text/template: add back pointer to Nodes for better error generation

ErrorContext now has all the information it needs from the Node,
rather than depending on the template that contains it. This makes
it easier for html/template to generate correct locations in its
error messages.

Updated html/template to use this ability where it is easy, which is
not everywhere, but more work can probably push it through.

Fixes #8577.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/130620043

3 files changed, 210 insertions, 105 deletions

diff --git a/src/pkg/text/template/parse/node.go b/src/pkg/text/template/parse/node.go
index dc6a3bb929..e6d6613250 100644
--- a/src/pkg/text/template/parse/node.go
+++ b/src/pkg/text/template/parse/node.go
@@ -26,8 +26,9 @@ 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()
+	// tree returns the containing *Tree.
+	// It is unexported so all implementations of Node are in this package.
+	tree() *Tree
 }
 
 // NodeType identifies the type of a parse tree node.
@@ -41,11 +42,6 @@ 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 {
@@ -81,17 +77,22 @@ const (
 type ListNode struct {
 	NodeType
 	Pos
+	tr    *Tree
 	Nodes []Node // The element nodes in lexical order.
 }
 
-func newList(pos Pos) *ListNode {
-	return &ListNode{NodeType: NodeList, Pos: pos}
+func (t *Tree) newList(pos Pos) *ListNode {
+	return &ListNode{tr: t, NodeType: NodeList, Pos: pos}
 }
 
 func (l *ListNode) append(n Node) {
 	l.Nodes = append(l.Nodes, n)
 }
 
+func (l *ListNode) tree() *Tree {
+	return l.tr
+}
+
 func (l *ListNode) String() string {
 	b := new(bytes.Buffer)
 	for _, n := range l.Nodes {
@@ -104,7 +105,7 @@ func (l *ListNode) CopyList() *ListNode {
 	if l == nil {
 		return l
 	}
-	n := newList(l.Pos)
+	n := l.tr.newList(l.Pos)
 	for _, elem := range l.Nodes {
 		n.append(elem.Copy())
 	}
@@ -119,32 +120,38 @@ func (l *ListNode) Copy() Node {
 type TextNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Text []byte // The text; may span newlines.
 }
 
-func newText(pos Pos, text string) *TextNode {
-	return &TextNode{NodeType: NodeText, Pos: pos, Text: []byte(text)}
+func (t *Tree) newText(pos Pos, text string) *TextNode {
+	return &TextNode{tr: t, NodeType: NodeText, Pos: pos, Text: []byte(text)}
 }
 
 func (t *TextNode) String() string {
 	return fmt.Sprintf(textFormat, t.Text)
 }
 
+func (t *TextNode) tree() *Tree {
+	return t.tr
+}
+
 func (t *TextNode) Copy() Node {
-	return &TextNode{NodeType: NodeText, Text: append([]byte{}, t.Text...)}
+	return &TextNode{tr: t.tr, NodeType: NodeText, Pos: t.Pos, Text: append([]byte{}, t.Text...)}
 }
 
 // PipeNode holds a pipeline with optional declaration
 type PipeNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Line int             // The line number in the input (deprecated; kept for compatibility)
 	Decl []*VariableNode // Variable declarations in lexical order.
 	Cmds []*CommandNode  // The commands in lexical order.
 }
 
-func newPipeline(pos Pos, line int, decl []*VariableNode) *PipeNode {
-	return &PipeNode{NodeType: NodePipe, Pos: pos, Line: line, Decl: decl}
+func (t *Tree) newPipeline(pos Pos, line int, decl []*VariableNode) *PipeNode {
+	return &PipeNode{tr: t, NodeType: NodePipe, Pos: pos, Line: line, Decl: decl}
 }
 
 func (p *PipeNode) append(command *CommandNode) {
@@ -171,6 +178,10 @@ func (p *PipeNode) String() string {
 	return s
 }
 
