Fix error inside Lombok generated code when dealing with @nullable @B…

…uilder.Default fields

nullaway/src/main/java/com/uber/nullaway/NullAway.java

@@ -822,8 +822,17 @@ private Description checkReturnExpression(
       // support)
       return Description.NO_MATCH;
     }
-    if (codeAnnotationInfo.isSymbolUnannotated(methodSymbol, config)
-        || Nullness.hasNullableAnnotation(methodSymbol, config)) {
+    final boolean isContainingMethodAnnotated =
+        !codeAnnotationInfo.isSymbolUnannotated(methodSymbol, config);
+    Nullness containingMethodReturnNullness =
+        Nullness.NULLABLE; // Permissive default for unannotated code.
+    if (isContainingMethodAnnotated && !Nullness.hasNullableAnnotation(methodSymbol, config)) {
+      containingMethodReturnNullness = Nullness.NONNULL;
+    }
+    containingMethodReturnNullness =
+        handler.onOverrideMethodInvocationReturnNullability(
+            methodSymbol, state, isContainingMethodAnnotated, containingMethodReturnNullness);
+    if (containingMethodReturnNullness.equals(Nullness.NULLABLE)) {
       return Description.NO_MATCH;
     }
     if (mayBeNullExpr(state, retExpr)) {

nullaway/src/main/java/com/uber/nullaway/handlers/Handlers.java

@@ -74,6 +74,7 @@ public static Handler buildDefault(Config config) {
     if (config.checkContracts()) {
       handlerListBuilder.add(new ContractCheckHandler(config));
     }
+    handlerListBuilder.add(new LombokHandler(config));
 
     return new CompositeHandler(handlerListBuilder.build());
   }

nullaway/src/main/java/com/uber/nullaway/handlers/LibraryModelsHandler.java

@@ -116,7 +116,9 @@ public Nullness onOverrideMethodInvocationReturnNullability(
       Nullness returnNullness) {
     OptimizedLibraryModels optLibraryModels = getOptLibraryModels(state.context);
     if (optLibraryModels.hasNonNullReturn(methodSymbol, state.getTypes(), !isAnnotated)) {
-      return Nullness.NONNULL;
+      return NONNULL;
+    } else if (optLibraryModels.hasNullableReturn(methodSymbol, state.getTypes(), !isAnnotated)) {
+      return NULLABLE;
     }
     return returnNullness;
   }

