cmd/compile: clean up ... Bound marker

This mostly a mechanical change.
However, the change in assignop (subr.go) is a bug fix.
The code didn’t match the comment,
and the comment was correct.
Nevertheless, this CL passes toolstash -cmp.

The last direct reference to dddBound outside
type.go (in typecheck.go) will go away
in a future CL.

Change-Id: Ifb1691e0a07f906712c18c4a4cd23060807a5da5
Reviewed-on: https://go-review.googlesource.com/21235Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -256,7 +256,7 @@ func dowidth(t *Type) {
 			w = int64(sizeof_Array)
 			checkwidth(t.Type)
 			t.Align = uint8(Widthptr)
-		} else if t.Bound == -100 {
+		} else if t.isDDDArray() {
 			if !t.Broke {
 				Yyerror("use of [...] array outside of array literal")
 				t.Broke = true

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

@@ -503,8 +503,10 @@ func (p *exporter) typ(t *Type) {
 	// otherwise we have a type literal
 	switch t.Etype {
 	case TARRAY:
-		// TODO(gri) define named constant for the -100
-		if t.Bound >= 0 || t.Bound == -100 {
+		if t.isDDDArray() {
+			Fatalf("array bounds should be known at export time: %v", t)
+		}
+		if t.Bound >= 0 {
 			p.tag(arrayTag)
 			p.int64(t.Bound)
 		} else {

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

@@ -589,7 +589,7 @@ func typefmt(t *Type, flag FmtFlag) string {
 		if t.Bound >= 0 {
 			return fmt.Sprintf("[%d]%v", t.Bound, t.Type)
 		}
-		if t.Bound == -100 {
+		if t.isDDDArray() {
 			return "[...]" + t.Type.String()
 		}
 		return "[]" + t.Type.String()

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

@@ -897,7 +897,7 @@ func assignop(src *Type, dst *Type, why *string) Op {
 	if src.Etype == TNIL {
 		switch dst.Etype {
 		case TARRAY:
-			if dst.Bound != -100 { // not slice
+			if !dst.IsSlice() {
 				break
 			}
 			fallthrough

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

@@ -70,6 +70,8 @@ const (
 	NTYPE
 )
 
+const dddBound = -100 // arrays declared as [...]T start life with Bound=dddBound
+
 // Types stores pointers to predeclared named types.
 //
 // It also stores pointers to several special types:
@@ -373,6 +375,13 @@ func (t *Type) SetFields(fields []*Field) {
 	t.Fields().Set(fields)
 }
 
+func (t *Type) isDDDArray() bool {
+	if t.Etype != TARRAY {
+		return false
+	}
+	return t.Bound == dddBound
+}
+
 func (t *Type) Size() int64 {
 	dowidth(t)
 	return t.Width

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

@@ -336,7 +336,7 @@ OpSwitch:
 		if l == nil {
 			t.Bound = -1 // slice
 		} else if l.Op == ODDD {
-			t.Bound = -100 // to be filled in
+			t.Bound = dddBound // to be filled in
 			if top&Ecomplit == 0 && n.Diag == 0 {
 				t.Broke = true
 				n.Diag = 1
@@ -385,7 +385,7 @@ OpSwitch:
 		n.Type = t
 		n.Left = nil
 		n.Right = nil
-		if t.Bound != -100 {
+		if !t.isDDDArray() {
 			checkwidth(t)
 		}
 
@@ -1267,7 +1267,7 @@ OpSwitch:
 		n.Left = defaultlit(n.Left, nil)
 		l = n.Left
 		if l.Op == OTYPE {
-			if n.Isddd || l.Type.Bound == -100 {
+			if n.Isddd || l.Type.isDDDArray() {
 				if !l.Type.Broke {
 					Yyerror("invalid use of ... in type conversion to %v", l.Type)
 				}
@@ -2991,7 +2991,7 @@ func typecheckcomplit(n *Node) *Node {
 			l.Right = assignconv(r, t.Type, "array or slice literal")
 		}
 
-		if t.Bound == -100 {
+		if t.isDDDArray() {
 			t.Bound = length
 		}
 		if t.Bound < 0 {