text/template: add an unexported method to Node

Protects the package a little against undesirable clients.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6624054

src/pkg/text/template/parse/node.go

@@ -13,7 +13,9 @@ import (
 	"strings"
 )
 
-// A node is an element in the parse tree. The interface is trivial.
+// A Node is an element in the parse tree. The interface is trivial.
+// The interface contains an unexported method so that only
+// types local to this package can satisfy it.
 type Node interface {
 	Type() NodeType
 	String() string
@@ -22,6 +24,8 @@ type Node interface {
 	// CopyXxx methods that return *XxxNode.
 	Copy() Node
 	Position() Pos // byte position of start of node in full original input string
+	// Make sure only functions in this package can create Nodes.
+	unexported()
 }
 
 // NodeType identifies the type of a parse tree node.
@@ -35,6 +39,11 @@ func (p Pos) Position() Pos {
 	return p
 }
 
+// unexported keeps Node implementations local to the package.
+// All implementations embed Pos, so this takes care of it.
+func (Pos) unexported() {
+}
+
 // Type returns itself and provides an easy default implementation
 // for embedding in a Node. Embedded in all non-trivial Nodes.
 func (t NodeType) Type() NodeType {