Add annotation to allow passthrough default methods on assisted factories

extensions/assistedinject/src/com/google/inject/assistedinject/FactoryModuleBuilder.java

@@ -305,8 +305,9 @@ public <T> FactoryModuleBuilder implement(Key<T> source, TypeLiteral<? extends T
    * Typically called via {@code withLookups(MethodHandles.lookup())}. Sets the MethodHandles.Lookup
    * that the factory implementation will use to call default methods on the factory interface.
    * While this is not always required, it is always OK to set it. It is required if the factory
-   * passed to {@link #build} is non-public and javac generated default methods while compiling it
-   * (which javac can sometimes do if the factory uses generic types).
+   * passed to {@link #build} is non-public and has default methods, either javac generated default
+   * methods (which javac can sometimes emit if the factory uses generic types), or user-specified
+   * default methods marked by {@link PassthroughDefaultMethods}.
    *
    * <p>Guice will try to work properly even if this method is not called (or called with a lookups
    * that doesn't have access to the factory), but doing so requires reflection into the JDK, which

extensions/assistedinject/src/com/google/inject/assistedinject/FactoryProvider2.java

@@ -271,9 +271,9 @@ public TypeLiteral<?> getImplementationType() {
           continue;
         }
 
-        // Skip default methods that java8 may have created.
-        if (isDefault(method) && (method.isBridge() || method.isSynthetic())) {
-          // Even synthetic default methods need the return type validation...
+        // Skip default methods that java8 may have created (or the user specifies to skip).
+        if (isSkippableDefaultMethod(factoryRawType, method)) {
+          // Even default methods need the return type validation...
           // unavoidable consequence of javac8. :-(
           validateFactoryReturnType(errors, method.getReturnType(), factoryRawType);
           defaultMethods.put(method.getName(), method);
@@ -388,7 +388,7 @@ public TypeLiteral<?> getImplementationType() {
           warnedAboutUserLookups = true;
           logger.log(
               Level.WARNING,
-              "AssistedInject factory {0} is non-public and has javac-generated default methods. "
+              "AssistedInject factory {0} is non-public and has default methods."
                   + " Please pass a `MethodHandles.lookup()` with"
                   + " FactoryModuleBuilder.withLookups when using this factory so that Guice can"
                   + " properly call the default methods. Guice will try to workaround this, but "
@@ -436,6 +436,10 @@ public TypeLiteral<?> getImplementationType() {
                     + " public.";
         if (handle != null) {
           methodHandleBuilder.put(defaultMethod, handle);
+        } else if (isUserSpecifiedDefaultMethod(factoryRawType, defaultMethod)) {
+          // Don't try to find matching signature for user-specified default methods
+          errors.addMessage(failureMsg.get());
+          throw new IllegalStateException("Can't find method compatible with: " + defaultMethod);
         } else if (!allowMethodHandleWorkaround) {
           errors.addMessage(failureMsg.get());
         } else {
@@ -452,8 +456,7 @@ public TypeLiteral<?> getImplementationType() {
             }
           }
           // We always expect to find at least one match, because we only deal with javac-generated
-          // default methods. If we ever allow user-specified default methods, this will need to
-          // change.
+          // default methods here.
           if (!foundMatch) {
             throw new IllegalStateException("Can't find method compatible with: " + defaultMethod);
           }
@@ -473,6 +476,19 @@ public TypeLiteral<?> getImplementationType() {
     }
   }
 
+  private static boolean isSkippableDefaultMethod(Class<?> factoryRawType, Method method) {
+    final boolean synthetic = method.isBridge() || method.isSynthetic();
+    final boolean annotated = method.isAnnotationPresent(PassthroughDefaultMethods.class)
+        || factoryRawType.isAnnotationPresent(PassthroughDefaultMethods.class);
+    return isDefault(method) && (synthetic || annotated);
+  }
+
+  private static boolean isUserSpecifiedDefaultMethod(Class<?> factoryRawType, Method defaultMethod) {
+    return defaultMethod.isAnnotationPresent(PassthroughDefaultMethods.class)
+        || (factoryRawType.isAnnotationPresent(PassthroughDefaultMethods.class)
+        && !defaultMethod.isBridge() && !defaultMethod.isSynthetic());
+  }
+
   static boolean isDefault(Method method) {
     // Per the javadoc, default methods are non-abstract, public, non-static.
     // They're also in interfaces, but we can guarantee that already since we only act

extensions/assistedinject/src/com/google/inject/assistedinject/PassthroughDefaultMethods.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2023 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.inject.assistedinject;
+
+import com.google.inject.BindingAnnotation;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+import java.lang.invoke.MethodHandles;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * Annotates a factory interface to indicate its default methods
+ * should be pass-through on the generated factory implementation,
+ * instead of treated as standard factory methods.
+ *
+ * <p>This annotation may also be used on individual default methods
+ * of factory interfaces, but it is named with the assumption that
+ * the general use case wants default methods treated in a uniform
+ * fashion for an entire factory.</p>
+ *
+ * @see FactoryModuleBuilder#withLookups(MethodHandles.Lookup)
+ */
+@BindingAnnotation
+@Target({METHOD, TYPE})
+@Retention(RUNTIME)
+public @interface PassthroughDefaultMethods {
+}

extensions/assistedinject/test/com/google/inject/assistedinject/PassthroughDefaultMethodsTest.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2023 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.inject.assistedinject;
+
+import com.google.inject.AbstractModule;
+import com.google.inject.Guice;
+import com.google.inject.Inject;
+import com.google.inject.Injector;
+import java.lang.invoke.MethodHandles;
+import junit.framework.TestCase;
+
+public class PassthroughDefaultMethodsTest extends TestCase {
+  private static class Thing {
+    final int i;
+
+    @Inject
+    Thing(@Assisted int i) {
+      this.i = i;
+    }
+  }
+
+  @PassthroughDefaultMethods
+  private interface Factory {
+    Thing create(int i);
+
+    default Thing one() {
+      return this.create(1);
+    }
+
+    default Thing createPow(int i, int pow) {
+      return this.create((int) Math.pow(i, pow));
+    }
+  }
+
+  public void testAssistedInjection() throws IllegalAccessException {
+    MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(Factory.class, MethodHandles.lookup());
+    Injector injector =
+      Guice.createInjector(
+        new AbstractModule() {
+          @Override
+          protected void configure() {
+            install(new FactoryModuleBuilder().withLookups(lookup).build(Factory.class));
+          }
+        });
+    Factory factory = injector.getInstance(Factory.class);
+    assertEquals(1, factory.create(1).i);
+    assertEquals(1, factory.one().i);
+    assertEquals(256, factory.createPow(2, 8).i);
+  }
+}