[cfe] Support implict this access to external extension fields

Closes #60270

Change-Id: Ic842740c8812a04a95f53556555a4f5c360eab55
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413783
Reviewed-by: Jens Johansen <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/kernel/body_builder.dart

@@ -3333,11 +3333,12 @@ class BodyBuilder extends StackListenerImpl
           declaration.parent as ExtensionBuilder;
       MemberBuilder? setterBuilder =
           _getCorrespondingSetterBuilder(scope, declaration, name, nameOffset);
-      if (declaration.isField) {
+      if (declaration.isField && !declaration.isExternal) {
         declaration = null;
       }
       if (setterBuilder != null &&
-          (setterBuilder.isField || setterBuilder.isStatic)) {
+          ((setterBuilder.isField && !setterBuilder.isExternal) ||
+              setterBuilder.isStatic)) {
         setterBuilder = null;
       }
       if ((declaration == null && setterBuilder == null) ||

pkg/front_end/lib/src/kernel/expression_generator.dart

@@ -1648,7 +1648,10 @@ class ExtensionInstanceAccessGenerator extends Generator {
     Procedure? readTarget;
     Procedure? invokeTarget;
     if (getterBuilder != null) {
-      if (getterBuilder.isGetter) {
+      if (getterBuilder.isField) {
+        assert(!getterBuilder.isStatic && getterBuilder.isExternal);
+        readTarget = getterBuilder.readTarget as Procedure?;
+      } else if (getterBuilder.isGetter) {
         assert(!getterBuilder.isStatic);
         readTarget = getterBuilder.readTarget as Procedure?;
       } else if (getterBuilder.isRegularMethod) {
@@ -1670,7 +1673,10 @@ class ExtensionInstanceAccessGenerator extends Generator {
     }
     Procedure? writeTarget;
     if (setterBuilder != null) {
-      if (setterBuilder.isSetter) {
+      if (setterBuilder.isField) {
+        assert(!setterBuilder.isStatic && setterBuilder.isExternal);
+        writeTarget = setterBuilder.writeTarget as Procedure?;
+      } else if (setterBuilder.isSetter) {
         assert(!setterBuilder.isStatic);
         writeTarget = setterBuilder.writeTarget as Procedure?;
         // Coverage-ignore-block(suite): Not run.

pkg/front_end/testcases/extensions/external_field_access.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+extension on int {
+  external int field;
+
+  bool get getter {
+    return field.isEven;
+  }
+
+  set setter(bool value) {
+    field = value ? 0 : 1;
+  }
+}

pkg/front_end/testcases/extensions/external_field_access.dart.strong.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension /* unnamed */ _extension#0 on core::int {
+  get field = self::_extension#0|get#field;
+  set field = self::_extension#0|set#field;
+  get getter = self::_extension#0|get#getter;
+  set setter = self::_extension#0|set#setter;
+}
+external static extension-member method _extension#0|get#field(lowered core::int #this) → core::int;
+external static extension-member method _extension#0|set#field(lowered core::int #this, synthesized core::int #externalFieldValue) → void;
+static extension-member method _extension#0|get#getter(lowered final core::int #this) → core::bool {
+  return self::_extension#0|get#field(#this).{core::int::isEven}{core::bool};
+}
+static extension-member method _extension#0|set#setter(lowered final core::int #this, core::bool value) → void {
+  self::_extension#0|set#field(#this, value ?{core::int} 0 : 1);
+}

pkg/front_end/testcases/extensions/external_field_access.dart.strong.modular.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension /* unnamed */ _extension#0 on core::int {
+  get field = self::_extension#0|get#field;
+  set field = self::_extension#0|set#field;
+  get getter = self::_extension#0|get#getter;
+  set setter = self::_extension#0|set#setter;
+}
+external static extension-member method _extension#0|get#field(lowered core::int #this) → core::int;
+external static extension-member method _extension#0|set#field(lowered core::int #this, synthesized core::int #externalFieldValue) → void;
+static extension-member method _extension#0|get#getter(lowered final core::int #this) → core::bool {
+  return self::_extension#0|get#field(#this).{core::int::isEven}{core::bool};
+}
+static extension-member method _extension#0|set#setter(lowered final core::int #this, core::bool value) → void {
+  self::_extension#0|set#field(#this, value ?{core::int} 0 : 1);
+}

pkg/front_end/testcases/extensions/external_field_access.dart.strong.outline.expect

@@ -0,0 +1,16 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension /* unnamed */ _extension#0 on core::int {
+  get field = self::_extension#0|get#field;
+  set field = self::_extension#0|set#field;
+  get getter = self::_extension#0|get#getter;
+  set setter = self::_extension#0|set#setter;
+}
+external static extension-member method _extension#0|get#field(lowered core::int #this) → core::int;
+external static extension-member method _extension#0|set#field(lowered core::int #this, synthesized core::int #externalFieldValue) → void;
+static extension-member method _extension#0|get#getter(lowered final core::int #this) → core::bool
+  ;
+static extension-member method _extension#0|set#setter(lowered final core::int #this, core::bool value) → void
+  ;

pkg/front_end/testcases/extensions/external_field_access.dart.strong.transformed.expect

@@ -0,0 +1,18 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension /* unnamed */ _extension#0 on core::int {
+  get field = self::_extension#0|get#field;
+  set field = self::_extension#0|set#field;
+  get getter = self::_extension#0|get#getter;
+  set setter = self::_extension#0|set#setter;
+}
+external static extension-member method _extension#0|get#field(lowered core::int #this) → core::int;
+external static extension-member method _extension#0|set#field(lowered core::int #this, synthesized core::int #externalFieldValue) → void;
+static extension-member method _extension#0|get#getter(lowered final core::int #this) → core::bool {
+  return self::_extension#0|get#field(#this).{core::int::isEven}{core::bool};
+}
+static extension-member method _extension#0|set#setter(lowered final core::int #this, core::bool value) → void {
+  self::_extension#0|set#field(#this, value ?{core::int} 0 : 1);
+}

pkg/front_end/testcases/extensions/external_field_access.dart.textual_outline.expect

@@ -0,0 +1,5 @@
+extension on int {
+  external int field;
+  bool get getter {}
+  set setter(bool value) {}
+}

pkg/front_end/testcases/extensions/external_field_access.dart.textual_outline_modelled.expect

@@ -0,0 +1,5 @@
+extension on int {
+  bool get getter {}
+  external int field;
+  set setter(bool value) {}
+}