cmd/compile: add nodeSeqIterator interface

I tried to write a program to convert *NodeList to Node, but ran into
too many problem cases.  I'm backing off and trying a more iterative
approach using interfaces.

This CL adds an interface for iteration over either a *NodeList or a
Nodes.  I changed typechecklist to use it, to show how it works.  After
NodeList is eliminated, we can change the typechecklist parameter type
to Nodes.

Passes toolstash -cmp.

Change-Id: I5c7593714b020d20868b99151b1e7cadbbdbc397
Reviewed-on: https://go-review.googlesource.com/20190
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -303,7 +303,7 @@ func genhash(sym *Sym, t *Type) {
 	Curfn = fn
 	fn.Func.Dupok = true
 	typecheck(&fn, Etop)
-	typecheckslice(fn.Nbody.Slice(), Etop)
+	typechecklist(fn.Nbody, Etop)
 	Curfn = nil
 
 	// Disable safemode while compiling this code: the code we
@@ -511,7 +511,7 @@ func geneq(sym *Sym, t *Type) {
 	Curfn = fn
 	fn.Func.Dupok = true
 	typecheck(&fn, Etop)
-	typecheckslice(fn.Nbody.Slice(), Etop)
+	typechecklist(fn.Nbody, Etop)
 	Curfn = nil
 
 	// Disable safemode while compiling this code: the code we

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

@@ -111,7 +111,7 @@ func typecheckclosure(func_ *Node, top int) {
 		Curfn = func_
 		olddd := decldepth
 		decldepth = 1
-		typecheckslice(func_.Nbody.Slice(), Etop)
+		typechecklist(func_.Nbody, Etop)
 		decldepth = olddd
 		Curfn = oldfn
 	}

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

@@ -87,7 +87,7 @@ func typecheckinl(fn *Node) {
 
 	savefn := Curfn
 	Curfn = fn
-	typecheckslice(fn.Func.Inl.Slice(), Etop)
+	typechecklist(fn.Func.Inl, Etop)
 	Curfn = savefn
 
 	safemode = save_safemode

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

@@ -406,7 +406,7 @@ func Main() {
 			Curfn = l.N
 			decldepth = 1
 			saveerrors()
-			typecheckslice(l.N.Nbody.Slice(), Etop)
+			typechecklist(l.N.Nbody, Etop)
 			checkreturn(l.N)
 			if nerrors != 0 {
 				l.N.Nbody.Set(nil) // type errors; do not compile

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

@@ -128,7 +128,7 @@ out:
 	}
 
 	decldepth++
-	typecheckslice(n.Nbody.Slice(), Etop)
+	typechecklist(n.Nbody, Etop)
 	decldepth--
 }
 
@@ -400,7 +400,7 @@ func memclrrange(n, v1, v2, a *Node) bool {
 	n.Nbody.Append(v1)
 
 	typecheck(&n.Left, Erv)
-	typecheckslice(n.Nbody.Slice(), Etop)
+	typechecklist(n.Nbody, Etop)
 	walkstmt(&n)
 	return true
 }

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

@@ -79,7 +79,7 @@ func typecheckselect(sel *Node) {
 			}
 		}
 
-		typecheckslice(ncase.Nbody.Slice(), Etop)
+		typechecklist(ncase.Nbody, Etop)
 	}
 
 	sel.Xoffset = int64(count)

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

@@ -2215,7 +2215,7 @@ func genwrapper(rcvr *Type, method *Type, newnam *Sym, iface int) {
 		fn.Func.Dupok = true
 	}
 	typecheck(&fn, Etop)
-	typecheckslice(fn.Nbody.Slice(), Etop)
+	typechecklist(fn.Nbody, Etop)
 
 	inlcalls(fn)
 	escAnalyze([]*Node{fn}, false)

