config: parse ${path.module}

config/config.go

@@ -201,17 +201,22 @@ func (c *Config) Validate() error {
 
 	// Check that all count variables are valid.
 	for source, vs := range vars {
-		for _, v := range vs {
-			cv, ok := v.(*CountVariable)
-			if !ok {
-				continue
-			}
-
-			if cv.Type == CountValueInvalid {
+		for _, rawV := range vs {
+			switch v := rawV.(type) {
+			case *CountVariable:
+				if v.Type == CountValueInvalid {
 					errs = append(errs, fmt.Errorf(
 						"%s: invalid count variable: %s",
 						source,
-					cv.FullKey()))
+						v.FullKey()))
+				}
+			case *PathVariable:
+				if v.Type == PathValueInvalid {
+					errs = append(errs, fmt.Errorf(
+						"%s: invalid path variable: %s",
+						source,
+						v.FullKey()))
+				}
 			}
 		}
 	}

config/config_test.go

@@ -137,6 +137,20 @@ func TestConfigValidate_outputBadField(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_pathVar(t *testing.T) {
+	c := testConfig(t, "validate-path-var")
+	if err := c.Validate(); err != nil {
+		t.Fatal("err: %s", err)
+	}
+}
+
+func TestConfigValidate_pathVarInvalid(t *testing.T) {
+	c := testConfig(t, "validate-path-var-invalid")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_unknownThing(t *testing.T) {
 	c := testConfig(t, "validate-unknownthing")
 	if err := c.Validate(); err == nil {

config/interpolate.go

@@ -75,6 +75,20 @@ type ModuleVariable struct {
 	key   string
 }
 
+// A PathVariable is a variable that references path information about the
+// module.
+type PathVariable struct {
+	Type PathValueType
+	key  string
+}
+
+type PathValueType byte
+
+const (
+	PathValueInvalid PathValueType = iota
+	PathValueModule
+)
+
 // A ResourceVariable is a variable that is referencing the field
 // of a resource, such as "${aws_instance.foo.ami}"
 type ResourceVariable struct {
@@ -101,6 +115,8 @@ type UserVariable struct {
 func NewInterpolatedVariable(v string) (InterpolatedVariable, error) {
 	if strings.HasPrefix(v, "count.") {
 		return NewCountVariable(v)
+	} else if strings.HasPrefix(v, "path.") {
+		return NewPathVariable(v)
 	} else if strings.HasPrefix(v, "var.") {
 		return NewUserVariable(v)
 	} else if strings.HasPrefix(v, "module.") {
@@ -206,6 +222,24 @@ func (v *ModuleVariable) FullKey() string {
 	return v.key
 }
 
+func NewPathVariable(key string) (*PathVariable, error) {
+	var fieldType PathValueType
+	parts := strings.SplitN(key, ".", 2)
+	switch parts[1] {
+	case "module":
+		fieldType = PathValueModule
+	}
+
+	return &PathVariable{
+		Type: fieldType,
+		key:  key,
+	}, nil
+}
+
+func (v *PathVariable) FullKey() string {
+	return v.key
+}
+
 func NewResourceVariable(key string) (*ResourceVariable, error) {
 	parts := strings.SplitN(key, ".", 3)
 	if len(parts) < 3 {

config/interpolate_test.go

@@ -45,6 +45,14 @@ func TestNewInterpolatedVariable(t *testing.T) {
 			},
 			false,
 		},
+		{
+			"path.module",
+			&PathVariable{
+				Type: PathValueModule,
+				key:  "path.module",
+			},
+			false,
+		},
 	}
 
 	for i, tc := range cases {

config/test-fixtures/validate-path-var-invalid/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "foo" {
+    foo = "${path.nope}"
+}

config/test-fixtures/validate-path-var/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "foo" {
+    foo = "${path.module}"
+}