[TypeChecker] Allow closures to assume nonisolated(nonsending)

Always infer `nonisolated(nonsending)` from context directly on
a closure unless the closure is marked as `@concurrent`, otherwise
the closure is not going to get correct isolation and going to hop
to the wrong executor in its preamble.

Resolves: rdar://149107104

Author: xedin

lib/Sema/CSApply.cpp

@@ -6514,18 +6514,31 @@ ArgumentList *ExprRewriter::coerceCallArguments(
   return ArgumentList::createTypeChecked(ctx, args, newArgs);
 }
 
-/// Whether the given expression is a closure that should inherit
-/// the actor context from where it was formed.
-static bool closureInheritsActorContext(Expr *expr) {
+/// Looks through any non-semantic expressions and a capture list
+/// to find out whether the given expression is an explicit closure.
+static ClosureExpr *isExplicitClosureExpr(Expr *expr) {
   if (auto IE = dyn_cast<IdentityExpr>(expr))
-    return closureInheritsActorContext(IE->getSubExpr());
+    return isExplicitClosureExpr(IE->getSubExpr());
 
   if (auto CLE = dyn_cast<CaptureListExpr>(expr))
-    return closureInheritsActorContext(CLE->getClosureBody());
+    return isExplicitClosureExpr(CLE->getClosureBody());
+
+  return dyn_cast<ClosureExpr>(expr);
+}
 
-  if (auto CE = dyn_cast<ClosureExpr>(expr))
+/// Whether the given expression is a closure that should inherit
+/// the actor context from where it was formed.
+static bool closureInheritsActorContext(Expr *expr) {
+  if (auto *CE = isExplicitClosureExpr(expr))
     return CE->inheritsActorContext();
+  return false;
+}
 
+/// Determine whether the given expression is a closure that
+/// is explicitly marked as `@concurrent`.
+static bool isClosureMarkedAsConcurrent(Expr *expr) {
+  if (auto *CE = isExplicitClosureExpr(expr))
+    return CE->getAttrs().hasAttribute<ConcurrentAttr>();
   return false;
 }
 
@@ -7767,6 +7780,23 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       }
     }
 
+    // If we have a ClosureExpr, then we can safely propagate the
+    // 'nonisolated(nonsending)' isolation if it's not explicitly
+    // marked as `@concurrent`.
+    if (toEI.getIsolation().isNonIsolatedCaller() &&
+        (fromEI.getIsolation().isNonIsolated() &&
+         !isClosureMarkedAsConcurrent(expr))) {
+      auto newFromFuncType = fromFunc->withIsolation(
+          FunctionTypeIsolation::forNonIsolatedCaller());
+      if (applyTypeToClosureExpr(cs, expr, newFromFuncType)) {
+        fromFunc = newFromFuncType->castTo<FunctionType>();
+        // Propagating 'nonisolated(nonsending)' might have satisfied the entire
+        // conversion. If so, we're done, otherwise keep converting.
+        if (fromFunc->isEqual(toType))
+          return expr;
+      }
+    }
+
     if (ctx.LangOpts.isDynamicActorIsolationCheckingEnabled()) {
       // Passing a synchronous global actor-isolated function value and
       // parameter that expects a synchronous non-isolated function type could

lib/Sema/TypeCheckConcurrency.cpp

@@ -4696,6 +4696,13 @@ ActorIsolation ActorIsolationChecker::determineClosureIsolation(
       }
     }
 
+    // `nonisolated(nonsending)` inferred from the context makes
+    // the closure caller isolated.
+    if (auto *closureTy = getType(closure)->getAs<FunctionType>()) {
+      if (closureTy->getIsolation().isNonIsolatedCaller())
+        return ActorIsolation::forCallerIsolationInheriting();
+    }
+
     // If a closure has an isolated parameter, it is isolated to that
     // parameter.
     for (auto param : *closure->getParameters()) {

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -425,3 +425,24 @@ func conversionsFromSyncToAsync(_ x: @escaping @Sendable (NonSendableKlass) -> V
   let _: @concurrent (SendableKlass) async -> Void = y
   let _: @concurrent (NonSendableKlass) async -> Void = z
 }
+
+func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) async -> Void) {
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaYCcfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+  let _: nonisolated(nonsending) () async -> Void = {
+    42
+  }
+
+  func testParam(_: nonisolated(nonsending) () async throws -> Void) {}
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaYCXEfU0_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> @error any Error
+  testParam { 42 }
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaXEfU1_ : $@convention(thin) @async () -> ()
+  testParam { @concurrent in 42 }
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFySiYaYCcfU2_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, Int) -> ()
+  fn = { _ in }
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFySiYacfU3_ : $@convention(thin) @async (Int) -> ()
+  fn = { @concurrent _ in }
+}