[clang] [Clang] Propagate elide safe context through [[clang::coro_must_await]] (PR #108474)

[Yuxuan Chen via cfe-commits]

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@@ -8261,12 +8261,19 @@ def CoroAwaitElidableDoc : Documentation {
 The ``[[clang::coro_await_elidable]]`` is a class attribute which can be applied
 to a coroutine return type.
 
-When a coroutine function that returns such a type calls another coroutine function,
-the compiler performs heap allocation elision when the call to the coroutine function
-is immediately co_awaited as a prvalue. In this case, the coroutine frame for the
-callee will be a local variable within the enclosing braces in the caller's stack
-frame. And the local variable, like other variables in coroutines, may be collected
-into the coroutine frame, which may be allocated on the heap.
+When a coroutine function returns such a type, a direct call expression therein
+that returns a prvalue of a type attributed ``[[clang::coro_await_elidable]]``
+is said to be under a safe elide context if one of the following is true:
----------------
yuxuanchen1997 wrote:

> Is checking if the callee coroutine is annotated actually necessary?

Depends on the new spec which I haven't thought about yet but most likely I'll say it's still necessary for simplicity. We can make `await_transform` and `operator co_await` propagate the safe elide context. Just having the awaiter type attributed, and asserting that a prvalue of the awaiter returned from operator co_await must satisfy the requirements *may* do the work. I haven't thought it through. 

https://github.com/llvm/llvm-project/pull/108474