Add cross-referencing support for Rust

Bug: 222044478
Test: run TARGET_BUILD_VARIANT=userdebug TARGET_PRODUCT=cf_x86_64_phone XREF_CORPUS=googleplex-android.googlesource.com/codesearch/android build/soong/build_kzip.bash
Change-Id: Ia12eed53fafd0cbbbf2cc499fa20a0f5a40031de
diff --git a/rust/testing.go b/rust/testing.go
index cb98bed..4796f69 100644
--- a/rust/testing.go
+++ b/rust/testing.go
@@ -194,6 +194,7 @@
 		ctx.BottomUp("rust_begin", BeginMutator).Parallel()
 	})
 	ctx.RegisterSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
+	ctx.RegisterSingletonType("kythe_rust_extract", kytheExtractRustFactory)
 	ctx.PostDepsMutators(func(ctx android.RegisterMutatorsContext) {
 		ctx.BottomUp("rust_sanitizers", rustSanitizerRuntimeMutator).Parallel()
 	})