expand: replace Config.NoGlob with Config.ReadDir

One could already disable globbing by using ReadDir==nil instead of
NoGlob==true, so make that the only way to disable globbing. NoGlob had
no advantage over a nil func, and NoGlob==false with ReadDir==nil
confusingly disabled globbing.

expand/expand.go

@@ -35,14 +35,6 @@ type Config struct {
 	// variables.
 	Env Environ
 
-	// TODO(mvdan): consider replacing NoGlob==true with ReadDir==nil.
-
-	// NoGlob corresponds to the shell option that disables globbing.
-	NoGlob bool
-	// GlobStar corresponds to the shell option that allows globbing with
-	// "**".
-	GlobStar bool
-
 	// CmdSubst expands a command substitution node, writing its standard
 	// output to the provided io.Writer.
 	//
@@ -54,6 +46,10 @@ type Config struct {
 	// Use ioutil.ReadDir to use the filesystem directly.
 	ReadDir func(string) ([]os.FileInfo, error)
 
+	// GlobStar corresponds to the shell option that allows globbing with
+	// "**".
+	GlobStar bool
+
 	bufferAlloc bytes.Buffer
 	fieldAlloc  [4]fieldPart
 	fieldsAlloc [4][]fieldPart
@@ -328,7 +324,7 @@ func Fields(cfg *Config, words ...*syntax.Word) ([]string, error) {
 			for _, field := range wfields {
 				path, doGlob := cfg.escapedGlobField(field)
 				var matches []string
-				if doGlob && !cfg.NoGlob {
+				if doGlob && cfg.ReadDir != nil {
 					matches, err = cfg.glob(dir, path)
 					if err != nil {
 						return nil, err
@@ -688,10 +684,6 @@ func (cfg *Config) glob(base, pattern string) ([]string, error) {
 }
 
 func (cfg *Config) globDir(base, dir string, rx *regexp.Regexp, wantDir bool, matches []string) ([]string, error) {
-	if cfg.ReadDir == nil {
-		// TODO(mvdan): check this at the beginning of a glob?
-		return nil, nil
-	}
 	fullDir := dir
 	if !filepath.IsAbs(dir) {
 		fullDir = filepath.Join(base, dir)

interp/interp.go

@@ -85,7 +85,6 @@ func (r *Runner) fillExpandConfig(ctx context.Context) {
 			r2.stmts(ctx, cs.StmtList)
 			return r2.err
 		},
-		ReadDir: ioutil.ReadDir,
 	}
 	r.updateExpandOpts()
 }
@@ -107,7 +106,11 @@ func catShortcutArg(stmt *syntax.Stmt) *syntax.Word {
 }
 
 func (r *Runner) updateExpandOpts() {
-	r.ecfg.NoGlob = r.opts[optNoGlob]
+	if r.opts[optNoGlob] {
+		r.ecfg.ReadDir = nil
+	} else {
+		r.ecfg.ReadDir = ioutil.ReadDir
+	}
 	r.ecfg.GlobStar = r.opts[optGlobStar]
 }