Merge pull request #36201 from etcwilde/ewilde/fix-crash-in-async-main

[Concurrency] Fix async-main crash

Author: etcwilde

include/swift/AST/DiagnosticsSema.def

@@ -4389,6 +4389,9 @@ ERROR(actorindependent_local_var,none,
 ERROR(concurrency_lib_missing,none,
       "missing '%0' declaration, probably because the '_Concurrency' "
       "module was not imported", (StringRef))
+ERROR(async_main_no_concurrency,none,
+      "'_Concurrency' module not imported, required for async main", ())
+
 ERROR(enqueue_partial_task_not_in_context,none,
       "'enqueue(partialTask:)' can only be implemented in the definition of "
       "actor class %0", (Type))

lib/Sema/TypeCheckAttr.cpp

@@ -1835,7 +1835,17 @@ synthesizeMainBody(AbstractFunctionDecl *fn, void *arg) {
     // Resulting $main looks like:
     // $main() { _runAsyncMain(main) }
     auto *concurrencyModule = context.getLoadedModule(context.Id_Concurrency);
-    assert(concurrencyModule != nullptr && "Failed to find Concurrency module");
+    if (!concurrencyModule) {
+      context.Diags.diagnose(mainFunction->getAsyncLoc(),
+                             diag::async_main_no_concurrency);
+      auto result = new (context) ErrorExpr(mainFunction->getSourceRange());
+      SmallVector<ASTNode, 1> stmts;
+      stmts.push_back(result);
+      auto body = BraceStmt::create(context, SourceLoc(), stmts,
+                                    SourceLoc(), /*Implicit*/true);
+
+      return std::make_pair(body, /*typechecked*/true);
+    }
 
     SmallVector<ValueDecl *, 1> decls;
     concurrencyModule->lookupQualified(

test/Concurrency/async_main_no_concurrency.swift

@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -parse-as-library %s
+// RUN: %target-typecheck-verify-swift -parse-as-library -enable-experimental-concurrency -parse-stdlib %s
+
+@main struct Main {
+  // expected-error@+1:22{{'_Concurrency' module not imported, required for async main}}
+  static func main() async {
+  }
+}