don't apply unchanged attributes from legacy diffs

builtin/providers/test/provider.go

@@ -23,6 +23,7 @@ func Provider() terraform.ResourceProvider {
 			"test_resource_timeout":          testResourceTimeout(),
 			"test_resource_diff_suppress":    testResourceDiffSuppress(),
 			"test_resource_force_new":        testResourceForceNew(),
+			"test_resource_nested":           testResourceNested(),
 		},
 		DataSourcesMap: map[string]*schema.Resource{
 			"test_data_source":    testDataSource(),

builtin/providers/test/resource_nested.go

@@ -0,0 +1,61 @@
+package test
+
+import (
+	"fmt"
+	"math/rand"
+
+	"github.com/hashicorp/terraform/helper/schema"
+)
+
+func testResourceNested() *schema.Resource {
+	return &schema.Resource{
+		Create: testResourceNestedCreate,
+		Read:   testResourceNestedRead,
+		Delete: testResourceNestedDelete,
+
+		Importer: &schema.ResourceImporter{
+			State: schema.ImportStatePassthrough,
+		},
+
+		Schema: map[string]*schema.Schema{
+			"optional": {
+				Type:     schema.TypeBool,
+				Optional: true,
+				ForceNew: true,
+			},
+			"nested": {
+				Type:     schema.TypeSet,
+				Optional: true,
+				ForceNew: true,
+				Elem: &schema.Resource{
+					Schema: map[string]*schema.Schema{
+						"string": {
+							Type:     schema.TypeString,
+							Optional: true,
+							ForceNew: true,
+						},
+						"optional": {
+							Type:     schema.TypeBool,
+							Optional: true,
+							ForceNew: true,
+						},
+					},
+				},
+			},
+		},
+	}
+}
+
+func testResourceNestedCreate(d *schema.ResourceData, meta interface{}) error {
+	d.SetId(fmt.Sprintf("%x", rand.Int63()))
+	return testResourceNestedRead(d, meta)
+}
+
+func testResourceNestedRead(d *schema.ResourceData, meta interface{}) error {
+	return nil
+}
+
+func testResourceNestedDelete(d *schema.ResourceData, meta interface{}) error {
+	d.SetId("")
+	return nil
+}

builtin/providers/test/resource_nested_test.go

@@ -0,0 +1,104 @@
+package test
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/hashicorp/terraform/addrs"
+	"github.com/hashicorp/terraform/helper/resource"
+	"github.com/hashicorp/terraform/terraform"
+)
+
+func TestResourceNested_basic(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+	nested {
+		string = "val"
+	}
+}
+				`),
+			},
+		},
+	})
+}
+
+func TestResourceNested_addRemove(t *testing.T) {
+	var id string
+	checkFunc := func(s *terraform.State) error {
+		root := s.ModuleByPath(addrs.RootModuleInstance)
+		res := root.Resources["test_resource_nested.foo"]
+		if res.Primary.ID == id {
+			return errors.New("expected new resource")
+		}
+		id = res.Primary.ID
+		return nil
+	}
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+	nested {
+		string = "val"
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+	optional = true
+	nested {
+		string = "val"
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+	nested {
+		string = "val"
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+	nested {
+		string = "val"
+		optional = true
+	}
+}
+				`),
+				Check: checkFunc,
+			},
+			resource.TestStep{
+				Config: strings.TrimSpace(`
+resource "test_resource_nested" "foo" {
+}
+				`),
+				Check: checkFunc,
+			},
+		},
+	})
+}

terraform/diff.go

@@ -457,6 +457,11 @@ func (d *InstanceDiff) ApplyToValue(base cty.Value, schema *configschema.Block)
 			continue
 		}
 
+		// sometimes helper/schema gives us values that aren't really a diff
+		if diff.Old == diff.New {
+			continue
+		}
+
 		attrs[attr] = diff.New
 	}