cmd/go: change to use struct in go test -cover

Go tool half of https://golang.org/cl/10271044

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/10272043

src/cmd/go/build.go

@@ -798,7 +798,7 @@ func (b *builder) build(a *action) (err error) {
 				continue
 			}
 			coverFile := filepath.Join(obj, file)
-			if err := b.cover(a, coverFile, sourceFile, 0666, cover.Count, cover.Pos); err != nil {
+			if err := b.cover(a, coverFile, sourceFile, 0666, cover.Var); err != nil {
 				return err
 			}
 			gofiles = append(gofiles, coverFile)
@@ -1110,13 +1110,12 @@ func (b *builder) copyFile(a *action, dst, src string, perm os.FileMode) error {
 }
 
 // cover runs, in effect,
-//	go tool cover -mode=b.coverMode -count="count" -pos="pos" -o dst.go src.go
-func (b *builder) cover(a *action, dst, src string, perm os.FileMode, count, pos string) error {
+//	go tool cover -mode=b.coverMode -var="varName" -o dst.go src.go
+func (b *builder) cover(a *action, dst, src string, perm os.FileMode, varName string) error {
 	return b.run(a.objdir, "cover "+a.p.ImportPath, nil,
 		tool("cover"),
 		"-mode", a.p.coverMode,
-		"-count", count,
-		"-pos", pos,
+		"-var", varName,
 		"-o", dst,
 		src)
 }

src/cmd/go/pkg.go

@@ -90,9 +90,8 @@ type Package struct {
 
 // CoverVar holds the name of the generated coverage variables targeting the named file.
 type CoverVar struct {
-	File  string // local file name
-	Count string // name of count array
-	Pos   string // name of position array
+	File string // local file name
+	Var  string // name of count struct
 }
 
 func (p *Package) copyBuild(pp *build.Package) {

src/cmd/go/test.go

@@ -682,9 +682,8 @@ func declareCoverVars(files ...string) map[string]*CoverVar {
 	coverVars := make(map[string]*CoverVar)
 	for _, file := range files {
 		coverVars[file] = &CoverVar{
-			File:  file,
-			Count: fmt.Sprintf("GoCoverCount_%d", coverIndex),
-			Pos:   fmt.Sprintf("GoCoverPos_%d", coverIndex),
+			File: file,
+			Var:  fmt.Sprintf("GoCover_%d", coverIndex),
 		}
 		coverIndex++
 	}
@@ -988,7 +987,7 @@ var (
 
 func init() {
 	{{range $file, $cover := .CoverVars}}
-	coverRegisterFile({{printf "%q" $file}}, _test.{{$cover.Count}}[:], _test.{{$cover.Pos}}[:]...)
+	coverRegisterFile({{printf "%q" $file}}, _test.{{$cover.Var}}.Count[:], _test.{{$cover.Var}}.Pos[:]...)
 	{{end}}
 }