core: error instead of panic on self var in wrong scope

Fixes #4808
Fixes #5174

terraform/interpolate.go

@@ -238,6 +238,10 @@ func (i *Interpolater) valueSelfVar(
 	n string,
 	v *config.SelfVariable,
 	result map[string]ast.Variable) error {
+	if scope == nil || scope.Resource == nil {
+		return fmt.Errorf(
+			"%s: invalid scope, self variables are only valid on resources", n)
+	}
 	rv, err := config.NewResourceVariable(fmt.Sprintf(
 		"%s.%s.%d.%s",
 		scope.Resource.Type,

terraform/interpolate_test.go

@@ -386,6 +386,24 @@ func TestInterpolator_resourceMultiAttributesComputed(t *testing.T) {
 	})
 }
 
+func TestInterpolater_selfVarWithoutResource(t *testing.T) {
+	i := &Interpolater{}
+
+	scope := &InterpolationScope{
+		Path: rootModulePath,
+	}
+
+	v, err := config.NewInterpolatedVariable("self.name")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	_, err = i.Values(scope, map[string]config.InterpolatedVariable{"foo": v})
+	if err == nil {
+		t.Fatalf("expected err, got none")
+	}
+}
+
 func getInterpolaterFixture(t *testing.T) *Interpolater {
 	lock := new(sync.RWMutex)
 	state := &State{