rust: Add missing Rust allow path modules.

Adds rust_benchmark and rust_fuzz to the list of Rust module types which
need to be in the allowed paths, and a comment about rust_bindgen and
rust_protobuf.

This also adds a downstream allow path list which helps prevent merge
conflicts if downstream wants to extend the list of allowed paths.

Bug: 191507775
Test: Paths are checked for the newly added module types.
Test: Paths in DownstreamRustAllowedPaths allow Rust modules.
Change-Id: Ida80c33a815d47ffdfb1f648125d71316a2a9d8a

rust/config/allowed_list.go

@@ -26,9 +26,18 @@ var (
 		"system/logging/rust",
 		"system/security",
 		"system/tools/aidl",
+		"tools/security/fuzzing/example_rust_fuzzer",
+	}
+
+	DownstreamRustAllowedPaths = []string{
+		// Add downstream allowed Rust paths here.
 	}
 
 	RustModuleTypes = []string{
+		// Don't add rust_bindgen or rust_protobuf as these are code generation modules
+		// and can be expected to be in paths without Rust code.
+		"rust_benchmark",
+		"rust_benchmark_host",
 		"rust_binary",
 		"rust_binary_host",
 		"rust_library",
@@ -37,6 +46,7 @@ var (
 		"rust_ffi",
 		"rust_ffi_shared",
 		"rust_ffi_static",
+		"rust_fuzz",
 		"rust_library_host",
 		"rust_library_host_dylib",
 		"rust_library_host_rlib",

rust/rust.go

@@ -35,7 +35,7 @@ func init() {
 
 	android.AddNeverAllowRules(
 		android.NeverAllow().
-			NotIn(config.RustAllowedPaths...).
+			NotIn(append(config.RustAllowedPaths, config.DownstreamRustAllowedPaths...)...).
 			ModuleType(config.RustModuleTypes...))
 
 	android.RegisterModuleType("rust_defaults", defaultsFactory)