+func (p *PipeNode) tree() *Tree {
+	return p.tr
+}
+
 func (p *PipeNode) CopyPipe() *PipeNode {
 	if p == nil {
 		return p
@@ -179,7 +190,7 @@ func (p *PipeNode) CopyPipe() *PipeNode {
 	for _, d := range p.Decl {
 		decl = append(decl, d.Copy().(*VariableNode))
 	}
-	n := newPipeline(p.Pos, p.Line, decl)
+	n := p.tr.newPipeline(p.Pos, p.Line, decl)
 	for _, c := range p.Cmds {
 		n.append(c.Copy().(*CommandNode))
 	}
@@ -196,12 +207,13 @@ func (p *PipeNode) Copy() Node {
 type ActionNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Line int       // The line number in the input (deprecated; kept for compatibility)
 	Pipe *PipeNode // The pipeline in the action.
 }
 
-func newAction(pos Pos, line int, pipe *PipeNode) *ActionNode {
-	return &ActionNode{NodeType: NodeAction, Pos: pos, Line: line, Pipe: pipe}
+func (t *Tree) newAction(pos Pos, line int, pipe *PipeNode) *ActionNode {
+	return &ActionNode{tr: t, NodeType: NodeAction, Pos: pos, Line: line, Pipe: pipe}
 }
 
 func (a *ActionNode) String() string {
@@ -209,8 +221,12 @@ func (a *ActionNode) String() string {
 
 }
 
+func (a *ActionNode) tree() *Tree {
+	return a.tr
+}
+
 func (a *ActionNode) Copy() Node {
-	return newAction(a.Pos, a.Line, a.Pipe.CopyPipe())
+	return a.tr.newAction(a.Pos, a.Line, a.Pipe.CopyPipe())
 
 }
 
@@ -218,11 +234,12 @@ func (a *ActionNode) Copy() Node {
 type CommandNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Args []Node // Arguments in lexical order: Identifier, field, or constant.
 }
 
-func newCommand(pos Pos) *CommandNode {
-	return &CommandNode{NodeType: NodeCommand, Pos: pos}
+func (t *Tree) newCommand(pos Pos) *CommandNode {
+	return &CommandNode{tr: t, NodeType: NodeCommand, Pos: pos}
 }
 
 func (c *CommandNode) append(arg Node) {
@@ -244,11 +261,15 @@ func (c *CommandNode) String() string {
 	return s
 }
 
+func (c *CommandNode) tree() *Tree {
+	return c.tr
+}
+
 func (c *CommandNode) Copy() Node {
 	if c == nil {
 		return c
 	}
-	n := newCommand(c.Pos)
+	n := c.tr.newCommand(c.Pos)
 	for _, c := range c.Args {
 		n.append(c.Copy())
 	}
@@ -259,6 +280,7 @@ func (c *CommandNode) Copy() Node {
 type IdentifierNode struct {
 	NodeType
 	Pos
+	tr    *Tree
 	Ident string // The identifier's name.
 }
 
@@ -275,12 +297,24 @@ func (i *IdentifierNode) SetPos(pos Pos) *IdentifierNode {
 	return i
 }
 
+// SetTree sets the parent tree for the node. NewIdentifier is a public method so we can't modify its signature.
+// Chained for convenience.
+// TODO: fix one day?
+func (i *IdentifierNode) SetTree(t *Tree) *IdentifierNode {
+	i.tr = t
+	return i
+}
+
 func (i *IdentifierNode) String() string {
 	return i.Ident
 }
 
+func (i *IdentifierNode) tree() *Tree {
+	return i.tr
+}
+
 func (i *IdentifierNode) Copy() Node {
-	return NewIdentifier(i.Ident).SetPos(i.Pos)
+	return NewIdentifier(i.Ident).SetTree(i.tr).SetPos(i.Pos)
 }
 
 // VariableNode holds a list of variable names, possibly with chained field
@@ -288,11 +322,12 @@ func (i *IdentifierNode) Copy() Node {
 type VariableNode struct {
 	NodeType
 	Pos
+	tr    *Tree
 	Ident []string // Variable name and fields in lexical order.
 }
 
-func newVariable(pos Pos, ident string) *VariableNode {
-	return &VariableNode{NodeType: NodeVariable, Pos: pos, Ident: strings.Split(ident, ".")}
+func (t *Tree) newVariable(pos Pos, ident string) *VariableNode {
+	return &VariableNode{tr: t, NodeType: NodeVariable, Pos: pos, Ident: strings.Split(ident, ".")}
 }
 
 func (v *VariableNode) String() string {
@@ -306,50 +341,58 @@ func (v *VariableNode) String() string {
 	return s
 }
 
