Parallelize singleton execution

Bug: 281536768
Test: manual, presubmits
Change-Id: I57fdc76ba6b277e88e196b506af87127a530fd37
diff --git a/rust/testing.go b/rust/testing.go
index 0a6a870..7f30569 100644
--- a/rust/testing.go
+++ b/rust/testing.go
@@ -200,8 +200,8 @@
 		ctx.BottomUp("rust_stdlinkage", LibstdMutator).Parallel()
 		ctx.BottomUp("rust_begin", BeginMutator).Parallel()
 	})
-	ctx.RegisterSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
-	ctx.RegisterSingletonType("kythe_rust_extract", kytheExtractRustFactory)
+	ctx.RegisterParallelSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
+	ctx.RegisterParallelSingletonType("kythe_rust_extract", kytheExtractRustFactory)
 	ctx.PostDepsMutators(func(ctx android.RegisterMutatorsContext) {
 		ctx.BottomUp("rust_sanitizers", rustSanitizerRuntimeMutator).Parallel()
 	})