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

@@ -181,7 +181,7 @@ func typecheckswitch(n *Node) {
 			}
 		}
 
-		typecheckslice(ncase.Nbody.Slice(), Etop)
+		typechecklist(ncase.Nbody, Etop)
 	}
 
 	lineno = lno

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

@@ -494,3 +494,72 @@ func (n *Nodes) AppendNodeList(l *NodeList) {
 		}
 	}
 }
+
+// nodeSeqIterator is an interface used to iterate over a sequence of nodes.
+// TODO(iant): Remove after conversion from NodeList to Nodes is complete.
+type nodeSeqIterator interface {
+	// Return whether iteration is complete.
+	Done() bool
+	// Advance to the next node.
+	Next()
+	// Return the current node.
+	N() *Node
+	// Return the address of the current node.
+	P() **Node
+}
+
+// nodeListIterator is a type that implements nodeSeqIterator using a
+// *NodeList.
+type nodeListIterator struct {
+	l *NodeList
+}
+
+func (nli *nodeListIterator) Done() bool {
+	return nli.l == nil
+}
+
+func (nli *nodeListIterator) Next() {
+	nli.l = nli.l.Next
+}
+
+func (nli *nodeListIterator) N() *Node {
+	return nli.l.N
+}
+
+func (nli *nodeListIterator) P() **Node {
+	return &nli.l.N
+}
+
+// nodesIterator implements nodeSeqIterator using a Nodes.
+type nodesIterator struct {
+	n Nodes
+	i int
+}
+
+func (ni *nodesIterator) Done() bool {
+	return ni.i >= len(ni.n.Slice())
+}
+
+func (ni *nodesIterator) Next() {
+	ni.i++
+}
+
+func (ni *nodesIterator) N() *Node {
+	return ni.n.Slice()[ni.i]
+}
+
+func (ni *nodesIterator) P() **Node {
+	return &ni.n.Slice()[ni.i]
+}
+
+// nodeSeqIterate returns an iterator over either a *Nodelist or a *Nodes.
+func nodeSeqIterate(ns interface{}) nodeSeqIterator {
+	switch ns := ns.(type) {
+	case *NodeList:
+		return &nodeListIterator{ns}
+	case Nodes:
+		return &nodesIterator{ns, 0}
+	default:
+		panic("can't happen")
+	}
+}

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

@@ -34,9 +34,9 @@ func resolve(n *Node) *Node {
 	return n
 }
 
-func typechecklist(l *NodeList, top int) {
-	for ; l != nil; l = l.Next {
-		typecheck(&l.N, top)
+func typechecklist(l interface{}, top int) {
+	for it := nodeSeqIterate(l); !it.Done(); it.Next() {
+		typecheck(it.P(), top)
 	}
 }
 
@@ -2073,7 +2073,7 @@ OpSwitch:
 			}
 		}
 		typecheck(&n.Right, Etop)
-		typecheckslice(n.Nbody.Slice(), Etop)
+		typechecklist(n.Nbody, Etop)
 		decldepth--
 		break OpSwitch
 
@@ -2087,7 +2087,7 @@ OpSwitch:
 				Yyerror("non-bool %v used as if condition", Nconv(n.Left, obj.FmtLong))
 			}
 		}
-		typecheckslice(n.Nbody.Slice(), Etop)
+		typechecklist(n.Nbody, Etop)
 		typechecklist(n.Rlist, Etop)
 		break OpSwitch
 
@@ -2137,7 +2137,7 @@ OpSwitch:
 	case OXCASE:
 		ok |= Etop
 		typechecklist(n.List, Erv)
-		typecheckslice(n.Nbody.Slice(), Etop)
+		typechecklist(n.Nbody, Etop)
 		break OpSwitch
 
 	case ODCLFUNC:

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

@@ -3965,7 +3965,7 @@ func walkprintfunc(np **Node, init **NodeList) {
 	funcbody(fn)
 
 	typecheck(&fn, Etop)
-	typecheckslice(fn.Nbody.Slice(), Etop)
+	typechecklist(fn.Nbody, Etop)
 	xtop = list(xtop, fn)
 	Curfn = oldfn