+func (v *VariableNode) tree() *Tree {
+	return v.tr
+}
+
 func (v *VariableNode) Copy() Node {
-	return &VariableNode{NodeType: NodeVariable, Pos: v.Pos, Ident: append([]string{}, v.Ident...)}
+	return &VariableNode{tr: v.tr, NodeType: NodeVariable, Pos: v.Pos, Ident: append([]string{}, v.Ident...)}
 }
 
 // DotNode holds the special identifier '.'.
 type DotNode struct {
+	NodeType
 	Pos
+	tr *Tree
 }
 
-func newDot(pos Pos) *DotNode {
-	return &DotNode{Pos: pos}
-}
-
-func (d *DotNode) Type() NodeType {
-	return NodeDot
+func (t *Tree) newDot(pos Pos) *DotNode {
+	return &DotNode{tr: t, NodeType: NodeDot, Pos: pos}
 }
 
 func (d *DotNode) String() string {
 	return "."
 }
 
+func (d *DotNode) tree() *Tree {
+	return d.tr
+}
+
 func (d *DotNode) Copy() Node {
-	return newDot(d.Pos)
+	return d.tr.newDot(d.Pos)
 }
 
 // NilNode holds the special identifier 'nil' representing an untyped nil constant.
 type NilNode struct {
+	NodeType
 	Pos
+	tr *Tree
 }
 
-func newNil(pos Pos) *NilNode {
-	return &NilNode{Pos: pos}
-}
-
-func (n *NilNode) Type() NodeType {
-	return NodeNil
+func (t *Tree) newNil(pos Pos) *NilNode {
+	return &NilNode{tr: t, NodeType: NodeNil, Pos: pos}
 }
 
 func (n *NilNode) String() string {
 	return "nil"
 }
 
+func (n *NilNode) tree() *Tree {
+	return n.tr
+}
+
 func (n *NilNode) Copy() Node {
-	return newNil(n.Pos)
+	return n.tr.newNil(n.Pos)
 }
 
 // FieldNode holds a field (identifier starting with '.').
@@ -358,11 +401,12 @@ func (n *NilNode) Copy() Node {
 type FieldNode struct {
 	NodeType
 	Pos
+	tr    *Tree
 	Ident []string // The identifiers in lexical order.
 }
 
-func newField(pos Pos, ident string) *FieldNode {
-	return &FieldNode{NodeType: NodeField, Pos: pos, Ident: strings.Split(ident[1:], ".")} // [1:] to drop leading period
+func (t *Tree) newField(pos Pos, ident string) *FieldNode {
+	return &FieldNode{tr: t, NodeType: NodeField, Pos: pos, Ident: strings.Split(ident[1:], ".")} // [1:] to drop leading period
 }
 
 func (f *FieldNode) String() string {
@@ -373,8 +417,12 @@ func (f *FieldNode) String() string {
 	return s
 }
 
+func (f *FieldNode) tree() *Tree {
+	return f.tr
+}
+
 func (f *FieldNode) Copy() Node {
-	return &FieldNode{NodeType: NodeField, Pos: f.Pos, Ident: append([]string{}, f.Ident...)}
+	return &FieldNode{tr: f.tr, NodeType: NodeField, Pos: f.Pos, Ident: append([]string{}, f.Ident...)}
 }
 
 // ChainNode holds a term followed by a chain of field accesses (identifier starting with '.').
@@ -383,12 +431,13 @@ func (f *FieldNode) Copy() Node {
 type ChainNode struct {
 	NodeType
 	Pos
+	tr    *Tree
 	Node  Node
 	Field []string // The identifiers in lexical order.
 }
 
-func newChain(pos Pos, node Node) *ChainNode {
-	return &ChainNode{NodeType: NodeChain, Pos: pos, Node: node}
+func (t *Tree) newChain(pos Pos, node Node) *ChainNode {
+	return &ChainNode{tr: t, NodeType: NodeChain, Pos: pos, Node: node}
 }
 
