addrs: All Referenceable implementations support String()

This gives us a convenient way to convert back to a string representation
that matches what the user would've written in configuration.

addrs/count_attr.go

@@ -6,3 +6,7 @@ type CountAttr struct {
 	referenceable
 	Name string
 }
+
+func (ca CountAttr) String() string {
+	return "count." + ca.Name
+}

addrs/input_variable.go

@@ -5,3 +5,7 @@ type InputVariable struct {
 	referenceable
 	Name string
 }
+
+func (v InputVariable) String() string {
+	return "var." + v.Name
+}

addrs/instance_key.go

@@ -17,6 +17,7 @@ import (
 // InstanceKey.
 type InstanceKey interface {
 	instanceKeySigil()
+	String() string
 }
 
 // ParseInstanceKey returns the instance key corresponding to the given value,
@@ -51,9 +52,19 @@ type IntKey int
 func (k IntKey) instanceKeySigil() {
 }
 
+func (k IntKey) String() string {
+	return fmt.Sprintf("[%d]", int(k))
+}
+
 // StringKey is the InstanceKey representation representing string indices, as
 // used when the "for_each" argument is specified with a map or object type.
 type StringKey string
 
 func (k StringKey) instanceKeySigil() {
 }
+
+func (k StringKey) String() string {
+	// FIXME: This isn't _quite_ right because Go's quoted string syntax is
+	// slightly different than HCL's, but we'll accept it for now.
+	return fmt.Sprintf("[%q]", string(k))
+}

addrs/local_value.go

@@ -5,3 +5,7 @@ type LocalValue struct {
 	referenceable
 	Name string
 }
+
+func (v LocalValue) String() string {
+	return "local." + v.Name
+}

addrs/module_call.go

@@ -1,5 +1,9 @@
 package addrs
 
+import (
+	"fmt"
+)
+
 // ModuleCall is the address of a call from the current module to a child
 // module.
 //
@@ -10,6 +14,10 @@ type ModuleCall struct {
 	Name string
 }
 
+func (c ModuleCall) String() string {
+	return "module." + c.Name
+}
+
 // Instance returns the address of an instance of the receiver identified by
 // the given key.
 func (c ModuleCall) Instance(key InstanceKey) ModuleCallInstance {
@@ -28,6 +36,13 @@ type ModuleCallInstance struct {
 	Key  InstanceKey
 }
 
+func (c ModuleCallInstance) String() string {
+	if c.Key == NoKey {
+		return c.Call.String()
+	}
+	return fmt.Sprintf("module.%s%s", c.Call.Name, c.Key)
+}
+
 // Output returns the address of an output of the receiver identified by its
 // name.
 func (c ModuleCallInstance) Output(name string) ModuleCallOutput {
@@ -44,3 +59,7 @@ type ModuleCallOutput struct {
 	Call ModuleCallInstance
 	Name string
 }
+
+func (co ModuleCallOutput) String() string {
+	return fmt.Sprintf("%s.%s", co.Call.String(), co.Name)
+}

addrs/path_attr.go

@@ -6,3 +6,7 @@ type PathAttr struct {
 	referenceable
 	Name string
 }
+
+func (pa PathAttr) String() string {
+	return "path." + pa.Name
+}

addrs/referenceable.go

@@ -3,7 +3,14 @@ package addrs
 // Referenceable is an interface implemented by all address types that can
 // appear as references in configuration language expressions.
 type Referenceable interface {
+	// All implementations of this interface must be covered by the type switch
+	// in lang.Scope.buildEvalContext.
 	referenceableSigil()
+
+	// String produces a string representation of the address that could be
+	// parsed as a HCL traversal and passed to ParseRef to produce an identical
+	// result.
+	String() string
 }
 
 type referenceable struct {

addrs/resource.go

@@ -1,5 +1,9 @@
 package addrs
 
+import (
+	"fmt"
+)
+
 // Resource is an address for a resource block within configuration, which
 // contains potentially-multiple resource instances if that configuration
 // block uses "count" or "for_each".
@@ -10,6 +14,17 @@ type Resource struct {
 	Name string
 }
 
+func (r Resource) String() string {
+	switch r.Mode {
+	case ManagedResourceMode:
+		return fmt.Sprintf("%s.%s", r.Type, r.Name)
+	case DataResourceMode:
+		return fmt.Sprintf("data.%s.%s", r.Type, r.Name)
+	default:
+		panic(fmt.Errorf("resource address with invalid mode %s", r.Mode))
+	}
+}
+
 // Instance produces the address for a specific instance of the receiver
 // that is idenfied by the given key.
 func (r Resource) Instance(key InstanceKey) ResourceInstance {
@@ -37,6 +52,13 @@ type ResourceInstance struct {
 	Key      InstanceKey
 }
 
+func (r ResourceInstance) String() string {
+	if r.Key == NoKey {
+		return r.Resource.String()
+	}
+	return r.Resource.String() + r.Key.String()
+}
+
 // Absolute returns an AbsResourceInstance from the receiver and the given module
 // instance address.
 func (r ResourceInstance) Absolute(module ModuleInstance) AbsResourceInstance {

addrs/self.go

@@ -8,3 +8,7 @@ type selfT int
 
 func (s selfT) referenceableSigil() {
 }
+
+func (s selfT) String() string {
+	return "self"
+}

addrs/terraform_attr.go

@@ -6,3 +6,7 @@ type TerraformAttr struct {
 	referenceable
 	Name string
 }
+
+func (ta TerraformAttr) String() string {
+	return "terraform." + ta.Name
+}