[Concurrency] Ban multiple isolated params, as per SE-0313

Author: ktoso

include/swift/AST/DiagnosticsSema.def

@@ -4832,6 +4832,9 @@ NOTE(protocol_isolated_to_global_actor_here,none,
 
 ERROR(isolated_parameter_not_actor,none,
       "'isolated' parameter has non-actor type %0", (Type))
+ERROR(isolated_parameter_second_isolated_param,none,
+      "function cannot have more than one 'isolated' parameter (%0 and %1)",
+      (DeclName, DeclName))
 
 WARNING(non_sendable_param_type,none,
         "non-sendable type %0 %select{passed in call to %4 %2 %3|"

lib/Sema/TypeCheckConcurrency.cpp

@@ -3086,6 +3086,30 @@ void swift::checkTopLevelActorIsolation(TopLevelCodeDecl *decl) {
     body->walk(checker);
 }
 
+static bool checkDuplicateIsolatedParameter(AbstractFunctionDecl *decl) {
+  // If we have already seen at least one 'isolated' parameter,
+  // and we see another one, this is invalid as only one 'isolated' param
+  // per function is allowed.
+  // See:
+  // https://github.com/apple/swift-evolution/blob/main/proposals/0313-actor-isolation-control.md#multiple-isolated-parameters
+  ParamDecl* previousIsolatedParam = nullptr;
+  for (size_t i = 0; i < decl->getParameters()->size(); ++i) {
+    auto param = decl->getParameters()->get(i);
+    if (param->isIsolated()) {
+      if (auto previous = previousIsolatedParam) {
+        decl->diagnose(diag::isolated_parameter_second_isolated_param,
+                       previous->getParameterName(),
+                       param->getParameterName());
+        return true;
+      }
+
+      previousIsolatedParam = const_cast<ParamDecl*>(param);
+    }
+  }
+
+  return false;
+}
+
 void swift::checkFunctionActorIsolation(AbstractFunctionDecl *decl) {
   // Disable this check for @LLDBDebuggerFunction functions.
   if (decl->getAttrs().hasAttribute<LLDBDebuggerFunctionAttr>())
@@ -3099,6 +3123,8 @@ void swift::checkFunctionActorIsolation(AbstractFunctionDecl *decl) {
     if (auto superInit = ctor->getSuperInitCall())
       superInit->walk(checker);
   }
+
+  checkDuplicateIsolatedParameter(decl);
   if (decl->getAttrs().hasAttribute<DistributedActorAttr>()) {
     if (auto func = dyn_cast<FuncDecl>(decl)) {
       checkDistributedFunction(func);

test/Concurrency/isolated_parameters.swift

@@ -12,7 +12,7 @@ extension Actor {
 }
 
 @available(SwiftStdlib 5.1, *)
-func testA<T: Actor>(
+func testA<T: Actor>( // expected-error{{function cannot have more than one 'isolated' parameter ('a' and 'b')}}
   a: isolated A,
   b: isolated T,
   c: isolated Int // expected-error{{'isolated' parameter has non-actor type 'Int'}}
@@ -22,6 +22,29 @@ func testA<T: Actor>(
   b.g()
 }
 
+actor Other {
+  func inc() {}
+  func pass(_ f: @Sendable  (Self) async -> Void) {}
+}
+actor Counter {
+  func inc() {}
+  func pass(_ f: @Sendable  (Self) async -> Void) {}
+}
+
+@available(SwiftStdlib 5.1, *)
+func testDoubleIsolatedParams(a: isolated Counter, b: isolated Other) async { // expected-error{{function cannot have more than one 'isolated' parameter ('a' and 'b')}}
+  a.inc()
+  b.inc()
+}
+
+func taaaa() async {
+  await Counter().pass { isoC in
+    await Other().pass { isoO in
+      await testDoubleIsolatedParams(a: isoC, b: isoO)
+    }
+  }
+}
+
 @available(SwiftStdlib 5.1, *)
 typealias Fn = (isolated A) -> Void
 
@@ -107,7 +130,7 @@ func redecl(_: isolated TestActor) { } // expected-error{{invalid redeclaration
 func tuplify<Ts>(_ fn: (Ts) -> Void) {} // expected-note {{in call to function 'tuplify'}}
 
 @available(SwiftStdlib 5.1, *)
-func testTuplingIsolated(_ a: isolated A, _ b: isolated A) {
+func testTuplingIsolated(_ a: isolated A, _ b: isolated A) { // expected-error{{function cannot have more than one 'isolated' parameter ('a' and 'b')}}
   tuplify(testTuplingIsolated)
   // expected-error@-1 {{generic parameter 'Ts' could not be inferred}}
   // expected-error@-2 {{cannot convert value of type '(isolated A, isolated A) -> ()' to expected argument type '(Ts) -> Void'}}