 // Add adds the named field (which should start with a period) to the end of the chain.
@@ -414,19 +463,24 @@ func (c *ChainNode) String() string {
 	return s
 }
 
+func (c *ChainNode) tree() *Tree {
+	return c.tr
+}
+
 func (c *ChainNode) Copy() Node {
-	return &ChainNode{NodeType: NodeChain, Pos: c.Pos, Node: c.Node, Field: append([]string{}, c.Field...)}
+	return &ChainNode{tr: c.tr, NodeType: NodeChain, Pos: c.Pos, Node: c.Node, Field: append([]string{}, c.Field...)}
 }
 
 // BoolNode holds a boolean constant.
 type BoolNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	True bool // The value of the boolean constant.
 }
 
-func newBool(pos Pos, true bool) *BoolNode {
-	return &BoolNode{NodeType: NodeBool, Pos: pos, True: true}
+func (t *Tree) newBool(pos Pos, true bool) *BoolNode {
+	return &BoolNode{tr: t, NodeType: NodeBool, Pos: pos, True: true}
 }
 
 func (b *BoolNode) String() string {
@@ -436,8 +490,12 @@ func (b *BoolNode) String() string {
 	return "false"
 }
 
+func (b *BoolNode) tree() *Tree {
+	return b.tr
+}
+
 func (b *BoolNode) Copy() Node {
-	return newBool(b.Pos, b.True)
+	return b.tr.newBool(b.Pos, b.True)
 }
 
 // NumberNode holds a number: signed or unsigned integer, float, or complex.
@@ -446,6 +504,7 @@ func (b *BoolNode) Copy() Node {
 type NumberNode struct {
 	NodeType
 	Pos
+	tr         *Tree
 	IsInt      bool       // Number has an integral value.
 	IsUint     bool       // Number has an unsigned integral value.
 	IsFloat    bool       // Number has a floating-point value.
@@ -457,8 +516,8 @@ type NumberNode struct {
 	Text       string     // The original textual representation from the input.
 }
 
-func newNumber(pos Pos, text string, typ itemType) (*NumberNode, error) {
-	n := &NumberNode{NodeType: NodeNumber, Pos: pos, Text: text}
+func (t *Tree) newNumber(pos Pos, text string, typ itemType) (*NumberNode, error) {
+	n := &NumberNode{tr: t, NodeType: NodeNumber, Pos: pos, Text: text}
 	switch typ {
 	case itemCharConstant:
 		rune, _, tail, err := strconv.UnquoteChar(text[1:], text[0])
@@ -559,6 +618,10 @@ func (n *NumberNode) String() string {
 	return n.Text
 }
 
+func (n *NumberNode) tree() *Tree {
+	return n.tr
+}
+
 func (n *NumberNode) Copy() Node {
 	nn := new(NumberNode)
 	*nn = *n // Easy, fast, correct.
@@ -569,53 +632,61 @@ func (n *NumberNode) Copy() Node {
 type StringNode struct {
 	NodeType
 	Pos
+	tr     *Tree
 	Quoted string // The original text of the string, with quotes.
 	Text   string // The string, after quote processing.
 }
 
-func newString(pos Pos, orig, text string) *StringNode {
-	return &StringNode{NodeType: NodeString, Pos: pos, Quoted: orig, Text: text}
+func (t *Tree) newString(pos Pos, orig, text string) *StringNode {
+	return &StringNode{tr: t, NodeType: NodeString, Pos: pos, Quoted: orig, Text: text}
 }
 
 func (s *StringNode) String() string {
 	return s.Quoted
 }
 
+func (s *StringNode) tree() *Tree {
+	return s.tr
+}
+
 func (s *StringNode) Copy() Node {
-	return newString(s.Pos, s.Quoted, s.Text)
+	return s.tr.newString(s.Pos, s.Quoted, s.Text)
 }
 
 // endNode represents an {{end}} action.
 // It does not appear in the final parse tree.
 type endNode struct {
+	NodeType
 	Pos
+	tr *Tree
 }
 
