[clang] [llvm] [Clang] C++20 Coroutines: Introduce Frontend Attribute [[clang::coro_await_elidable]] (PR #99282)

[Yuxuan Chen via cfe-commits]

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@@ -8119,6 +8119,35 @@ but do not pass them to the underlying coroutine or pass them by value.
 }];
 }
 
+def CoroAwaitElidableDoc : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+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.
----------------
yuxuanchen1997 wrote:

> Then the question is, if the call is immediately co_awaited, in what cases it won't be a prvalue.

There probably aren't any. But I was still not confident of what "immediately" means in general. 

The term "immediate subexpression" is defined to be a different meaning in the standard: https://eel.is/c++draft/intro.execution#3

But I don't have a strong opinion of this. Probably not ambiguous in this context. This isn't the first time we use an overloaded term and won't be the last either, but I still like interfacing with developer with the term `prvalue` better. In my experience it's better understood. 

https://github.com/llvm/llvm-project/pull/99282