cmd/compile: fix isStaticCompositeLiteral

Previously, isStaticCompositeLiteral would
return the wrong value for literals like:

[1]struct{ b []byte }{b: []byte{1}}

Note that the outermost component is an array,
but once we recurse into isStaticCompositeLiteral,
we never check again that arrays are actually arrays.

Instead of adding more logic to the guts of
isStaticCompositeLiteral, allow it to accept
any Node and return the correct answer.

Change-Id: I6af7814a9037bbc7043da9a96137fbee067bbe0e
Reviewed-on: https://go-review.googlesource.com/22247Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -564,8 +564,18 @@ func getdyn(n *Node, top int) initGenType {
 }
 
 // isStaticCompositeLiteral reports whether n is a compile-time constant.
-// n must be a struct or array literal.
 func isStaticCompositeLiteral(n *Node) bool {
+	switch n.Op {
+	case OARRAYLIT:
+		if n.Type.IsSlice() {
+			return false
+		}
+	case OSTRUCTLIT:
+	case OLITERAL:
+		return true
+	default:
+		return false
+	}
 	for _, r := range n.List.Slice() {
 		if r.Op != OKEY {
 			Fatalf("isStaticCompositeLiteral: rhs not OKEY: %v", r)
@@ -575,16 +585,9 @@ func isStaticCompositeLiteral(n *Node) bool {
 			return false
 		}
 		value := r.Right
-		switch value.Op {
-		case OSTRUCTLIT, OARRAYLIT:
 		if !isStaticCompositeLiteral(value) {
 			return false
 		}
-		default:
-			if value.Op != OLITERAL {
-				return false
-			}
-		}
 	}
 	return true
 }
@@ -1031,7 +1034,7 @@ func anylit(ctxt int, n *Node, var_ *Node, init *Nodes) {
 	t := n.Type
 	switch n.Op {
 	default:
-		Fatalf("anylit: not lit")
+		Fatalf("anylit: not lit, op=%v node=%v", opnames[n.Op], n)
 
 	case OPTRLIT:
 		if !t.IsPtr() {

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

@@ -1531,7 +1531,7 @@ opswitch:
 		n = r
 
 	case OARRAYLIT, OMAPLIT, OSTRUCTLIT, OPTRLIT:
-		if (n.Op == OSTRUCTLIT || (n.Op == OARRAYLIT && !n.Type.IsSlice())) && isStaticCompositeLiteral(n) {
+		if isStaticCompositeLiteral(n) {
 			// n can be directly represented in the read-only data section.
 			// Make direct reference to the static data. See issue 12841.
 			vstat := staticname(n.Type, 0)