-func newEnd(pos Pos) *endNode {
-	return &endNode{Pos: pos}
-}
-
-func (e *endNode) Type() NodeType {
-	return nodeEnd
+func (t *Tree) newEnd(pos Pos) *endNode {
+	return &endNode{tr: t, NodeType: nodeEnd, Pos: pos}
 }
 
 func (e *endNode) String() string {
 	return "{{end}}"
 }
 
+func (e *endNode) tree() *Tree {
+	return e.tr
+}
+
 func (e *endNode) Copy() Node {
-	return newEnd(e.Pos)
+	return e.tr.newEnd(e.Pos)
 }
 
 // elseNode represents an {{else}} action. Does not appear in the final tree.
 type elseNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Line int // The line number in the input (deprecated; kept for compatibility)
 }
 
-func newElse(pos Pos, line int) *elseNode {
-	return &elseNode{NodeType: nodeElse, Pos: pos, Line: line}
+func (t *Tree) newElse(pos Pos, line int) *elseNode {
+	return &elseNode{tr: t, NodeType: nodeElse, Pos: pos, Line: line}
 }
 
 func (e *elseNode) Type() NodeType {
@@ -626,14 +697,19 @@ func (e *elseNode) String() string {
 	return "{{else}}"
 }
 
+func (e *elseNode) tree() *Tree {
+	return e.tr
+}
+
 func (e *elseNode) Copy() Node {
-	return newElse(e.Pos, e.Line)
+	return e.tr.newElse(e.Pos, e.Line)
 }
 
 // BranchNode is the common representation of if, range, and with.
 type BranchNode struct {
 	NodeType
 	Pos
+	tr       *Tree
 	Line     int       // The line number in the input (deprecated; kept for compatibility)
 	Pipe     *PipeNode // The pipeline to be evaluated.
 	List     *ListNode // What to execute if the value is non-empty.
@@ -658,17 +734,34 @@ func (b *BranchNode) String() string {
 	return fmt.Sprintf("{{%s %s}}%s{{end}}", name, b.Pipe, b.List)
 }
 
+func (b *BranchNode) tree() *Tree {
+	return b.tr
+}
+
+func (b *BranchNode) Copy() Node {
+	switch b.NodeType {
+	case NodeIf:
+		return b.tr.newIf(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	case NodeRange:
+		return b.tr.newRange(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	case NodeWith:
+		return b.tr.newWith(b.Pos, b.Line, b.Pipe, b.List, b.ElseList)
+	default:
+		panic("unknown branch type")
+	}
+}
+
 // IfNode represents an {{if}} action and its commands.
 type IfNode struct {
 	BranchNode
 }
 
-func newIf(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *IfNode {
-	return &IfNode{BranchNode{NodeType: NodeIf, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+func (t *Tree) newIf(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *IfNode {
+	return &IfNode{BranchNode{tr: t, NodeType: NodeIf, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
 }
 
 func (i *IfNode) Copy() Node {
-	return newIf(i.Pos, i.Line, i.Pipe.CopyPipe(), i.List.CopyList(), i.ElseList.CopyList())
+	return i.tr.newIf(i.Pos, i.Line, i.Pipe.CopyPipe(), i.List.CopyList(), i.ElseList.CopyList())
 }
 
 // RangeNode represents a {{range}} action and its commands.
@@ -676,12 +769,12 @@ type RangeNode struct {
 	BranchNode
 }
 
-func newRange(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *RangeNode {
-	return &RangeNode{BranchNode{NodeType: NodeRange, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+func (t *Tree) newRange(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *RangeNode {
+	return &RangeNode{BranchNode{tr: t, NodeType: NodeRange, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
 }
 
 func (r *RangeNode) Copy() Node {
-	return newRange(r.Pos, r.Line, r.Pipe.CopyPipe(), r.List.CopyList(), r.ElseList.CopyList())
+	return r.tr.newRange(r.Pos, r.Line, r.Pipe.CopyPipe(), r.List.CopyList(), r.ElseList.CopyList())
 }
 
 // WithNode represents a {{with}} action and its commands.
@@ -689,25 +782,26 @@ type WithNode struct {
 	BranchNode
 }
 
