go/printer: don't print newlines for empty statements

Fixes #3466.

gofmt -w src misc causes no changes.

R=rsc
CC=golang-dev
https://golang.org/cl/6206073

src/pkg/go/printer/nodes.go

@@ -868,28 +868,32 @@ func (p *printer) expr(x ast.Expr) {
 // Print the statement list indented, but without a newline after the last statement.
 // Extra line breaks between statements in the source are respected but at most one
 // empty line is printed between statements.
-func (p *printer) stmtList(list []ast.Stmt, _indent int, nextIsRBrace bool) {
-	// TODO(gri): fix _indent code
-	if _indent > 0 {
+func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
+	if nindent > 0 {
 		p.print(indent)
 	}
 	multiLine := false
-	for i, s := range list {
+	i := 0
+	for _, s := range list {
+		// ignore empty statements (was issue 3466)
+		if _, isEmpty := s.(*ast.EmptyStmt); !isEmpty {
 			// _indent == 0 only for lists of switch/select case clauses;
 			// in those cases each clause is a new section
-		p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || _indent == 0 || multiLine)
+			p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || multiLine)
 			p.stmt(s, nextIsRBrace && i == len(list)-1)
 			multiLine = p.isMultiLine(s)
+			i++
 		}
-	if _indent > 0 {
+	}
+	if nindent > 0 {
 		p.print(unindent)
 	}
 }
 
 // block prints an *ast.BlockStmt; it always spans at least two lines.
-func (p *printer) block(s *ast.BlockStmt, indent int) {
+func (p *printer) block(s *ast.BlockStmt, nindent int) {
 	p.print(s.Pos(), token.LBRACE)
-	p.stmtList(s.List, indent, true)
+	p.stmtList(s.List, nindent, true)
 	p.linebreak(p.lineFor(s.Rbrace), 1, ignore, true)
 	p.print(s.Rbrace, token.RBRACE)
 }

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

@@ -527,3 +527,29 @@ AVeryLongLabelThatShouldNotAffectFormatting:
 	// There should be a single empty line before this comment.
 	MoreCode()
 }
+
+// Formatting of empty statements.
+func _() {
+
+}
+
+func _() {
+}
+
+func _() {
+}
+
+func _() {
+	f()
+}
+
+func _() {
+L:
+	;
+}
+
+func _() {
+L:
+	;
+	f()
+}

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

@@ -468,3 +468,27 @@ AVeryLongLabelThatShouldNotAffectFormatting:
 	// There should be a single empty line before this comment.
 	MoreCode()
 }
+
+
+// Formatting of empty statements.
+func _() {
+	;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;}
+
+func _() {
+f();;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+	f()
+}