prune unused nodes from a destroy plan graph

We may need to prune nodes from a full destroy plan graph which cannot
be evaluated if there is no current state.

Add missing method to nodeExpandPlannableResource to ensure planned
resource are handled correctly when pruning nodes.

Author: jbardin

internal/terraform/context_plan2_test.go

@@ -3518,3 +3518,44 @@ resource "test_object" "b" {
 		t.Fatalf("no cycle error found:\n got: %s\n", msg)
 	}
 }
+
+// plan a destroy with no state where configuration could fail to evaluate
+// expansion indexes.
+func TestContext2Plan_emptyDestroy(t *testing.T) {
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+locals {
+  enable = true
+  value  = local.enable ? module.example[0].out : null
+}
+
+module "example" {
+  count  = local.enable ? 1 : 0
+  source = "./example"
+}
+`,
+		"example/main.tf": `
+resource "test_resource" "x" {
+}
+
+output "out" {
+  value = test_resource.x
+}
+`,
+	})
+
+	p := testProvider("test")
+	state := states.NewState()
+
+	ctx := testContext2(t, &ContextOpts{
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+	})
+
+	_, diags := ctx.Plan(m, state, &PlanOpts{
+		Mode: plans.DestroyMode,
+	})
+
+	assertNoErrors(t, diags)
+}

internal/terraform/graph_builder_plan.go

@@ -170,6 +170,10 @@ func (b *PlanGraphBuilder) Steps() []GraphTransformer {
 		// TargetsTransformer can determine which nodes to keep in the graph.
 		&DestroyEdgeTransformer{},
 
+		&pruneUnusedNodesTransformer{
+			skip: b.Operation != walkPlanDestroy,
+		},
+
 		// Target
 		&TargetsTransformer{Targets: b.Targets},
 

internal/terraform/node_resource_plan.go

@@ -53,12 +53,16 @@ var (
 	_ GraphNodeAttachResourceConfig = (*nodeExpandPlannableResource)(nil)
 	_ GraphNodeAttachDependencies   = (*nodeExpandPlannableResource)(nil)
 	_ GraphNodeTargetable           = (*nodeExpandPlannableResource)(nil)
+	_ graphNodeExpandsInstances     = (*nodeExpandPlannableResource)(nil)
 )
 
 func (n *nodeExpandPlannableResource) Name() string {
 	return n.NodeAbstractResource.Name() + " (expand)"
 }
 
+func (n *nodeExpandPlannableResource) expandsInstances() {
+}
+
 // GraphNodeAttachDependencies
 func (n *nodeExpandPlannableResource) AttachDependencies(deps []addrs.ConfigResource) {
 	n.dependencies = deps

internal/terraform/transform_destroy_edge.go

@@ -170,9 +170,18 @@ func (t *DestroyEdgeTransformer) Transform(g *Graph) error {
 // closers also need to disable their use of expansion if the module itself is
 // no longer present.
 type pruneUnusedNodesTransformer struct {
+	// The plan graph builder will skip this transformer except during a full
+	// destroy. Planing normally involves all nodes, but during a destroy plan
+	// we may need to prune things which are in the configuration but do not
+	// exist in state to evaluate.
+	skip bool
 }
 
 func (t *pruneUnusedNodesTransformer) Transform(g *Graph) error {
+	if t.skip {
+		return nil
+	}
+
 	// We need a reverse depth first walk of modules, processing them in order
 	// from the leaf modules to the root. This allows us to remove unneeded
 	// dependencies from child modules, freeing up nodes in the parent module