Interpolation also skipped during Validate phase

Adding walkValidate to the EvalTree operations, and removing the
walkValidate guard from the Interpolater.valueModuleVar allows the
values to be interpolated for Validate.

terraform/context_validate_test.go

@@ -749,3 +749,30 @@ func TestContext2Validate_varRefFilled(t *testing.T) {
 		t.Fatalf("bad: %#v", value)
 	}
 }
+
+// Module variables weren't being interpolated during Validate phase.
+// related to https://github.com/hashicorp/terraform/issues/5322
+func TestContext2Validate_interpolateVar(t *testing.T) {
+	input := new(MockUIInput)
+
+	m := testModule(t, "input-interpolate-var")
+	p := testProvider("null")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"template": testProviderFuncFixed(p),
+		},
+		UIInput: input,
+	})
+
+	w, e := ctx.Validate()
+	if w != nil {
+		t.Log("warnings:", w)
+	}
+	if e != nil {
+		t.Fatal("err:", e)
+	}
+}

terraform/graph_config_node_output.go

@@ -44,7 +44,8 @@ func (n *GraphNodeConfigOutput) DependentOn() []string {
 // GraphNodeEvalable impl.
 func (n *GraphNodeConfigOutput) EvalTree() EvalNode {
 	return &EvalOpFilter{
-		Ops: []walkOperation{walkRefresh, walkPlan, walkApply, walkDestroy, walkInput},
+		Ops: []walkOperation{walkRefresh, walkPlan, walkApply,
+			walkDestroy, walkInput, walkValidate},
 		Node: &EvalSequence{
 			Nodes: []EvalNode{
 				&EvalWriteOutput{

terraform/interpolate.go

@@ -130,16 +130,6 @@ func (i *Interpolater) valueModuleVar(
 	v *config.ModuleVariable,
 	result map[string]ast.Variable) error {
 
-	// If we're computing all dynamic fields, then module vars count
-	// and we mark it as computed.
-	if i.Operation == walkValidate {
-		result[n] = ast.Variable{
-			Value: config.UnknownVariableValue,
-			Type:  ast.TypeString,
-		}
-		return nil
-	}
-
 	// Build the path to the child module we want
 	path := make([]string, len(scope.Path), len(scope.Path)+1)
 	copy(path, scope.Path)