globalref.Reference.ResourceAttr

Convert a global reference to a specific AbsResource and attribute pair.
The hcl.Traversal is converted to a cty.Path at this point because plan
rendering is based on cty values.

internal/lang/globalref/analyzer_contributing_resources_test.go

@@ -1,6 +1,7 @@
 package globalref
 
 import (
+	"sort"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -94,3 +95,96 @@ func TestAnalyzerContributingResources(t *testing.T) {
 		})
 	}
 }
+
+func TestAnalyzerContributingResourceAttrs(t *testing.T) {
+	azr := testAnalyzer(t, "contributing-resources")
+
+	tests := map[string]struct {
+		StartRefs func() []Reference
+		WantAttrs []string
+	}{
+		"root output 'network'": {
+			func() []Reference {
+				return azr.ReferencesFromOutputValue(
+					addrs.OutputValue{Name: "network"}.Absolute(addrs.RootModuleInstance),
+				)
+			},
+			[]string{
+				`data.test_thing.environment.any.base_cidr_block`,
+				`data.test_thing.environment.any.subnet_count`,
+				`module.network.test_thing.subnet`,
+				`module.network.test_thing.vpc.string`,
+			},
+		},
+		"root output 'c10s_url'": {
+			func() []Reference {
+				return azr.ReferencesFromOutputValue(
+					addrs.OutputValue{Name: "c10s_url"}.Absolute(addrs.RootModuleInstance),
+				)
+			},
+			[]string{
+				`data.test_thing.environment.any.base_cidr_block`,
+				`data.test_thing.environment.any.subnet_count`,
+				`module.compute.test_thing.load_balancer.string`,
+				`module.network.test_thing.subnet`,
+				`module.network.test_thing.vpc.string`,
+			},
+		},
+		"module.compute.test_thing.load_balancer": {
+			func() []Reference {
+				return azr.ReferencesFromResourceInstance(
+					addrs.Resource{
+						Mode: addrs.ManagedResourceMode,
+						Type: "test_thing",
+						Name: "load_balancer",
+					}.Instance(addrs.NoKey).Absolute(addrs.RootModuleInstance.Child("compute", addrs.NoKey)),
+				)
+			},
+			[]string{
+				`data.test_thing.environment.any.base_cidr_block`,
+				`data.test_thing.environment.any.subnet_count`,
+				`module.compute.test_thing.controller`,
+				`module.network.test_thing.subnet`,
+				`module.network.test_thing.vpc.string`,
+			},
+		},
+		"data.test_thing.environment": {
+			func() []Reference {
+				return azr.ReferencesFromResourceInstance(
+					addrs.Resource{
+						Mode: addrs.DataResourceMode,
+						Type: "test_thing",
+						Name: "environment",
+					}.Instance(addrs.NoKey).Absolute(addrs.RootModuleInstance),
+				)
+			},
+			[]string{
+				// Nothing! This one only refers to an input variable.
+			},
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			startRefs := test.StartRefs()
+			refs := azr.ContributingResourceReferences(startRefs...)
+
+			want := test.WantAttrs
+			got := make([]string, len(refs))
+			for i, ref := range refs {
+				resAttr, ok := ref.ResourceAttr()
+				if !ok {
+					t.Errorf("%s is not a resource attr reference", resAttr.DebugString())
+					continue
+				}
+				got[i] = resAttr.DebugString()
+			}
+
+			sort.Strings(got)
+
+			if diff := cmp.Diff(want, got); diff != "" {
+				t.Errorf("wrong addresses\n%s", diff)
+			}
+		})
+	}
+}

internal/lang/globalref/reference.go

@@ -3,7 +3,10 @@ package globalref
 import (
 	"fmt"
 
+	"github.com/hashicorp/hcl/v2"
 	"github.com/hashicorp/terraform/internal/addrs"
+	"github.com/hashicorp/terraform/internal/tfdiags"
+	"github.com/zclconf/go-cty/cty"
 )
 
 // Reference combines an addrs.Reference with the address of the module
@@ -125,6 +128,45 @@ func (r Reference) DebugString() string {
 	return r.ContainerAddr.String() + "::" + r.LocalRef.DisplayString()
 }
 
+// ResourceAttr converts the Reference value to a more specific ResourceAttr
+// value.
+//
+// Because not all references belong to resources, the extra boolean return
+// value indicates whether the returned address is valid.
+func (r Reference) ResourceAttr() (ResourceAttr, bool) {
+	res, ok := r.ResourceAddr()
+	if !ok {
+		return ResourceAttr{}, ok
+	}
+
+	traversal := r.LocalRef.Remaining
+
+	path := make(cty.Path, len(traversal))
+	for si, step := range traversal {
+		switch ts := step.(type) {
+		case hcl.TraverseRoot:
+			path[si] = cty.GetAttrStep{
+				Name: ts.Name,
+			}
+		case hcl.TraverseAttr:
+			path[si] = cty.GetAttrStep{
+				Name: ts.Name,
+			}
+		case hcl.TraverseIndex:
+			path[si] = cty.IndexStep{
+				Key: ts.Key,
+			}
+		default:
+			panic(fmt.Sprintf("unsupported traversal step %#v", step))
+		}
+	}
+
+	return ResourceAttr{
+		Resource: res,
+		Attr:     path,
+	}, true
+}
+
 // addrKey returns the referenceAddrKey value for the item that
 // this reference refers to, discarding any source location information.
 //
@@ -146,3 +188,15 @@ func (r Reference) addrKey() referenceAddrKey {
 // make it easier to see when we're intentionally using strings to uniquely
 // identify absolute reference addresses.
 type referenceAddrKey string
+
+// ResourceAttr represents a global resource and attribute reference.
+// This is a more specific form of the Reference type since it can only refer
+// to a specific AbsResource and one of its attributes.
+type ResourceAttr struct {
+	Resource addrs.AbsResource
+	Attr     cty.Path
+}
+
+func (r ResourceAttr) DebugString() string {
+	return r.Resource.String() + tfdiags.FormatCtyPath(r.Attr)
+}