cmd/compile/internal/gc: split SetInterface from SetFields

Change-Id: I4e568414faf64d3d47b1795382f0615f6caf53bc
Reviewed-on: https://go-review.googlesource.com/38390
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -533,7 +533,7 @@ func (p *importer) typ() *Type {
 			t = Types[TINTER]
 		} else {
 			t = p.newtyp(TINTER)
-			t.SetFields(ml)
+			t.SetInterface(ml)
 		}
 		checkwidth(t)
 

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

@@ -897,7 +897,7 @@ func tointerface0(t *Type, l []*Node) *Type {
 		}
 	}
 	sort.Sort(methcmp(fields))
-	t.SetFields(fields)
+	t.SetInterface(fields)
 
 	checkdupfields("method", t)
 	checkwidth(t)

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

@@ -865,6 +865,7 @@ func (t *Type) FieldSlice() []*Field {
 
 // SetFields sets struct/interface type t's fields/methods to fields.
 func (t *Type) SetFields(fields []*Field) {
+	t.wantEtype(TSTRUCT)
 	for _, f := range fields {
 		// If type T contains a field F with a go:notinheap
 		// type, then T must also be go:notinheap. Otherwise,
@@ -879,6 +880,11 @@ func (t *Type) SetFields(fields []*Field) {
 	t.Fields().Set(fields)
 }
 
+func (t *Type) SetInterface(methods []*Field) {
+	t.wantEtype(TINTER)
+	t.Fields().Set(methods)
+}
+
 func (t *Type) isDDDArray() bool {
 	if t.Etype != TARRAY {
 		return false

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

@@ -382,7 +382,7 @@ func makeErrorInterface() *Type {
 	field.Type = f
 
 	t := typ(TINTER)
-	t.SetFields([]*Field{field})
+	t.SetInterface([]*Field{field})
 	return t
 }