addrs: ModuleInstance and AbsResourceInstance are UniqueKeyers

Since these address types are not directly comparable themselves, we use
an unexported named type around the string representation, whereby the
special type can avoid any ambiguity between string representations of
different types and thus each type only needs to worry about possible
ambiguity of its _own_ string representation.

internal/addrs/module_instance.go

@@ -275,6 +275,14 @@ func (m ModuleInstance) String() string {
 	return buf.String()
 }
 
+type moduleInstanceKey string
+
+func (m ModuleInstance) UniqueKey() UniqueKey {
+	return moduleInstanceKey(m.String())
+}
+
+func (mk moduleInstanceKey) uniqueKeySigil() {}
+
 // Equal returns true if the receiver and the given other value
 // contains the exact same parts.
 func (m ModuleInstance) Equal(o ModuleInstance) bool {

internal/addrs/resource.go

@@ -292,6 +292,14 @@ func (r AbsResourceInstance) Less(o AbsResourceInstance) bool {
 	}
 }
 
+type absResourceInstanceKey string
+
+func (r AbsResourceInstance) UniqueKey() UniqueKey {
+	return absResourceInstanceKey(r.String())
+}
+
+func (r absResourceInstanceKey) uniqueKeySigil() {}
+
 func (r AbsResourceInstance) absMoveableSigil() {
 	// AbsResourceInstance is moveable
 }

internal/addrs/unique_key_test.go

@@ -46,6 +46,14 @@ func TestUniqueKeyer(t *testing.T) {
 			},
 			Key: IntKey(1),
 		},
+		RootModuleInstance,
+		RootModuleInstance.Child("foo", NoKey),
+		RootModuleInstance.ResourceInstance(
+			DataResourceMode,
+			"boop",
+			"beep",
+			NoKey,
+		),
 		Self,
 	}