-func newWith(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *WithNode {
-	return &WithNode{BranchNode{NodeType: NodeWith, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
+func (t *Tree) newWith(pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) *WithNode {
+	return &WithNode{BranchNode{tr: t, NodeType: NodeWith, Pos: pos, Line: line, Pipe: pipe, List: list, ElseList: elseList}}
 }
 
 func (w *WithNode) Copy() Node {
-	return newWith(w.Pos, w.Line, w.Pipe.CopyPipe(), w.List.CopyList(), w.ElseList.CopyList())
+	return w.tr.newWith(w.Pos, w.Line, w.Pipe.CopyPipe(), w.List.CopyList(), w.ElseList.CopyList())
 }
 
 // TemplateNode represents a {{template}} action.
 type TemplateNode struct {
 	NodeType
 	Pos
+	tr   *Tree
 	Line int       // The line number in the input (deprecated; kept for compatibility)
 	Name string    // The name of the template (unquoted).
 	Pipe *PipeNode // The command to evaluate as dot for the template.
 }
 
-func newTemplate(pos Pos, line int, name string, pipe *PipeNode) *TemplateNode {
-	return &TemplateNode{NodeType: NodeTemplate, Line: line, Pos: pos, Name: name, Pipe: pipe}
+func (t *Tree) newTemplate(pos Pos, line int, name string, pipe *PipeNode) *TemplateNode {
+	return &TemplateNode{tr: t, NodeType: NodeTemplate, Pos: pos, Line: line, Name: name, Pipe: pipe}
 }
 
 func (t *TemplateNode) String() string {
@@ -717,6 +811,10 @@ func (t *TemplateNode) String() string {
 	return fmt.Sprintf("{{template %q %s}}", t.Name, t.Pipe)
 }
 
+func (t *TemplateNode) tree() *Tree {
+	return t.tr
+}
+
 func (t *TemplateNode) Copy() Node {
-	return newTemplate(t.Pos, t.Line, t.Name, t.Pipe.CopyPipe())
+	return t.tr.newTemplate(t.Pos, t.Line, t.Name, t.Pipe.CopyPipe())
 }
diff --git a/src/pkg/text/template/parse/parse.go b/src/pkg/text/template/parse/parse.go
index 34112fb7b3..af33880c15 100644
--- a/src/pkg/text/template/parse/parse.go
+++ b/src/pkg/text/template/parse/parse.go
@@ -129,9 +129,15 @@ func New(name string, funcs ...map[string]interface{}) *Tree {
 }
 
 // ErrorContext returns a textual representation of the location of the node in the input text.
+// The receiver is only used when the node does not have a pointer to the tree inside,
+// which can occur in old code.
 func (t *Tree) ErrorContext(n Node) (location, context string) {
 	pos := int(n.Position())
-	text := t.text[:pos]
+	tree := n.tree()
+	if tree == nil {
+		tree = t
+	}
+	text := tree.text[:pos]
 	byteNum := strings.LastIndex(text, "\n")
 	if byteNum == -1 {
 		byteNum = pos // On first line.
@@ -144,7 +150,7 @@ func (t *Tree) ErrorContext(n Node) (location, context string) {
 	if len(context) > 20 {
 		context = fmt.Sprintf("%.20s...", context)
 	}
-	return fmt.Sprintf("%s:%d:%d", t.ParseName, lineNum, byteNum), context
+	return fmt.Sprintf("%s:%d:%d", tree.ParseName, lineNum, byteNum), context
 }
 
