Help compiler understand that co_yield co_error() in a Task coroutine never returns

Summary:
Mark the `await_resume()` method of the `final_suspend()` awaiter as `[[noreturn]]`.

This helps the compiler to dead-code eliminate this code, and in particular helps
the compiler to determine that code cannot continue execution after a
`co_yield co_error(ex)` expression (which calls `final_suspend()`).

This enables us to write code that has a `co_yield co_error()` statement as the
last line in a non-void `Task` coroutine without the compiler emitting a warning
about control running off the end of the coroutine.

Reviewed By: yfeldblum

Differential Revision: D22229020

fbshipit-source-id: b7a63b030cb42653198731d542ffa9bbf90daa83

folly/experimental/coro/Task.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <exception>
 #include <experimental/coroutine>
 #include <type_traits>
 
@@ -39,6 +40,7 @@
 #include <folly/experimental/coro/detail/Traits.h>
 #include <folly/futures/Future.h>
 #include <folly/io/async/Request.h>
+#include <folly/lang/Assume.h>
 
 namespace folly {
 namespace coro {
@@ -65,7 +67,9 @@ class TaskPromiseBase {
       return promise.continuation_;
     }
 
-    void await_resume() noexcept {}
+    [[noreturn]] void await_resume() noexcept {
+      folly::assume_unreachable();
+    }
   };
 
   friend class FinalAwaiter;

folly/experimental/coro/test/CoroTest.cpp

@@ -673,15 +673,17 @@ TEST(Coro, CoThrow) {
   EXPECT_THROW(
       folly::coro::blockingWait([]() -> folly::coro::Task<int> {
         co_yield folly::coro::co_error(ExpectedException());
-        EXPECT_TRUE(false) << "unreachable";
-        co_return 42;
+        ADD_FAILURE() << "unreachable";
+        // Intential lack of co_return statement to check
+        // that compiler treats code after co_yield co_error()
+        // as unreachable.
       }()),
       ExpectedException);
 
   EXPECT_THROW(
       folly::coro::blockingWait([]() -> folly::coro::Task<void> {
         co_yield folly::coro::co_error(ExpectedException());
-        EXPECT_TRUE(false) << "unreachable";
+        ADD_FAILURE() << "unreachable";
       }()),
       ExpectedException);
 }