don't count empty containers in diff.Apply

If there were no matching keys, and there was no diff at all, don't set
a zero count for the container. Normally Providers can't reliably detect
empty vs unset here, but there are some cases that worked.

builtin/providers/test/resource_test.go

@@ -24,10 +24,96 @@ resource "test_resource" "foo" {
 	}
 }
 				`),
-				Check: func(s *terraform.State) error {
-					return nil
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckNoResourceAttr(
+						"test_resource.foo", "list.#",
+					),
+				),
 			},
 		},
+	})
+}
+
+func TestResource_changedList(t *testing.T) {
+	resource.UnitTest(t, resource.TestCase{
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckResourceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckNoResourceAttr(
+						"test_resource.foo", "list.#",
+					),
+				),
+			},
+			{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+	list = ["a"]
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.#", "1",
+					),
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.0", "a",
+					),
+				),
+			},
+			{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+	list = ["a", "b"]
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.#", "2",
+					),
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.0", "a",
+					),
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.1", "b",
+					),
+				),
+			},
+			{
+				Config: strings.TrimSpace(`
+resource "test_resource" "foo" {
+	required = "yep"
+	required_map = {
+	    key = "value"
+	}
+	list = ["b"]
+}
+				`),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.#", "1",
+					),
+					resource.TestCheckResourceAttr(
+						"test_resource.foo", "list.0", "b",
+					),
+				),
+			},
 		},
 	})
 }
@@ -165,9 +251,6 @@ resource "test_resource" "foo" {
   }
 }
 				`),
-				Check: func(s *terraform.State) error {
-					return nil
-				},
 			},
 			resource.TestStep{
 				Config: strings.TrimSpace(`

terraform/diff.go

@@ -454,10 +454,10 @@ func (d *InstanceDiff) Apply(attrs map[string]string, schema *configschema.Block
 		return result, nil
 	}
 
-	return d.blockDiff(nil, attrs, schema)
+	return d.applyBlockDiff(nil, attrs, schema)
 }
 
-func (d *InstanceDiff) blockDiff(path []string, attrs map[string]string, schema *configschema.Block) (map[string]string, error) {
+func (d *InstanceDiff) applyBlockDiff(path []string, attrs map[string]string, schema *configschema.Block) (map[string]string, error) {
 	result := map[string]string{}
 
 	name := ""
@@ -564,7 +564,7 @@ func (d *InstanceDiff) blockDiff(path []string, attrs map[string]string, schema
 		}
 
 		for k := range candidateKeys {
-			newAttrs, err := d.blockDiff(append(path, n, k), attrs, &block.Block)
+			newAttrs, err := d.applyBlockDiff(append(path, n, k), attrs, &block.Block)
 			if err != nil {
 				return result, err
 			}
@@ -735,24 +735,37 @@ func (d *InstanceDiff) applyCollectionDiff(path []string, attrs map[string]strin
 	}
 
 	// collect all the keys from the diff and the old state
+	noDiff := true
 	keys := map[string]bool{}
 	for k := range d.Attributes {
+		if !strings.HasPrefix(k, currentKey+".") {
+			continue
+		}
+		noDiff = false
 		keys[k] = true
 	}
+
+	noAttrs := true
 	for k := range attrs {
+		if !strings.HasPrefix(k, currentKey+".") {
+			continue
+		}
+		noAttrs = false
 		keys[k] = true
 	}
 
+	// If there's no diff and no attrs, then there's no value at all.
+	// This prevents an unexpected zero-count attribute in the attributes.
+	if noDiff && noAttrs {
+		return result, nil
+	}
+
 	idx := "#"
 	if attrSchema.Type.IsMapType() {
 		idx = "%"
 	}
 
 	for k := range keys {
-		if !strings.HasPrefix(k, currentKey+".") {
-			continue
-		}
-
 		// generate an schema placeholder for the values
 		elSchema := &configschema.Attribute{
 			Type: attrSchema.Type.ElementType(),