config: add LiteralInterpolation

config/interpolate.go

@@ -43,6 +43,14 @@ type FunctionInterpolation struct {
 	key string
 }
 
+// LiteralInterpolation implements Interpolation for literals. Ex:
+// ${"foo"} will equal "foo".
+type LiteralInterpolation struct {
+	Literal string
+
+	key string
+}
+
 // VariableInterpolation implements Interpolation for simple variable
 // interpolation. Ex: "${var.foo}" or "${aws_instance.foo.bar}"
 type VariableInterpolation struct {
@@ -174,6 +182,19 @@ func (i *FunctionInterpolation) Variables() map[string]InterpolatedVariable {
 	return result
 }
 
+func (i *LiteralInterpolation) FullString() string {
+	return i.key
+}
+
+func (i *LiteralInterpolation) Interpolate(
+	map[string]string) (string, error) {
+	return i.Literal, nil
+}
+
+func (i *LiteralInterpolation) Variables() map[string]InterpolatedVariable {
+	return nil
+}
+
 func (i *VariableInterpolation) FullString() string {
 	return i.key
 }

config/interpolate_test.go

@@ -199,6 +199,33 @@ func TestFunctionInterpolation(t *testing.T) {
 	}
 }
 
+func TestLiteralInterpolation_impl(t *testing.T) {
+	var _ Interpolation = new(LiteralInterpolation)
+}
+
+func TestLiteralInterpolation(t *testing.T) {
+	i := &LiteralInterpolation{
+		Literal: "bar",
+		key:     "foo",
+	}
+	if i.FullString() != "foo" {
+		t.Fatalf("err: %#v", i)
+	}
+
+	if i.Variables() != nil {
+		t.Fatalf("bad: %#v", i.Variables())
+	}
+
+	actual, err := i.Interpolate(nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if actual != "bar" {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
 func TestResourceVariable_impl(t *testing.T) {
 	var _ InterpolatedVariable = new(ResourceVariable)
 }