Deprecate Application-Specific Main Entrypoint Attributes

Issue a deprecation warning in Swift 5 and an error in Swift 6 when we encounter @UIApplicationMain and @NSApplicationMain. These attributes are unnecessary now that @main works with UIApplicationDelegate and NSApplicationDelegate. As these conformances are required when working with the corresponding attributes, we can migrate users off of them by replacing them with @main.

Author: CodaFi

include/swift/AST/DiagnosticsSema.def

@@ -3399,6 +3399,9 @@ ERROR(attr_ApplicationMain_with_script,none,
       "%" SELECT_APPLICATION_MAIN "0 attribute cannot be used in a module that contains "
       "top-level code",
       (unsigned))
+ERROR(attr_ApplicationMain_deprecated,none,
+      "%" SELECT_APPLICATION_MAIN "0 is deprecated and will be removed in a future release; use '@main' instead",
+      (unsigned))
 
 NOTE(attr_ApplicationMain_parse_as_library,none,
      "pass '-parse-as-library' to compiler invocation if this is intentional",

lib/Sema/TypeCheckAttr.cpp

@@ -2249,6 +2249,13 @@ void AttributeChecker::checkApplicationMainAttribute(DeclAttribute *attr,
     attr->setInvalid();
   }
 
+  diagnose(attr->getLocation(),
+           diag::attr_ApplicationMain_deprecated,
+           applicationMainKind)
+    .warnUntilSwiftVersion(6)
+    .fixItReplace(attr->getRange(), "@main");
+
+
   if (attr->isInvalid())
     return;
 

test/attr/ApplicationMain/attr_NSApplicationMain.swift

@@ -4,6 +4,6 @@
 
 import AppKit
 
-@NSApplicationMain
+@NSApplicationMain // expected-warning {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate: NSObject, NSApplicationDelegate {
 }

test/attr/ApplicationMain/attr_NSApplicationMain_inherited.swift

@@ -6,6 +6,6 @@ import AppKit
 
 class DelegateBase : NSObject, NSApplicationDelegate { }
 
-@NSApplicationMain
+@NSApplicationMain // expected-warning {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate : DelegateBase { }
 

test/attr/ApplicationMain/attr_NSApplicationMain_multi_file/another_delegate.swift

@@ -9,6 +9,7 @@
 import AppKit
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class EvilDelegate: NSObject, NSApplicationDelegate {
 }
 

test/attr/ApplicationMain/attr_NSApplicationMain_multi_file/delegate.swift

@@ -8,6 +8,7 @@
 import AppKit
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate: NSObject, NSApplicationDelegate {
 }
 

test/attr/ApplicationMain/attr_NSApplicationMain_multiple.swift

@@ -5,13 +5,16 @@
 import AppKit
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate1: NSObject, NSApplicationDelegate {
 }
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate2: NSObject, NSApplicationDelegate {
 }
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate3: NSObject, NSApplicationDelegate {
 }

test/attr/ApplicationMain/attr_NSApplicationMain_not_NSApplicationDelegate.swift

@@ -5,5 +5,6 @@
 import AppKit
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' class must conform to the 'NSApplicationDelegate' protocol}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyNonDelegate {
 }

test/attr/ApplicationMain/attr_NSApplicationMain_swift6.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -swift-version 6 -typecheck -parse-as-library -verify %s
+
+// REQUIRES: objc_interop
+
+import AppKit
+
+@NSApplicationMain // expected-error {{'NSApplicationMain' is deprecated and will be removed in a future release}}
+class MyDelegate: NSObject, NSApplicationDelegate {
+}

test/attr/ApplicationMain/attr_NSApplicationMain_with_main/delegate.swift

@@ -8,6 +8,7 @@
 import AppKit
 
 @NSApplicationMain // expected-error{{'NSApplicationMain' attribute cannot be used in a module that contains top-level code}}
+// expected-warning@-1 {{'NSApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate: NSObject, NSApplicationDelegate {
 }
 

test/attr/ApplicationMain/attr_UIApplicationMain.swift

@@ -4,6 +4,6 @@
 
 import UIKit
 
-@UIApplicationMain
+@UIApplicationMain // expected-warning {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate: NSObject, UIApplicationDelegate {
 }

test/attr/ApplicationMain/attr_UIApplicationMain_inherited.swift

@@ -6,6 +6,6 @@ import UIKit
 
 class DelegateBase : NSObject, UIApplicationDelegate { }
 
-@UIApplicationMain
+@UIApplicationMain // expected-warning {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate : DelegateBase { }
 

test/attr/ApplicationMain/attr_UIApplicationMain_multiple.swift

@@ -5,13 +5,16 @@
 import UIKit
 
 @UIApplicationMain // expected-error{{'UIApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate1: NSObject, UIApplicationDelegate {
 }
 
 @UIApplicationMain // expected-error{{'UIApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate2: NSObject, UIApplicationDelegate {
 }
 
 @UIApplicationMain // expected-error{{'UIApplicationMain' attribute can only apply to one class in a module}}
+// expected-warning@-1 {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate3: NSObject, UIApplicationDelegate {
 }

test/attr/ApplicationMain/attr_UIApplicationMain_not_UIApplicationDelegate.swift

@@ -5,5 +5,6 @@
 import UIKit
 
 @UIApplicationMain // expected-error{{'UIApplicationMain' class must conform to the 'UIApplicationDelegate' protocol}}
+// expected-warning@-1 {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyNonDelegate {
 }

test/attr/ApplicationMain/attr_UIApplicationMain_swift6.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -swift-version 6 -typecheck -parse-as-library -verify %s
+
+// REQUIRES: objc_interop
+
+import UIKit
+
+@UIApplicationMain // expected-error {{'UIApplicationMain' is deprecated and will be removed in a future release}}
+class MyDelegate: NSObject, UIApplicationDelegate {
+}

test/attr/ApplicationMain/attr_UIApplicationMain_with_main/delegate.swift

@@ -8,6 +8,7 @@
 import UIKit
 
 @UIApplicationMain // expected-error{{'UIApplicationMain' attribute cannot be used in a module that contains top-level code}}
+// expected-warning@-1 {{'UIApplicationMain' is deprecated and will be removed in a future release}}
 class MyDelegate: NSObject, UIApplicationDelegate {
 }