 // errorf formats the error and terminates processing.
@@ -268,7 +274,7 @@ func IsEmptyTree(n Node) bool {
 // as itemList except it also parses {{define}} actions.
 // It runs to EOF.
 func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
-	t.Root = newList(t.peek().pos)
+	t.Root = t.newList(t.peek().pos)
 	for t.peek().typ != itemEOF {
 		if t.peek().typ == itemLeftDelim {
 			delim := t.next()
@@ -316,7 +322,7 @@ func (t *Tree) parseDefinition(treeSet map[string]*Tree) {
 //	textOrAction*
 // Terminates at {{end}} or {{else}}, returned separately.
 func (t *Tree) itemList() (list *ListNode, next Node) {
-	list = newList(t.peekNonSpace().pos)
+	list = t.newList(t.peekNonSpace().pos)
 	for t.peekNonSpace().typ != itemEOF {
 		n := t.textOrAction()
 		switch n.Type() {
@@ -334,7 +340,7 @@ func (t *Tree) itemList() (list *ListNode, next Node) {
 func (t *Tree) textOrAction() Node {
 	switch token := t.nextNonSpace(); token.typ {
 	case itemText:
-		return newText(token.pos, token.val)
+		return t.newText(token.pos, token.val)
 	case itemLeftDelim:
 		return t.action()
 	default:
@@ -365,7 +371,7 @@ func (t *Tree) action() (n Node) {
 	}
 	t.backup()
 	// Do not pop variables; they persist until "end".
-	return newAction(t.peek().pos, t.lex.lineNumber(), t.pipeline("command"))
+	return t.newAction(t.peek().pos, t.lex.lineNumber(), t.pipeline("command"))
 }
 
 // Pipeline:
@@ -384,7 +390,7 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 			tokenAfterVariable := t.peek()
 			if next := t.peekNonSpace(); next.typ == itemColonEquals || (next.typ == itemChar && next.val == ",") {
 				t.nextNonSpace()
-				variable := newVariable(v.pos, v.val)
+				variable := t.newVariable(v.pos, v.val)
 				decl = append(decl, variable)
 				t.vars = append(t.vars, v.val)
 				if next.typ == itemChar && next.val == "," {
@@ -401,7 +407,7 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 		}
 		break
 	}
-	pipe = newPipeline(pos, t.lex.lineNumber(), decl)
+	pipe = t.newPipeline(pos, t.lex.lineNumber(), decl)
 	for {
 		switch token := t.nextNonSpace(); token.typ {
 		case itemRightDelim, itemRightParen:
@@ -442,7 +448,7 @@ func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int
 			// TODO: Should we allow else-if in with and range?
 			if t.peek().typ == itemIf {
 				t.next() // Consume the "if" token.
-				elseList = newList(next.Position())
+				elseList = t.newList(next.Position())
 				elseList.append(t.ifControl())
 				// Do not consume the next item - only one {{end}} required.
 				break
@@ -461,7 +467,7 @@ func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int
 //	{{if pipeline}} itemList {{else}} itemList {{end}}
 // If keyword is past.
 func (t *Tree) ifControl() Node {
-	return newIf(t.parseControl(true, "if"))
+	return t.newIf(t.parseControl(true, "if"))
 }
 
 // Range:
@@ -469,7 +475,7 @@ func (t *Tree) ifControl() Node {
 //	{{range pipeline}} itemList {{else}} itemList {{end}}
 // Range keyword is past.
 func (t *Tree) rangeControl() Node {
-	return newRange(t.parseControl(false, "range"))
+	return t.newRange(t.parseControl(false, "range"))
 }
 
 // With:
@@ -477,14 +483,14 @@ func (t *Tree) rangeControl() Node {
 //	{{with pipeline}} itemList {{else}} itemList {{end}}
 // If keyword is past.
 func (t *Tree) withControl() Node {
-	return newWith(t.parseControl(false, "with"))
+	return t.newWith(t.parseControl(false, "with"))
 }
 
 // End:
 //	{{end}}
 // End keyword is past.
 func (t *Tree) endControl() Node {
-	return newEnd(t.expect(itemRightDelim, "end").pos)
+	return t.newEnd(t.expect(itemRightDelim, "end").pos)
 }
 
 // Else:
@@ -495,9 +501,9 @@ func (t *Tree) elseControl() Node {
 	peek := t.peekNonSpace()
 	if peek.typ == itemIf {
 		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
-		return newElse(peek.pos, t.lex.lineNumber())
+		return t.newElse(peek.pos, t.lex.lineNumber())
 	}
-	return newElse(t.expect(itemRightDelim, "else").pos, t.lex.lineNumber())
+	return t.newElse(t.expect(itemRightDelim, "else").pos, t.lex.lineNumber())
 }
 
 // Template:
@@ -523,7 +529,7 @@ func (t *Tree) templateControl() Node {
 		// Do not pop variables; they persist until "end".
 		pipe = t.pipeline("template")
 	}
-	return newTemplate(token.pos, t.lex.lineNumber(), name, pipe)
+	return t.newTemplate(token.pos, t.lex.lineNumber(), name, pipe)
 }
 
