cmd/compile: don't walk field-name syntax in esc.go

Walking the field name as if it were an expression
caused a called to haspointers with a TFIELD, which panics.
Trigger was a field at a large offset within a large struct,
combined with a struct literal expression mentioning that
field.

Fixes #14405

Change-Id: I4589badae27cf3d7cf365f3a66c13447512f41f9
Reviewed-on: https://go-review.googlesource.com/19699Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/esc.go

@@ -576,6 +576,12 @@ func esc(e *EscState, n *Node, up *Node) {
 	if n == nil {
 		return
 	}
+	if n.Type != nil && n.Type.Etype == TFIELD {
+		// This is the left side of x:y in a struct literal.
+		// x is syntax, not an expression.
+		// See #14405.
+		return
+	}
 
 	lno := int(setlineno(n))
 
@@ -602,9 +608,10 @@ func esc(e *EscState, n *Node, up *Node) {
 
 	// Big stuff escapes unconditionally
 	// "Big" conditions that were scattered around in walk have been gathered here
-	if n.Esc != EscHeap && n.Type != nil && (n.Type.Width > MaxStackVarSize ||
-		n.Op == ONEW && n.Type.Type.Width >= 1<<16 ||
-		n.Op == OMAKESLICE && !isSmallMakeSlice(n)) {
+	if n.Esc != EscHeap && n.Type != nil &&
+		(n.Type.Width > MaxStackVarSize ||
+			n.Op == ONEW && n.Type.Type.Width >= 1<<16 ||
+			n.Op == OMAKESLICE && !isSmallMakeSlice(n)) {
 		if Debug['m'] > 1 {
 			Warnl(int(n.Lineno), "%v is too large for stack", n)
 		}

src/cmd/compile/internal/gc/fmt.go

@@ -1542,7 +1542,7 @@ func nodedump(n *Node, flag int) string {
 		} else {
 			fmt.Fprintf(&buf, "%v%v", Oconv(int(n.Op), 0), Jconv(n, 0))
 		}
-		if recur && n.Type == nil && n.Name.Param.Ntype != nil {
+		if recur && n.Type == nil && n.Name != nil && n.Name.Param != nil && n.Name.Param.Ntype != nil {
 			indent(&buf)
 			fmt.Fprintf(&buf, "%v-ntype%v", Oconv(int(n.Op), 0), n.Name.Param.Ntype)
 		}

test/fixedbugs/issue14405.go

+// compile
+
+// Copyright 2016 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Mention of field with large offset in struct literal causes crash
+package p
+
+type T struct {
+	Slice [1 << 20][]int
+	Ptr   *int
+}
+
+func New(p *int) *T {
+	return &T{Ptr: p}
+}