walkDestroy is a form of "apply"

When computing the count value, make sure to include walkDestroy with
walkApply, as the former is only a special case of the latter.

When applying a saved plan, the computed count values are lost and we
can no longer query the state for those values. The apply walk was
already considered in the `resourceCountMax` function, but the destroy
walk was not.  This worked when destroying in a single operation
("terraform destroy"), since the state would still be updated with the
latest counts from the plan.

terraform/context_apply_test.go

@@ -9578,6 +9578,89 @@ func TestContext2Apply_plannedInterpolatedCount(t *testing.T) {
 	}
 }
 
+func TestContext2Apply_plannedDestroyInterpolatedCount(t *testing.T) {
+	m := testModule(t, "plan-destroy-interpolated-count")
+
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+
+	providerResolver := ResourceProviderResolverFixed(
+		map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	)
+
+	s := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.a.0": {
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "foo",
+						},
+						Provider: "provider.aws",
+					},
+					"aws_instance.a.1": {
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "foo",
+						},
+						Provider: "provider.aws",
+					},
+				},
+				Outputs: map[string]*OutputState{
+					"out": {
+						Type:  "list",
+						Value: []string{"foo", "foo"},
+					},
+				},
+			},
+		},
+	}
+
+	ctx := testContext2(t, &ContextOpts{
+		Module:           m,
+		ProviderResolver: providerResolver,
+		State:            s,
+		Destroy:          true,
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("plan failed: %s", err)
+	}
+
+	// We'll marshal and unmarshal the plan here, to ensure that we have
+	// a clean new context as would be created if we separately ran
+	// terraform plan -out=tfplan && terraform apply tfplan
+	var planBuf bytes.Buffer
+	err = WritePlan(plan, &planBuf)
+	if err != nil {
+		t.Fatalf("failed to write plan: %s", err)
+	}
+	plan, err = ReadPlan(&planBuf)
+	if err != nil {
+		t.Fatalf("failed to read plan: %s", err)
+	}
+
+	ctx, err = plan.Context(&ContextOpts{
+		ProviderResolver: providerResolver,
+		Destroy:          true,
+	})
+	if err != nil {
+		t.Fatalf("failed to create context for plan: %s", err)
+	}
+
+	// Applying the plan should now succeed
+	_, err = ctx.Apply()
+	if err != nil {
+		t.Fatalf("apply failed: %s", err)
+	}
+}
+
 func TestContext2Apply_scaleInMultivarRef(t *testing.T) {
 	m := testModule(t, "apply-resource-scale-in")
 

terraform/interpolate.go

@@ -789,7 +789,8 @@ func (i *Interpolater) resourceCountMax(
 	// If we're NOT applying, then we assume we can read the count
 	// from the state. Plan and so on may not have any state yet so
 	// we do a full interpolation.
-	if i.Operation != walkApply {
+	// Don't forget walkDestroy, which is a special case of walkApply
+	if !(i.Operation == walkApply || i.Operation == walkDestroy) {
 		if cr == nil {
 			return 0, nil
 		}

terraform/test-fixtures/plan-destroy-interpolated-count/main.tf

@@ -0,0 +1,11 @@
+variable "list" {
+  default = ["1", "2"]
+}
+
+resource "aws_instance" "a" {
+  count = "${length(var.list)}"
+}
+
+output "out" {
+  value = "${aws_instance.a.*.id}"
+}