Rust: Reimplement type inference for impl Traits and await expressions

rust/tools/builtins/await.rs

@@ -0,0 +1,7 @@
+use std::future::Future;
+
+fn await_type_matching<T1, T2: Future<Output = T1>>(x: T2) -> T1 {
+    panic!(
+        "This function exists only in order to implement type inference for `.await` expressions."
+    );
+}