core: treat refs to unknown set resource attrs as unknown

References to computed list-ish attributes (set, list, map) were being
improperly resolved as an empty list `[]` during the plan phase (when
the value of the reference is not yet known) instead of as an
UnknownValue.

A "diffs didn't match" failure in an AWS DirectoryServices test led to
this discovery (and this commit fixes the failing test):

https://travis-ci.org/hashicorp/terraform/jobs/104812951

Refs hashicorp#2157 which has the original work to support computed list
attributes at all. This is just a simple tweak to that work.

/cc @radeksimko

terraform/interpolate.go

@@ -519,6 +519,16 @@ func (i *Interpolater) interpolateListAttribute(
 	log.Printf("[DEBUG] Interpolating computed list attribute %s (%s)",
 		resourceID, attr)
 
+	// In Terraform's internal dotted representation of list-like attributes, the
+	// ".#" count field is marked as unknown to indicate "this whole list is
+	// unknown". We must honor that meaning here so computed references can be
+	// treated properly during the plan phase.
+	if attr == config.UnknownVariableValue {
+		return attr, nil
+	}
+
+	// Otherwise we gather the values from the list-like attribute and return
+	// them.
 	var members []string
 	numberedListMember := regexp.MustCompile("^" + resourceID + "\\.[0-9]+$")
 	for id, value := range attributes {

terraform/interpolate_test.go

@@ -348,6 +348,44 @@ func TestInterpolator_resourceMultiAttributesWithResourceCount(t *testing.T) {
 	})
 }
 
+func TestInterpolator_resourceMultiAttributesComputed(t *testing.T) {
+	lock := new(sync.RWMutex)
+	// The state would never be written with an UnknownVariableValue in it, but
+	// it can/does exist that way in memory during the plan phase.
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_route53_zone.yada": &ResourceState{
+						Type: "aws_route53_zone",
+						Primary: &InstanceState{
+							ID: "z-abc123",
+							Attributes: map[string]string{
+								"name_servers.#": config.UnknownVariableValue,
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+	i := &Interpolater{
+		Module:    testModule(t, "interpolate-multi-vars"),
+		StateLock: lock,
+		State:     state,
+	}
+
+	scope := &InterpolationScope{
+		Path: rootModulePath,
+	}
+
+	testInterpolate(t, i, scope, "aws_route53_zone.yada.name_servers", ast.Variable{
+		Value: config.UnknownVariableValue,
+		Type:  ast.TypeString,
+	})
+}
+
 func getInterpolaterFixture(t *testing.T) *Interpolater {
 	lock := new(sync.RWMutex)
 	state := &State{