nullaway/src/main/java/com/uber/nullaway/handlers/LombokHandler.java

@@ -0,0 +1,92 @@
+package com.uber.nullaway.handlers;
+
+import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+import com.google.errorprone.VisitorState;
+import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.ExpressionTree;
+import com.sun.source.tree.Tree;
+import com.sun.tools.javac.code.Symbol;
+import com.uber.nullaway.Config;
+import com.uber.nullaway.NullAway;
+import com.uber.nullaway.Nullness;
+import java.util.stream.StreamSupport;
+import javax.annotation.Nullable;
+import javax.lang.model.element.ElementKind;
+
+/**
+ * A general handler for Lombok generated code and its internal semantics.
+ *
+ * <p>Currently used to propagate @Nullable in cases where the Lombok annotation processor fails to
+ * do so consistently.
+ */
+public class LombokHandler extends BaseNoOpHandler {
+
+  private static String LOMBOK_GENERATED_ANNOTATION_NAME = "lombok.Generated";
+  private static String LOMBOK_BUILDER_DEFAULT_METHOD_PREFIX = "$default$";
+
+  private final Config config;
+
+  public LombokHandler(Config config) {
+    this.config = config;
+  }
+
+  private boolean isLombokMethodWithMissingNullableAnnotation(
+      Symbol.MethodSymbol methodSymbol, VisitorState state) {
+    if (!ASTHelpers.hasAnnotation(methodSymbol, LOMBOK_GENERATED_ANNOTATION_NAME, state)) {
+      return false;
+    }
+    String methodNameString = methodSymbol.name.toString();
+    if (!methodNameString.startsWith(LOMBOK_BUILDER_DEFAULT_METHOD_PREFIX)) {
+      return false;
+    }
+    String originalFieldName =
+        methodNameString.substring(LOMBOK_BUILDER_DEFAULT_METHOD_PREFIX.length());
+    ImmutableList<Symbol> matchingMembers =
+        StreamSupport.stream(
+                ASTHelpers.scope(methodSymbol.enclClass().members())
+                    .getSymbols(
+                        sym ->
+                            sym.name.contentEquals(originalFieldName)
+                                && sym.getKind().equals(ElementKind.FIELD))
+                    .spliterator(),
+                false)
+            .collect(ImmutableList.toImmutableList());
+    Preconditions.checkArgument(
+        matchingMembers.size() == 1,
+        String.format(
+            "Found %d fields matching Lombok generated builder default method %s",
+            matchingMembers.size(), methodNameString));
+    return Nullness.hasNullableAnnotation(matchingMembers.get(0), config);
+  }
+
+  @Override
+  public boolean onOverrideMayBeNullExpr(
+      NullAway analysis,
+      ExpressionTree expr,
+      @Nullable Symbol exprSymbol,
+      VisitorState state,
+      boolean exprMayBeNull) {
+    if (exprMayBeNull) {
+      return true;
+    }
+    Tree.Kind exprKind = expr.getKind();
+    if (exprSymbol != null && exprKind == Tree.Kind.METHOD_INVOCATION) {
+      Symbol.MethodSymbol methodSymbol = (Symbol.MethodSymbol) exprSymbol;
+      return isLombokMethodWithMissingNullableAnnotation(methodSymbol, state);
+    }
+    return false;
+  }
+
+  @Override
+  public Nullness onOverrideMethodInvocationReturnNullability(
+      Symbol.MethodSymbol methodSymbol,
+      VisitorState state,
+      boolean isAnnotated,
+      Nullness returnNullness) {
+    if (isLombokMethodWithMissingNullableAnnotation(methodSymbol, state)) {
+      return Nullness.NULLABLE;
+    }
+    return returnNullness;
+  }
+}

nullaway/src/test/java/com/uber/nullaway/NullAwayCustomLibraryModelsTests.java

@@ -53,20 +53,22 @@ public void allowLibraryModelsOverrideAnnotations() {
                 temporaryFolder.getRoot().getAbsolutePath(),
                 "-XepOpt:NullAway:AnnotatedPackages=com.uber"))
         .addSourceLines(
-            "Foo.java",
+            "AnnotatedWithModels.java",
             "package com.uber;",
-            "public class Foo {",
+            "public class AnnotatedWithModels {",
             "   Object field = new Object();",
-            "   Object bar() {",
-            "      return new Object();",
+            "   // implicitly @Nullable due to library model",
+            "   Object returnsNullFromModel() {",
+            "      // null is valid here only because of the library model",
+            "      return null;",
             "   }",
             "   Object nullableReturn() {",
             "       // BUG: Diagnostic contains: returning @Nullable",
-            "       return bar();",
+            "       return returnsNullFromModel();",
             "   }",
             "   void run() {",
             "       // just to make sure, flow analysis is also impacted by library models information",
-            "      Object temp = bar();",
+            "      Object temp = returnsNullFromModel();",
             "       // BUG: Diagnostic contains: assigning @Nullable",
             "      this.field = temp;",
             "   }",

test-java-lib-lombok/src/main/java/com/uber/lombok/LombokDTO.java

@@ -36,4 +36,5 @@ public class LombokDTO {
   private String field;
   @Builder.Default private String fieldWithDefault = "Default";
   @Nullable private String nullableField;
+  @Nullable @Builder.Default private String fieldWithNullDefault = null;
 }

test-library-models/src/main/java/com/uber/nullaway/testlibrarymodels/TestLibraryModels.java

@@ -66,7 +66,7 @@ public ImmutableSetMultimap<MethodRef, Integer> nullImpliesNullParameters() {
   @Override
   public ImmutableSet<MethodRef> nullableReturns() {
     return ImmutableSet.of(
-        methodRef("com.uber.Foo", "bar()"),
+        methodRef("com.uber.AnnotatedWithModels", "returnsNullFromModel()"),
         methodRef("com.uber.lib.unannotated.UnannotatedWithModels", "returnsNullUnannotated()"),
         methodRef("com.uber.lib.unannotated.UnannotatedWithModels", "returnsNullUnannotated2()"));
   }