cmd/compile/internal/syntax: generalize error about var decls in init clauses

Change-Id: I62f9748b97bec245338ebf9686fbf6ad6dc6a9c2
Reviewed-on: https://go-review.googlesource.com/36931
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1703,8 +1703,8 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 
 	if p.tok != _Semi {
 		// accept potential varDecl but complain
-		if keyword == _For && p.got(_Var) {
-			p.syntax_error("var declaration not allowed in for initializer")
+		if p.got(_Var) {
+			p.syntax_error(fmt.Sprintf("var declaration not allowed in %s initializer", keyword.String()))
 		}
 		init = p.simpleStmt(nil, keyword == _For)
 		// If we have a range clause, we are done (can only happen for keyword == _For).

test/syntax/forvar.go → test/syntax/initvar.go

@@ -7,5 +7,9 @@
 package main
 
 func main() {
-	var x int // avoid undefined: x error below with recursive-descent parser
-	for var x = 0; x < 10; x++ {	// ERROR "var declaration not allowed in for initializer"
+	if var x = 0; x < 10 {}    // ERROR "var declaration not allowed in if initializer"
+
+	switch var x = 0; x {}     // ERROR "var declaration not allowed in switch initializer"
+
+	for var x = 0; x < 10; {}  // ERROR "var declaration not allowed in for initializer"
+}