helper/schema: show in diff when no config is going to empty set

helper/schema/schema.go

@@ -702,6 +702,7 @@ func (m schemaMap) diffSet(
 		return nil
 	}
 
+	oRaw := o
 	if o == nil {
 		o = &Set{F: schema.Set}
 	}
@@ -750,7 +751,7 @@ func (m schemaMap) diffSet(
 
 	// If the counts are not the same, then record that diff
 	changed := oldLen != newLen
-	if changed || all {
+	if changed || all || oRaw == nil {
 		countSchema := &Schema{
 			Type:     TypeInt,
 			Computed: schema.Computed,

helper/schema/schema_test.go

@@ -2032,6 +2032,37 @@ func TestSchemaMap_Diff(t *testing.T) {
 
 			Err: false,
 		},
+
+		// #51 - A set computed element shouldn't cause a diff
+		{
+			Schema: map[string]*Schema{
+				"instances": &Schema{
+					Type:     TypeSet,
+					Elem:     &Schema{Type: TypeString},
+					Optional: true,
+					ForceNew: true,
+					Set: func(v interface{}) int {
+						return len(v.(string))
+					},
+				},
+			},
+
+			State: nil,
+
+			Config: map[string]interface{}{},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"instances.#": &terraform.ResourceAttrDiff{
+						Old:         "0",
+						New:         "0",
+						RequiresNew: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
 	}
 
 	for i, tc := range cases {