[Concurrency] Downgrade missing await errors to warnings when referencing @preconcurrency declarations.

lib/Sema/TypeCheckEffects.cpp

@@ -365,6 +365,22 @@ class AbstractFunction {
     return decomposeFunction(fn);
   }
 
+  bool isPreconcurrency() const {
+    switch (getKind()) {
+    case Kind::Closure: {
+      auto *closure = dyn_cast<ClosureExpr>(getClosure());
+      return closure && closure->isIsolatedByPreconcurrency();
+    }
+
+    case Kind::Function:
+      return getActorIsolation(getFunction()).preconcurrency();
+
+    case Kind::Opaque:
+    case Kind::Parameter:
+      return false;
+    }
+  }
+
   static AbstractFunction decomposeFunction(Expr *fn) {
     assert(fn->getValueProvidingExpr() == fn);
 
@@ -1054,8 +1070,11 @@ class ApplyClassifier {
   }
 
   /// Whether a missing 'await' error on accessing an async var should be
-  /// downgraded to a warning. This is only the case for synchronous access
-  /// to isolated global or static 'let' variables.
+  /// downgraded to a warning.
+  ///
+  /// Missing 'await' errors are downgraded for synchronous access to isolated
+  /// global or static 'let' variables, which was previously accepted in
+  /// compiler versions before 5.10, or for declarations marked preconcurrency.
   bool downgradeAsyncAccessToWarning(Decl *decl) {
     if (auto *var = dyn_cast<VarDecl>(decl)) {
       ActorReferenceResult::Options options = llvm::None;
@@ -1067,7 +1086,7 @@ class ApplyClassifier {
       }
     }
 
-    return false;
+    return decl->preconcurrency();
   }
 
   Classification classifyLookup(LookupExpr *E) {
@@ -1192,6 +1211,10 @@ class ApplyClassifier {
         ctx, E->isImplicitlyAsync().has_value(), E->implicitlyThrows(),
         PotentialEffectReason::forApply());
 
+    // Downgrade missing 'await' errors for preconcurrency references.
+    result.setDowngradeToWarning(
+        result.hasAsync() && fnRef.isPreconcurrency());
+
     auto classifyApplyEffect = [&](EffectKind kind) {
       if (!fnType->hasEffect(kind) &&
           !(kind == EffectKind::Async && E->isImplicitlyAsync()) &&

test/Concurrency/global_actor_from_ordinary_context.swift

@@ -154,7 +154,7 @@ class Sub: Super {
 
   func g2() {
     Task.detached {
-      self.f() // expected-error{{expression is 'async' but is not marked with 'await'}}
+      self.f() // expected-warning{{expression is 'async' but is not marked with 'await'}}
       // expected-note@-1{{calls to instance method 'f()' from outside of its actor context are implicitly asynchronous}}
     }
   }

test/Concurrency/predates_concurrency.swift

@@ -108,14 +108,14 @@ func testCalls(x: X) {
 }
 
 func testCallsWithAsync() async {
-  onMainActorAlways() // expected-error{{expression is 'async' but is not marked with 'await'}}
+  onMainActorAlways() // expected-warning{{expression is 'async' but is not marked with 'await'}}
   // expected-note@-1{{calls to global function 'onMainActorAlways()' from outside of its actor context are implicitly asynchronous}}
 
   let _: () -> Void = onMainActorAlways // expected-warning {{converting function value of type '@MainActor () -> ()' to '() -> Void' loses global actor 'MainActor'}}
 
-  let c = MyModelClass() // expected-error{{expression is 'async' but is not marked with 'await'}}
+  let c = MyModelClass() // expected-warning{{expression is 'async' but is not marked with 'await'}}
   // expected-note@-1{{calls to initializer 'init()' from outside of its actor context are implicitly asynchronous}}
-  c.f() // expected-error{{expression is 'async' but is not marked with 'await'}}
+  c.f() // expected-warning{{expression is 'async' but is not marked with 'await'}}
   // expected-note@-1{{calls to instance method 'f()' from outside of its actor context are implicitly asynchronous}}
 }