cmd/compile: consolidate Type construction and copying code

This should is preparatory cleanup to make it easier to use separate
types to represent each kind of Go type, rather than a single omnibus
Type struct with heavily overloaded fields.

Also, add TODO comments marking assignments that change an existing
Type's kind, as they need to be removed before we can factor Type.

Change-Id: If4b551fdea4ae045b10b1a3de2ee98f5cf32a517
Reviewed-on: https://go-review.googlesource.com/20494Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -454,6 +454,7 @@ func (p *importer) param(named bool) *Node {
 	isddd := false
 	if typ.Etype == T_old_DARRAY {
 		// T_old_DARRAY indicates ... type
+		// TODO(mdempsky): Fix Type rekinding.
 		typ.Etype = TARRAY
 		isddd = true
 	}

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

@@ -697,8 +697,7 @@ func arraylit(ctxt int, pass int, n *Node, var_ *Node, init *Nodes) {
 
 func slicelit(ctxt int, n *Node, var_ *Node, init *Nodes) {
 	// make an array type
-	t := shallow(n.Type)
-
+	t := n.Type.Copy()
 	t.Bound = Mpgetfix(n.Right.Val().U.(*Mpint))
 	t.Width = 0
 	t.Sym = nil

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

@@ -387,15 +387,6 @@ func maptype(key *Type, val *Type) *Type {
 	return t
 }
 
-func typ(et EType) *Type {
-	t := new(Type)
-	t.Etype = et
-	t.Width = BADWIDTH
-	t.Lineno = lineno
-	t.Orig = t
-	return t
-}
-
 // methcmp sorts by symbol, then by package path for unexported symbols.
 type methcmp []*Type
 
@@ -1194,18 +1185,6 @@ func Noconv(t1 *Type, t2 *Type) bool {
 	return false
 }
 
-func shallow(t *Type) *Type {
-	if t == nil {
-		return nil
-	}
-	nt := typ(0)
-	*nt = *t
-	if t.Orig == t {
-		nt.Orig = nt
-	}
-	return nt
-}
-
 func deep(t *Type) *Type {
 	if t == nil {
 		return nil
@@ -1217,32 +1196,32 @@ func deep(t *Type) *Type {
 		nt = t // share from here down
 
 	case TANY:
-		nt = shallow(t)
+		nt = t.Copy()
 		nt.Copyany = true
 
 	case TPTR32, TPTR64, TCHAN, TARRAY:
-		nt = shallow(t)
+		nt = t.Copy()
 		nt.Type = deep(t.Type)
 
 	case TMAP:
-		nt = shallow(t)
+		nt = t.Copy()
 		nt.Down = deep(t.Down)
 		nt.Type = deep(t.Type)
 
 	case TFUNC:
-		nt = shallow(t)
+		nt = t.Copy()
 		*nt.RecvP() = deep(t.Recv())
 		*nt.ResultsP() = deep(t.Results())
 		*nt.ParamsP() = deep(t.Params())
 
 	case TSTRUCT:
-		nt = shallow(t)
-		nt.Type = shallow(t.Type)
+		nt = t.Copy()
+		nt.Type = t.Type.Copy()
 		xt := nt.Type
 
 		for t = t.Type; t != nil; t = t.Down {
 			xt.Type = deep(t.Type)
-			xt.Down = shallow(t.Down)
+			xt.Down = t.Down.Copy()
 			xt = xt.Down
 		}
 	}
@@ -1863,9 +1842,7 @@ func expandmeth(t *Type) {
 	for sl := slist; sl != nil; sl = sl.link {
 		if sl.good {
 			// add it to the base type method list
-			f = typ(TFIELD)
-
-			*f = *sl.field
+			f := sl.field.Copy()
 			f.Embedded = 1 // needs a trampoline
 			if sl.followptr {
 				f.Embedded = 2

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

@@ -164,6 +164,31 @@ type Type struct {
 	Lastfn *Node // for usefield
 }
 
+// typ returns a new Type of the specified kind.
+func typ(et EType) *Type {
+	t := &Type{
+		Etype:  et,
+		Width:  BADWIDTH,
+		Lineno: lineno,
+	}
+	t.Orig = t
+	return t
+}
+
+// Copy returns a shallow copy of the Type.
+func (t *Type) Copy() *Type {
+	if t == nil {
+		return nil
+	}
+	nt := new(Type)
+	*nt = *t
+	// TODO(mdempsky): Find out why this is necessary and explain.
+	if t.Orig == t {
+		nt.Orig = nt
+	}
+	return nt
+}
+
 // Iter provides an abstraction for iterating across struct fields and
 // interface methods.
 type Iter struct {

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

@@ -3485,8 +3485,8 @@ func domethod(n *Node) {
 	typecheck(&nt, Etype)
 	if nt.Type == nil {
 		// type check failed; leave empty func
+		// TODO(mdempsky): Fix Type rekinding.
 		n.Type.Etype = TFUNC
-
 		n.Type.Nod = nil
 		return
 	}
@@ -3505,6 +3505,7 @@ func domethod(n *Node) {
 		}
 	}
 
+	// TODO(mdempsky): Fix Type rekinding.
 	*n.Type = *nt.Type
 	n.Type.Nod = nil
 	checkwidth(n.Type)
@@ -3522,8 +3523,9 @@ func copytype(n *Node, t *Type) {
 
 	maplineno := int(n.Type.Maplineno)
 	embedlineno := int(n.Type.Embedlineno)
-
 	l := n.Type.Copyto
+
+	// TODO(mdempsky): Fix Type rekinding.
 	*n.Type = *t
 
 	t = n.Type