Add MaxItems attribute to Schema

 * MaxItems defines a maximum amount of items that can exist within a
   TypeSet or TypeList. Specific use cases would be if a TypeSet is being
   used to wrap a complex structure, however more than one instance would
   cause instability.

helper/schema/schema.go

@@ -99,7 +99,13 @@ type Schema struct {
 	// TypeList, and represents what the element type is. If it is *Schema,
 	// the element type is just a simple value. If it is *Resource, the
 	// element type is a complex structure, potentially with its own lifecycle.
-	Elem interface{}
+	//
+	// MaxItems defines a maximum amount of items that can exist within a
+	// TypeSet or TypeList. Specific use cases would be if a TypeSet is being
+	// used to wrap a complex structure, however more than one instance would
+	// cause instability.
+	Elem     interface{}
+	MaxItems int
 
 	// The following fields are only valid for a TypeSet type.
 	//
@@ -541,6 +547,10 @@ func (m schemaMap) InternalValidate(topSchemaMap schemaMap) error {
 						"%s: Elem must have only Type set", k)
 				}
 			}
+		} else {
+			if v.MaxItems > 0 {
+				return fmt.Errorf("%s: MaxItems is only supported on lists or sets", k)
+			}
 		}
 
 		if v.ValidateFunc != nil {
@@ -1049,6 +1059,12 @@ func (m schemaMap) validateList(
 			"%s: should be a list", k)}
 	}
 
+	// Validate length
+	if schema.MaxItems > 0 && rawV.Len() > schema.MaxItems {
+		return nil, []error{fmt.Errorf(
+			"%s: attribute supports %d item maximum, config has %d declared", k, schema.MaxItems, rawV.Len())}
+	}
+
 	// Now build the []interface{}
 	raws := make([]interface{}, rawV.Len())
 	for i, _ := range raws {

helper/schema/schema_test.go

@@ -3647,3 +3647,94 @@ func TestSchemaMap_Validate(t *testing.T) {
 		}
 	}
 }
+
+func TestSchemaSet_ValidateMaxItems(t *testing.T) {
+	cases := map[string]struct {
+		Schema          map[string]*Schema
+		State           *terraform.InstanceState
+		Config          map[string]interface{}
+		ConfigVariables map[string]string
+		Diff            *terraform.InstanceDiff
+		Err             bool
+		Errors          []error
+	}{
+		"#0": {
+			Schema: map[string]*Schema{
+				"aliases": &Schema{
+					Type:     TypeSet,
+					Optional: true,
+					MaxItems: 1,
+					Elem:     &Schema{Type: TypeString},
+				},
+			},
+			State: nil,
+			Config: map[string]interface{}{
+				"aliases": []interface{}{"foo", "bar"},
+			},
+			Diff: nil,
+			Err:  true,
+			Errors: []error{
+				fmt.Errorf("aliases: attribute supports 1 item maximum, config has 2 declared"),
+			},
+		},
+		"#1": {
+			Schema: map[string]*Schema{
+				"aliases": &Schema{
+					Type:     TypeSet,
+					Optional: true,
+					Elem:     &Schema{Type: TypeString},
+				},
+			},
+			State: nil,
+			Config: map[string]interface{}{
+				"aliases": []interface{}{"foo", "bar"},
+			},
+			Diff:   nil,
+			Err:    false,
+			Errors: nil,
+		},
+		"#2": {
+			Schema: map[string]*Schema{
+				"aliases": &Schema{
+					Type:     TypeSet,
+					Optional: true,
+					MaxItems: 1,
+					Elem:     &Schema{Type: TypeString},
+				},
+			},
+			State: nil,
+			Config: map[string]interface{}{
+				"aliases": []interface{}{"foo"},
+			},
+			Diff:   nil,
+			Err:    false,
+			Errors: nil,
+		},
+	}
+
+	for tn, tc := range cases {
+		c, err := config.NewRawConfig(tc.Config)
+		if err != nil {
+			t.Fatalf("%q: err: %s", tn, err)
+		}
+		_, es := schemaMap(tc.Schema).Validate(terraform.NewResourceConfig(c))
+
+		if len(es) > 0 != tc.Err {
+			if len(es) == 0 {
+				t.Errorf("%q: no errors", tn)
+			}
+
+			for _, e := range es {
+				t.Errorf("%q: err: %s", tn, e)
+			}
+
+			t.FailNow()
+		}
+
+		if tc.Errors != nil {
+			if !reflect.DeepEqual(es, tc.Errors) {
+				t.Fatalf("%q: expected: %q\ngot: %q", tn, tc.Errors, es)
+			}
+		}
+	}
+}