Top-level variables are on the MainActor

This is an interesting patch that may raise some eyebrows. The machinery
we have now can completely handle arbitrary global-actor isolation on
the global variables. I'm forcing the variables to be on the main actor
because in an ideal future, the variables won't be global and will
actually be local to the implicit `async_Main` function, where they will
be isolated to the main actor by virtue of the `async_Main` function
being run by the main actor executor. For that to work, we can't really
allow top-level variables to have global actor isolation since local
variables can't have global actor isolation. So while there is no
technical reason we can't handle it now, to avoid source-breaking
changes in the future, I'm removing the ability to explicitly declare a
global actor on top-level global variables and forcing them all to be on
the MainActor implicitly.

Author: etcwilde

include/swift/AST/DiagnosticsSema.def

@@ -4771,7 +4771,9 @@ ERROR(global_actor_non_unsafe_init,none,
       "global actor attribute %0 argument can only be '(unsafe)'", (Type))
 ERROR(global_actor_non_final_class,none,
       "non-final class %0 cannot be a global actor", (DeclName))
-      
+ERROR(global_actor_top_level_var,none,
+      "top-level code variables cannot have a global actor", ())
+
 ERROR(actor_isolation_multiple_attr,none,
       "%0 %1 has multiple actor-isolation attributes ('%2' and '%3')",
       (DescriptiveDeclKind, DeclName, StringRef, StringRef))

lib/Sema/TypeCheckConcurrency.cpp

@@ -271,7 +271,12 @@ GlobalActorAttributeRequest::evaluate(
   } else if (auto storage = dyn_cast<AbstractStorageDecl>(decl)) {
     // Subscripts and properties are fine...
     if (auto var = dyn_cast<VarDecl>(storage)) {
-      if (var->getDeclContext()->isLocalContext() && !var->isTopLevelGlobal()) {
+      if (var->isTopLevelGlobal() &&
+          var->getASTContext().LangOpts.EnableExperimentalAsyncTopLevel) {
+        var->diagnose(diag::global_actor_top_level_var)
+            .highlight(globalActorAttr->getRangeWithAt());
+        return None;
+      } else if (var->getDeclContext()->isLocalContext()) {
         var->diagnose(diag::global_actor_on_local_variable, var->getName())
             .highlight(globalActorAttr->getRangeWithAt());
         return None;
@@ -3505,6 +3510,14 @@ ActorIsolation ActorIsolationRequest::evaluate(
   }
 
   if (auto var = dyn_cast<VarDecl>(value)) {
+    ASTContext &ctx = var->getASTContext();
+    if (var->isTopLevelGlobal() &&
+        ctx.LangOpts.EnableExperimentalAsyncTopLevel) {
+      if (Type mainActor = ctx.getMainActorType())
+        return inferredIsolation(
+            ActorIsolation::forGlobalActor(mainActor,
+                                           /*unsafe=*/false));
+    }
     if (auto isolation = getActorIsolationFromWrappedProperty(var))
       return inferredIsolation(isolation);
   }