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

[Yuxuan Chen via cfe-commits]

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@@ -880,14 +898,12 @@ ExprResult Sema::BuildUnresolvedCoawaitExpr(SourceLocation Loc, Expr *Operand,
   }
 
   auto *RD = Promise->getType()->getAsCXXRecordDecl();
-  bool AwaitElidable =
-      isCoroAwaitElidableCall(Operand) &&
-      isAttributedCoroAwaitElidable(
-          getCurFunctionDecl(/*AllowLambda=*/true)->getReturnType());
-
-  if (AwaitElidable)
-    if (auto *Call = dyn_cast<CallExpr>(Operand->IgnoreImplicit()))
-      Call->setCoroElideSafe();
+
+  bool CurFnAwaitElidable = isAttributedCoroAwaitElidable(
+      getCurFunctionDecl(/*AllowLambda=*/true)->getReturnType());
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yuxuanchen1997 wrote:

This is not the piece of code that performs this check. See `applySafeElideContext`.

But what you said is potentially a good point, we actually skipped checking two things: `operator co_await` on the callee return object, and the `await_transform`. As these operations may not be safe. The original intention was to make this a user's liability to ensure early caller destruction cannot happen in both customization points. It's not as granular as I'd like. 

With this patch, we are actually making it possible. `await_transform` and `operator co_await` here are just function calls whose arguments can be attributed with `[[clang::coro_must_await]]`. 

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