go/parser: better error sync. if commas are missing

R=rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5756045

src/pkg/go/parser/parser.go

@@ -362,9 +362,9 @@ func (p *parser) expect(tok token.Token) token.Pos {
 // expectClosing is like expect but provides a better error message
 // for the common case of a missing comma before a newline.
 //
-func (p *parser) expectClosing(tok token.Token, construct string) token.Pos {
+func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
 	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
-		p.error(p.pos, "missing ',' before newline in "+construct)
+		p.error(p.pos, "missing ',' before newline in "+context)
 		p.next()
 	}
 	return p.expect(tok)
@@ -376,6 +376,18 @@ func (p *parser) expectSemi() {
 	}
 }
 
+func (p *parser) seesComma(context string) bool {
+	if p.tok == token.COMMA {
+		return true
+	}
+	if p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.error(p.pos, "missing ',' before newline in "+context)
+		return true // "insert" the comma and continue
+
+	}
+	return false
+}
+
 func assert(cond bool, msg string) {
 	if !cond {
 		panic("go/parser internal error: " + msg)
@@ -647,7 +659,7 @@ func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
 	// accept them all for more robust parsing and complain later
 	for typ := p.parseVarType(isParam); typ != nil; {
 		list = append(list, typ)
-		if p.tok != token.COMMA {
+		if !p.seesComma("variable list") {
 			break
 		}
 		p.next()
@@ -688,7 +700,7 @@ func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params [
 			// Go spec: The scope of an identifier denoting a function
 			// parameter or result variable is the function body.
 			p.declare(field, nil, scope, ast.Var, idents...)
-			if p.tok != token.COMMA {
+			if !p.seesComma("parameter list") {
 				break
 			}
 			p.next()
@@ -1078,7 +1090,7 @@ func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
 			ellipsis = p.pos
 			p.next()
 		}
-		if p.tok != token.COMMA {
+		if !p.seesComma("argument list") {
 			break
 		}
 		p.next()
@@ -1118,7 +1130,7 @@ func (p *parser) parseElementList() (list []ast.Expr) {
 
 	for p.tok != token.RBRACE && p.tok != token.EOF {
 		list = append(list, p.parseElement(true))
-		if p.tok != token.COMMA {
+		if !p.seesComma("composite literal") {
 			break
 		}
 		p.next()