add PruneUnusedValuesTransformer

Since outputs and local nodes are always evaluated, if the reference a
resource form the configuration that isn't in the state, the
interpolation could fail.

Prune any local or output values that have no references in the graph.

terraform/context_apply_test.go

@@ -7641,6 +7641,8 @@ func TestContext2Apply_destroyProvisionerWithLocals(t *testing.T) {
 	}
 }
 
+// this also tests a local value in the config referencing a resource that
+// wasn't in the state during destroy.
 func TestContext2Apply_destroyProvisionerWithMultipleLocals(t *testing.T) {
 	m := testModule(t, "apply-provisioner-destroy-multiple-locals")
 	p := testProvider("aws")

terraform/graph_builder_apply.go

@@ -119,16 +119,19 @@ func (b *ApplyGraphBuilder) Steps() []GraphTransformer {
 		// Connect references so ordering is correct
 		&ReferenceTransformer{},
 
+		// Handle destroy time transformations for output and local values.
 		// Reverse the edges from outputs and locals, so that
 		// interpolations don't fail during destroy.
+		// Create a destroy node for outputs to remove them from the state.
+		// Prune unreferenced values, which may have interpolations that can't
+		// be resolved.
 		GraphTransformIf(
 			func() bool { return b.Destroy },
-			&DestroyValueReferenceTransformer{},
-		),
-
-		GraphTransformIf(
-			func() bool { return b.Destroy },
-			&DestroyOutputTransformer{},
+			GraphTransformMulti(
+				&DestroyValueReferenceTransformer{},
+				&DestroyOutputTransformer{},
+				&PruneUnusedValuesTransformer{},
+			),
 		),
 
 		// Add the node to fix the state count boundaries

terraform/transform_reference.go

@@ -112,6 +112,30 @@ func (t *DestroyValueReferenceTransformer) Transform(g *Graph) error {
 	return nil
 }
 
+// PruneUnusedValuesTransformer is s GraphTransformer that removes local and
+// output values which are not referenced in the graph. Since outputs and
+// locals always need to be evaluated, if they reference a resource that is not
+// available in the state the interpolation could fail.
+type PruneUnusedValuesTransformer struct{}
+
+func (t *PruneUnusedValuesTransformer) Transform(g *Graph) error {
+	vs := g.Vertices()
+	for _, v := range vs {
+		switch v.(type) {
+		case *NodeApplyableOutput, *NodeLocal:
+			// OK
+		default:
+			continue
+		}
+
+		if len(g.EdgesTo(v)) == 0 {
+			g.Remove(v)
+		}
+	}
+
+	return nil
+}
+
 // ReferenceMap is a structure that can be used to efficiently check
 // for references on a graph.
 type ReferenceMap struct {