go/ast: fix bugs in SortImports

Tests are in gofix, since the bugs arise in rewritten ASTs.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5365043

src/cmd/gofix/import_test.go

@@ -264,6 +264,90 @@ import (
 	"io"
 	"os"
 )
+`,
+	},
+	{
+		Name: "import.14",
+		Fn:   rewriteImportFn("asn1", "encoding/asn1"),
+		In: `package main
+
+import (
+	"asn1"
+	"crypto"
+	"crypto/rsa"
+	_ "crypto/sha1"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"time"
+)
+
+var x = 1
+`,
+		Out: `package main
+
+import (
+	"crypto"
+	"crypto/rsa"
+	_ "crypto/sha1"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/asn1"
+	"time"
+)
+
+var x = 1
+`,
+	},
+	{
+		Name: "import.15",
+		Fn:   rewriteImportFn("url", "net/url"),
+		In: `package main
+
+import (
+	"bufio"
+	"net"
+	"path"
+	"url"
+)
+
+var x = 1 // comment on x, not on url
+`,
+		Out: `package main
+
+import (
+	"bufio"
+	"net"
+	"net/url"
+	"path"
+)
+
+var x = 1 // comment on x, not on url
+`,
+	},
+	{
+		Name: "import.16",
+		Fn:   rewriteImportFn("http", "net/http", "template", "text/template"),
+		In: `package main
+
+import (
+	"flag"
+	"http"
+	"log"
+	"template"
+)
+
+var addr = flag.String("addr", ":1718", "http service address") // Q=17, R=18
+`,
+		Out: `package main
+
+import (
+	"flag"
+	"log"
+	"net/http"
+	"text/template"
+)
+
+var addr = flag.String("addr", ":1718", "http service address") // Q=17, R=18
 `,
 	},
 }
@@ -288,12 +372,15 @@ func deleteImportFn(path string) func(*ast.File) bool {
 	}
 }
 
-func rewriteImportFn(old, new string) func(*ast.File) bool {
+func rewriteImportFn(oldnew ...string) func(*ast.File) bool {
 	return func(f *ast.File) bool {
-		if imports(f, old) {
-			rewriteImport(f, old, new)
-			return true
-		}
-		return false
+		fixed := false
+		for i := 0; i < len(oldnew); i += 2 {
+			if imports(f, oldnew[i]) {
+				rewriteImport(f, oldnew[i], oldnew[i+1])
+				fixed = true
+			}
+		}
+		return fixed
 	}
 }

src/pkg/go/ast/import.go

@@ -67,7 +67,12 @@ func sortSpecs(fset *token.FileSet, f *File, specs []Spec) {
 	// Record positions for specs.
 	pos := make([]posSpan, len(specs))
 	for i, s := range specs {
-		pos[i] = posSpan{s.Pos(), s.End()}
+		// Cannot use s.End(), because it looks at len(s.Path.Value),
+		// and that string might have gotten longer or shorter.
+		// Instead, use s.Pos()+1, which is guaranteed to be > s.Pos()
+		// and still before the original end of the string, since any
+		// string literal must be at least 2 characters ("" or ``).
+		pos[i] = posSpan{s.Pos(), s.Pos() + 1}
 	}
 
 	// Identify comments in this range.
@@ -107,6 +112,9 @@ func sortSpecs(fset *token.FileSet, f *File, specs []Spec) {
 	sort.Sort(byImportPath(specs))
 	for i, s := range specs {
 		s := s.(*ImportSpec)
+		if s.Name != nil {
+			s.Name.NamePos = pos[i].Start
+		}
 		s.Path.ValuePos = pos[i].Start
 		s.EndPos = pos[i].End
 		for _, g := range importComment[s] {