config: if any var is computed, the entire interpolation is computed

config/interpolate_walk.go

@@ -152,12 +152,12 @@ func (w *interpolationWalker) Primitive(v reflect.Value) error {
 		if w.loc == reflectwalk.SliceElem {
 			parts := strings.Split(replaceVal, InterpSplitDelim)
 			for _, p := range parts {
-				if strings.Contains(p, UnknownVariableValue) {
+				if p == UnknownVariableValue {
 					remove = true
 					break
 				}
 			}
-		} else if strings.Contains(replaceVal, UnknownVariableValue) {
+		} else if replaceVal == UnknownVariableValue {
 			remove = true
 		}
 		if remove {

config/interpolate_walk_test.go

@@ -5,7 +5,6 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/hashicorp/terraform/config/lang"
 	"github.com/hashicorp/terraform/config/lang/ast"
 	"github.com/mitchellh/reflectwalk"
 )
@@ -125,7 +124,7 @@ func TestInterpolationWalker_replace(t *testing.T) {
 				"foo": "hello, ${var.foo}",
 			},
 			Output: map[string]interface{}{
-				"foo": "hello, bar",
+				"foo": "bar",
 			},
 			Value: "bar",
 		},
@@ -171,38 +170,11 @@ func TestInterpolationWalker_replace(t *testing.T) {
 			Output: map[string]interface{}{},
 			Value:  UnknownVariableValue + InterpSplitDelim + "baz",
 		},
-
-		{
-			Input: map[string]interface{}{
-				"foo": []interface{}{
-					"${var.foo}/32",
-					"bing",
-				},
-			},
-			Output: map[string]interface{}{},
-			Value:  UnknownVariableValue,
-		},
 	}
 
 	for i, tc := range cases {
-		config := &lang.EvalConfig{
-			GlobalScope: &ast.BasicScope{
-				VarMap: map[string]ast.Variable{
-					"var.foo": ast.Variable{
-						Value: tc.Value,
-						Type:  ast.TypeString,
-					},
-				},
-			},
-		}
-
-		fn := func(root ast.Node) (string, error) {
-			value, _, err := lang.Eval(root, config)
-			if err != nil {
-				return "", err
-			}
-
-			return value.(string), nil
+		fn := func(ast.Node) (string, error) {
+			return tc.Value, nil
 		}
 
 		w := &interpolationWalker{F: fn, Replace: true}

config/raw_config.go

@@ -83,6 +83,29 @@ func (r *RawConfig) Config() map[string]interface{} {
 func (r *RawConfig) Interpolate(vs map[string]ast.Variable) error {
 	config := langEvalConfig(vs)
 	return r.interpolate(func(root ast.Node) (string, error) {
+		// We detect the variables again and check if the value of any
+		// of the variables is the computed value. If it is, then we
+		// treat this entire value as computed.
+		//
+		// We have to do this here before the `lang.Eval` because
+		// if any of the variables it depends on are computed, then
+		// the interpolation can fail at runtime for other reasons. Example:
+		// `${count.index+1}`: in a world where `count.index` is computed,
+		// this would fail a type check since the computed placeholder is
+		// a string, but realistically the whole value is just computed.
+		vars, err := DetectVariables(root)
+		if err != nil {
+			return "", err
+		}
+		for _, v := range vars {
+			varVal, ok := vs[v.FullKey()]
+			if ok && varVal.Value == UnknownVariableValue {
+				return UnknownVariableValue, nil
+			}
+		}
+
+		// None of the variables we need are computed, meaning we should
+		// be able to properly evaluate.
 		out, _, err := lang.Eval(root, config)
 		if err != nil {
 			return "", err

config/raw_config_test.go

@@ -238,6 +238,39 @@ func TestRawConfig_unknown(t *testing.T) {
 	}
 }
 
+func TestRawConfig_unknownPartial(t *testing.T) {
+	raw := map[string]interface{}{
+		"foo": "${var.bar}/32",
+	}
+
+	rc, err := NewRawConfig(raw)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	vars := map[string]ast.Variable{
+		"var.bar": ast.Variable{
+			Value: UnknownVariableValue,
+			Type:  ast.TypeString,
+		},
+	}
+	if err := rc.Interpolate(vars); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := rc.Config()
+	expected := map[string]interface{}{}
+
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad: %#v", actual)
+	}
+
+	expectedKeys := []string{"foo"}
+	if !reflect.DeepEqual(rc.UnknownKeys(), expectedKeys) {
+		t.Fatalf("bad: %#v", rc.UnknownKeys())
+	}
+}
+
 func TestRawConfigValue(t *testing.T) {
 	raw := map[string]interface{}{
 		"foo": "${var.bar}",