syntax,interp: fix "for i; do ..." loops

They aren't no-op loops like "for i in; do ..."; they range over the
shell's positional paramters. That is, it's a short version of:

	for i in "$@"; do ...

Fixes mvdan#341.

interp/interp.go

@@ -820,7 +820,11 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 		switch y := x.Loop.(type) {
 		case *syntax.WordIter:
 			name := y.Name.Value
-			for _, field := range r.fields(y.Items...) {
+			items := r.Params // for i; do ...
+			if y.InPos.IsValid() {
+				items = r.fields(y.Items...) // for i in ...; do ...
+			}
+			for _, field := range items {
 				r.setVarString(name, field)
 				if r.loopStmtsBroken(ctx, x.Do) {
 					break

interp/interp_test.go

@@ -551,6 +551,14 @@ var fileCases = []struct {
 		"for ((i=0; i<3; i++)); do :; done; echo $i",
 		"3\n",
 	},
+	{
+		"set -- a 'b c'; for i in; do echo $i; done",
+		"",
+	},
+	{
+		"set -- a 'b c'; for i; do echo $i; done",
+		"a\nb c\n",
+	},
 
 	// block
 	{

syntax/filetests_test.go

@@ -4384,6 +4384,9 @@ func clearPosRecurse(tb testing.TB, src string, v interface{}) {
 		recurse(x.Do)
 	case *WordIter:
 		recurse(x.Name)
+		if x.InPos.IsValid() {
+			setPos(&x.InPos, "in")
+		}
 		recurse(x.Items)
 	case *CStyleLoop:
 		setPos(&x.Lparen, "((")

syntax/nodes.go

@@ -373,14 +373,21 @@ func (*WordIter) loopNode()   {}
 func (*CStyleLoop) loopNode() {}
 
 // WordIter represents the iteration of a variable over a series of words in a
-// for clause.
+// for clause. If InPos is an invalid position, the "in" token was missing, so
+// the iteration is over the shell's positional parameters.
 type WordIter struct {
 	Name  *Lit
+	InPos Pos // position of "in"
 	Items []*Word
 }
 
 func (w *WordIter) Pos() Pos { return w.Name.Pos() }
-func (w *WordIter) End() Pos { return posMax(w.Name.End(), wordLastEnd(w.Items)) }
+func (w *WordIter) End() Pos {
+	if len(w.Items) > 0 {
+		return wordLastEnd(w.Items)
+	}
+	return posMax(w.Name.End(), posAddCol(w.InPos, 2))
+}
 
 // CStyleLoop represents the behaviour of a for clause similar to the C
 // language.

syntax/parser.go

@@ -2017,7 +2017,8 @@ func (p *Parser) wordIter(ftok string, fpos Pos) *WordIter {
 		return wi
 	}
 	p.got(_Newl)
-	if _, ok := p.gotRsrv("in"); ok {
+	if pos, ok := p.gotRsrv("in"); ok {
+		wi.InPos = pos
 		for !stopToken(p.tok) {
 			if w := p.getWord(); w == nil {
 				p.curErr("word list can only contain words")

syntax/printer.go

@@ -620,7 +620,7 @@ func (p *Printer) loop(loop Loop) {
 	switch x := loop.(type) {
 	case *WordIter:
 		p.WriteString(x.Name.Value)
-		if len(x.Items) > 0 {
+		if x.InPos.IsValid() {
 			p.spacedString(" in", Pos{})
 			p.wordJoin(x.Items)
 		}

syntax/printer_test.go

@@ -462,6 +462,8 @@ var printTests = []printCase{
 		"if foo # inline\nthen\n\tbar\nfi",
 		"if foo; then # inline\n\tbar\nfi",
 	},
+	samePrint("for i; do echo $i; done"),
+	samePrint("for i in; do echo $i; done"),
 	{
 		"for foo in a b # inline\ndo\n\tbar\ndone",
 		"for foo in a b; do # inline\n\tbar\ndone",