 // command:
@@ -531,7 +537,7 @@ func (t *Tree) templateControl() Node {
 // space-separated arguments up to a pipeline character or right delimiter.
 // we consume the pipe character but leave the right delim to terminate the action.
 func (t *Tree) command() *CommandNode {
-	cmd := newCommand(t.peekNonSpace().pos)
+	cmd := t.newCommand(t.peekNonSpace().pos)
 	for {
 		t.peekNonSpace() // skip leading spaces.
 		operand := t.operand()
@@ -568,7 +574,7 @@ func (t *Tree) operand() Node {
 		return nil
 	}
 	if t.peek().typ == itemField {
-		chain := newChain(t.peek().pos, node)
+		chain := t.newChain(t.peek().pos, node)
 		for t.peek().typ == itemField {
 			chain.Add(t.next().val)
 		}
@@ -578,9 +584,9 @@ func (t *Tree) operand() Node {
 		// TODO: Switch to Chains always when we can.
 		switch node.Type() {
 		case NodeField:
-			node = newField(chain.Position(), chain.String())
+			node = t.newField(chain.Position(), chain.String())
 		case NodeVariable:
-			node = newVariable(chain.Position(), chain.String())
+			node = t.newVariable(chain.Position(), chain.String())
 		default:
 			node = chain
 		}
@@ -605,19 +611,19 @@ func (t *Tree) term() Node {
 		if !t.hasFunction(token.val) {
 			t.errorf("function %q not defined", token.val)
 		}
-		return NewIdentifier(token.val).SetPos(token.pos)
+		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
 	case itemDot:
-		return newDot(token.pos)
+		return t.newDot(token.pos)
 	case itemNil:
-		return newNil(token.pos)
+		return t.newNil(token.pos)
 	case itemVariable:
 		return t.useVar(token.pos, token.val)
 	case itemField:
-		return newField(token.pos, token.val)
+		return t.newField(token.pos, token.val)
 	case itemBool:
-		return newBool(token.pos, token.val == "true")
+		return t.newBool(token.pos, token.val == "true")
 	case itemCharConstant, itemComplex, itemNumber:
-		number, err := newNumber(token.pos, token.val, token.typ)
+		number, err := t.newNumber(token.pos, token.val, token.typ)
 		if err != nil {
 			t.error(err)
 		}
@@ -633,7 +639,7 @@ func (t *Tree) term() Node {
 		if err != nil {
 			t.error(err)
 		}
-		return newString(token.pos, token.val, s)
+		return t.newString(token.pos, token.val, s)
 	}
 	t.backup()
 	return nil
@@ -660,7 +666,7 @@ func (t *Tree) popVars(n int) {
 // useVar returns a node for a variable reference. It errors if the
 // variable is not defined.
 func (t *Tree) useVar(pos Pos, name string) Node {
-	v := newVariable(pos, name)
+	v := t.newVariable(pos, name)
 	for _, varName := range t.vars {
 		if varName == v.Ident[0] {
 			return v
diff --git a/src/pkg/text/template/parse/parse_test.go b/src/pkg/text/template/parse/parse_test.go
index ba1a18ec54..fa6790bef9 100644
--- a/src/pkg/text/template/parse/parse_test.go
+++ b/src/pkg/text/template/parse/parse_test.go
@@ -77,6 +77,7 @@ func TestNumberParse(t *testing.T) {
 		// because imaginary comes out as a number.
 		var c complex128
 		typ := itemNumber
+		var tree *Tree
 		if test.text[0] == '\'' {
 			typ = itemCharConstant
 		} else {
@@ -85,7 +86,7 @@ func TestNumberParse(t *testing.T) {
 				typ = itemComplex
 			}
 		}
-		n, err := newNumber(0, test.text, typ)
+		n, err := tree.newNumber(0, test.text, typ)
 		ok := test.isInt || test.isUint || test.isFloat || test.isComplex
 		if ok && err != nil {
 			t.Errorf("unexpected error for %q: %s", test.text, err)