go/*: permit "for range x"

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/112970044

src/pkg/go/ast/ast.go

@@ -699,9 +699,9 @@ type (
 	// A RangeStmt represents a for statement with a range clause.
 	RangeStmt struct {
 		For        token.Pos   // position of "for" keyword
-		Key, Value Expr        // Value may be nil
-		TokPos     token.Pos   // position of Tok
-		Tok        token.Token // ASSIGN, DEFINE
+		Key, Value Expr        // Key, Value may be nil
+		TokPos     token.Pos   // position of Tok; invalid if Key == nil
+		Tok        token.Token // ILLEGAL if Key == nil, ASSIGN, DEFINE
 		X          Expr        // value to range over
 		Body       *BlockStmt
 	}

src/pkg/go/parser/parser.go

@@ -2041,7 +2041,16 @@ func (p *parser) parseForStmt() ast.Stmt {
 		prevLev := p.exprLev
 		p.exprLev = -1
 		if p.tok != token.SEMICOLON {
-			s2, isRange = p.parseSimpleStmt(rangeOk)
+			if p.tok == token.RANGE {
+				// "for range x" (nil lhs in assignment)
+				pos := p.pos
+				p.next()
+				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+				s2 = &ast.AssignStmt{Rhs: y}
+				isRange = true
+			} else {
+				s2, isRange = p.parseSimpleStmt(rangeOk)
+			}
 		}
 		if !isRange && p.tok == token.SEMICOLON {
 			p.next()
@@ -2066,12 +2075,14 @@ func (p *parser) parseForStmt() ast.Stmt {
 		// check lhs
 		var key, value ast.Expr
 		switch len(as.Lhs) {
-		case 2:
-			key, value = as.Lhs[0], as.Lhs[1]
+		case 0:
+			// nothing to do
 		case 1:
 			key = as.Lhs[0]
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
 		default:
-			p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
+			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
 			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
 		}
 		// parseSimpleStmt returned a right-hand side that

src/pkg/go/parser/short_test.go

@@ -38,6 +38,7 @@ var valids = []string{
 	`package p; func ((T),) m() {}`,
 	`package p; func ((*T),) m() {}`,
 	`package p; func (*(T),) m() {}`,
+	`package p; func _(x []int) { for range x {} }`,
 }
 
 func TestValid(t *testing.T) {

src/pkg/go/printer/nodes.go

@@ -1216,14 +1216,17 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
 
 	case *ast.RangeStmt:
 		p.print(token.FOR, blank)
-		p.expr(s.Key)
-		if s.Value != nil {
-			// use position of value following the comma as
-			// comma position for correct comment placement
-			p.print(s.Value.Pos(), token.COMMA, blank)
-			p.expr(s.Value)
+		if s.Key != nil {
+			p.expr(s.Key)
+			if s.Value != nil {
+				// use position of value following the comma as
+				// comma position for correct comment placement
+				p.print(s.Value.Pos(), token.COMMA, blank)
+				p.expr(s.Value)
+			}
+			p.print(blank, s.TokPos, s.Tok, blank)
 		}
-		p.print(blank, s.TokPos, s.Tok, blank, token.RANGE, blank)
+		p.print(token.RANGE, blank)
 		p.expr(stripParens(s.X))
 		p.print(blank)
 		p.block(s.Body, 1)

src/pkg/go/printer/testdata/statements.golden

@@ -309,6 +309,9 @@ func _() {
 	for x := expr; expr; expr = false {
 		use(x)
 	}
+	for range []int{} {
+		println("foo")
+	}
 	for x := range []int{} {
 		use(x)
 	}
@@ -338,6 +341,12 @@ func _() {
 		a[i] = i
 	}	// multiple lines
 
+	for range a {
+	}
+	for _ = range a {
+	}
+	for _, _ = range a {
+	}
 	for i := range a {
 	}
 	for i := range a {

src/pkg/go/printer/testdata/statements.input

@@ -269,6 +269,8 @@ func _() {
 	for x := expr;expr;expr = false {
 	use(x)
 	}
+	for range []int{} {
+	println("foo")}
 	for x := range []int{} {
 	use(x) }
 	for x := range (([]int{})) {
@@ -289,6 +291,9 @@ func _() {
 	for i := 0; i < len(a); 1++ { a[i] = i
 	} // multiple lines
 
+	for range a{}
+	for _ = range a{}
+	for _, _ = range a{}
 	for i := range a {}
 	for i := range a { a[i] = i }
 	for i := range a { a[i] = i