Allow stubs implementation to be omitted

When defining a stubs library, allow specifying that the
implementation library does not need to be installed. This allows for
cases where the implementation is deployed in some non-standard way -
e.g. inside a Microdroid virtual machine.

Without this, we get build errors like: "TARGET module
com.android.compos requires non-existent TARGET module: libvm_payload".

Default behavior is unchanged. The change is protected by an allowlist
to limit usage to the immediate use case.

Bug: 243512108
Test: builds; soong tests pass
Test: Remove allowlist, see build failure
Change-Id: Iaae75f2e93b842f5944a7518cc95069d62c5a638
diff --git a/android/neverallow.go b/android/neverallow.go
index 293bac8..ad9880a 100644
--- a/android/neverallow.go
+++ b/android/neverallow.go
@@ -59,6 +59,7 @@
 	AddNeverAllowRules(createInitFirstStageRules()...)
 	AddNeverAllowRules(createProhibitFrameworkAccessRules()...)
 	AddNeverAllowRules(createBp2BuildRule())
+	AddNeverAllowRules(createCcStubsRule())
 }
 
 // Add a NeverAllow rule to the set of rules to apply.
@@ -214,6 +215,17 @@
 	}
 }
 
+func createCcStubsRule() Rule {
+	ccStubsImplementationInstallableProjectsAllowedList := []string{
+		"packages/modules/Virtualization/vm_payload",
+	}
+
+	return NeverAllow().
+		NotIn(ccStubsImplementationInstallableProjectsAllowedList...).
+		WithMatcher("stubs.implementation_installable", isSetMatcherInstance).
+		Because("implementation_installable can only be used in allowed projects.")
+}
+
 func createUncompressDexRules() []Rule {
 	return []Rule{
 		NeverAllow().
diff --git a/android/neverallow_test.go b/android/neverallow_test.go
index 4772799..5f5f9a1 100644
--- a/android/neverallow_test.go
+++ b/android/neverallow_test.go
@@ -367,6 +367,22 @@
 			"framework can't be used when building against SDK",
 		},
 	},
+	// Test for the rule restricting use of implementation_installable
+	{
+		name: `"implementation_installable" outside allowed list`,
+		fs: map[string][]byte{
+			"Android.bp": []byte(`
+				cc_library {
+					name: "outside_allowed_list",
+					stubs: {
+                                                implementation_installable: true,
+					},
+				}`),
+		},
+		expectedErrors: []string{
+			`module "outside_allowed_list": violates neverallow`,
+		},
+	},
 }
 
 var prepareForNeverAllowTest = GroupFixturePreparers(
@@ -419,6 +435,10 @@
 	Platform struct {
 		Shared_libs []string
 	}
+
+	Stubs struct {
+		Implementation_installable *bool
+	}
 }
 
 type mockCcLibraryModule struct {