terraform: Add dependencies field to modules

terraform/state.go

@@ -185,6 +185,20 @@ type ModuleState struct {
 	// N instances underneath, although a user only needs to think
 	// about the 1:1 case.
 	Resources map[string]*ResourceState `json:"resources"`
+
+	// Dependencies are a list of things that this module relies on
+	// existing to remain intact. For example: an module may depend
+	// on a VPC ID given by an aws_vpc resource.
+	//
+	// Terraform uses this information to build valid destruction
+	// orders and to warn the user if they're destroying a module that
+	// another resource depends on.
+	//
+	// Things can be put into this list that may not be managed by
+	// Terraform. If Terraform doesn't find a matching ID in the
+	// overall state, then it assumes it isn't managed and doesn't
+	// worry about it.
+	Dependencies []string `json:"depends_on,omitempty"`
 }
 
 // IsRoot says whether or not this module diff is for the root module.

terraform/state_test.go

@@ -180,6 +180,9 @@ func TestReadWriteState(t *testing.T) {
 		Modules: []*ModuleState{
 			&ModuleState{
 				Path: rootModulePath,
+				Dependencies: []string{
+					"aws_instance.bar",
+				},
 				Resources: map[string]*ResourceState{
 					"foo": &ResourceState{
 						Primary: &InstanceState{