {
+	fmt.Println("Hello, world")
+}
+`,
+		out: `package main
+
+import "fmt"
+
+// A comment
+func main() {
+	fmt.Println("Hello, world")
+}
 `,
 	},
 }

imports/imports.go

@@ -45,7 +45,7 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 		return nil, err
 	}
 
-	_, err = fixImports(file)
+	_, err = fixImports(fileSet, file)
 	if err != nil {
 		return nil, err
 	}