config/lang: FixedValueTransform

config/lang/transform_fixed.go

@@ -0,0 +1,26 @@
+package lang
+
+import (
+	"github.com/hashicorp/terraform/config/lang/ast"
+)
+
+// FixedValueTransform transforms an AST to return a fixed value for
+// all interpolations. i.e. you can make "hello ${anything}" always
+// turn into "hello foo".
+func FixedValueTransform(root ast.Node, Value *ast.LiteralNode) ast.Node {
+	// We visit the nodes in top-down order
+	result := root
+	switch n := result.(type) {
+	case *ast.Concat:
+		for i, v := range n.Exprs {
+			n.Exprs[i] = FixedValueTransform(v, Value)
+		}
+	case *ast.LiteralNode:
+		// We keep it as-is
+	default:
+		// Anything else we replace
+		result = Value
+	}
+
+	return result
+}

config/lang/transform_fixed_test.go

@@ -0,0 +1,48 @@
+package lang
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/hashicorp/terraform/config/lang/ast"
+)
+
+func TestFixedValueTransform(t *testing.T) {
+	cases := []struct {
+		Input  ast.Node
+		Output ast.Node
+	}{
+		{
+			&ast.LiteralNode{Value: 42},
+			&ast.LiteralNode{Value: 42},
+		},
+
+		{
+			&ast.VariableAccess{Name: "bar"},
+			&ast.LiteralNode{Value: "foo"},
+		},
+
+		{
+			&ast.Concat{
+				Exprs: []ast.Node{
+					&ast.VariableAccess{Name: "bar"},
+					&ast.LiteralNode{Value: 42},
+				},
+			},
+			&ast.Concat{
+				Exprs: []ast.Node{
+					&ast.LiteralNode{Value: "foo"},
+					&ast.LiteralNode{Value: 42},
+				},
+			},
+		},
+	}
+
+	value := &ast.LiteralNode{Value: "foo"}
+	for _, tc := range cases {
+		actual := FixedValueTransform(tc.Input, value)
+		if !reflect.DeepEqual(actual, tc.Output) {
+			t.Fatalf("bad: %#v\n\nInput: %#v", actual, tc.Input)
+		}
+	}
+}