JSpecify: Handle @nullable assignments to @nonnull arrays (uber#929)

Handles cases in JSpecify mode where a `@Nullable` element is assigned
to an unannotated array. Added relevant unit tests.

**Current Behavior**

Both are valid assignments.

```
String [] foo = new String[10];
@nullable String [] bar = new String [10];
foo[1] = null;
bar[1] = null;
```

**New Behavior**

Assignment to `foo` generates an error since array elements are
`@Nonnull`.

```
String [] foo = new String[10];
@nullable String [] bar = new String [10];
foo[1] = null;
bar[1]=null;
```

---------

Co-authored-by: Manu Sridharan <[email protected]>

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

@@ -58,6 +58,7 @@ public enum MessageTypes {
     PASS_NULLABLE_GENERIC,
     WRONG_OVERRIDE_RETURN_GENERIC,
     WRONG_OVERRIDE_PARAM_GENERIC,
+    ASSIGN_NULLABLE_TO_NONNULL_ARRAY,
   }
 
   public String getMessage() {

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

@@ -32,6 +32,7 @@
 import static com.uber.nullaway.ASTHelpersBackports.isStatic;
 import static com.uber.nullaway.ErrorBuilder.errMsgForInitializer;
 import static com.uber.nullaway.NullabilityUtil.castToNonNull;
+import static com.uber.nullaway.NullabilityUtil.isArrayElementNullable;
 
 import com.google.auto.service.AutoService;
 import com.google.auto.value.AutoValue;
@@ -497,6 +498,30 @@ public Description matchAssignment(AssignmentTree tree, VisitorState state) {
       GenericsChecks.checkTypeParameterNullnessForAssignability(tree, this, state);
     }
 
+    if (config.isJSpecifyMode() && tree.getVariable() instanceof ArrayAccessTree) {
+      // check for a write of a @Nullable value into @NonNull array contents
+      ArrayAccessTree arrayAccess = (ArrayAccessTree) tree.getVariable();
+      ExpressionTree arrayExpr = arrayAccess.getExpression();
+      ExpressionTree expression = tree.getExpression();
+      Symbol arraySymbol = ASTHelpers.getSymbol(arrayExpr);
+      if (arraySymbol != null) {
+        boolean isElementNullable = isArrayElementNullable(arraySymbol, config);
+        if (!isElementNullable && mayBeNullExpr(state, expression)) {
+          final String message = "Writing @Nullable expression into array with @NonNull contents.";
+          ErrorMessage errorMessage =
+              new ErrorMessage(MessageTypes.ASSIGN_NULLABLE_TO_NONNULL_ARRAY, message);
+          // Future enhancements which auto-fix such warnings will require modification to this
+          // logic
+          return errorBuilder.createErrorDescription(
+              errorMessage,
+              expression,
+              buildDescription(tree),
+              state,
+              ASTHelpers.getSymbol(tree.getVariable()));
+        }
+      }
+    }
+
     Symbol assigned = ASTHelpers.getSymbol(tree.getVariable());
     if (assigned == null || assigned.getKind() != ElementKind.FIELD) {
       // not a field of nullable type

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

@@ -135,6 +135,83 @@ public void arrayContentsAndTopLevelAnnotation() {
         .doTest();
   }
 
+  @Test
+  public void nullableAssignmentNonnullArray() {
+    makeHelper()
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import org.jspecify.annotations.Nullable;",
+            "class Test {",
+            "  static String [] foo = new String[10];",
+            "  static void foo() {",
+            "    // BUG: Diagnostic contains: Writing @Nullable expression into array with @NonNull contents",
+            "    foo[1] = null;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void nullableAssignmentNullableArray() {
+    makeHelper()
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import org.jspecify.annotations.Nullable;",
+            "class Test {",
+            "  static @Nullable String [] foo = new String[10];",
+            "  static void foo() {",
+            "    // OK: since array elements are @Nullable",
+            "    foo[1] = null;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void nullableAssignmentLocalArray() {
+    makeHelper()
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import org.jspecify.annotations.Nullable;",
+            "class Test {",
+            "  static void foo() {",
+            "    String [] nonNullArray = new String[10];",
+            "    @Nullable String [] nullableArray = new String[10];",
+            "    // BUG: Diagnostic contains: Writing @Nullable expression into array with @NonNull contents",
+            "    nonNullArray[1] = null;",
+            "    // OK: since array elements are @Nullable",
+            "    nullableArray[1] = null;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  @Test
+  public void nullableAssignmentParameterArray() {
+    makeHelper()
+        .addSourceLines(
+            "Test.java",
+            "package com.uber;",
+            "import org.jspecify.annotations.Nullable;",
+            "class Test {",
+            "  static void fizz(String[] nonNullArray, @Nullable String[] nullableArray) {",
+            "    // BUG: Diagnostic contains: Writing @Nullable expression into array with @NonNull contents",
+            "    nonNullArray[1] = null;",
+            "    // OK: since array elements are @Nullable",
+            "    nullableArray[1] = null;",
+            "  }",
+            "  public static void main(String[] args) {",
+            "    String[] foo = new String[10];",
+            "    @Nullable String[] bar = new String[10];",
+            "    fizz(foo, bar);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
   private CompilationTestHelper makeHelper() {
     return makeTestHelperWithArgs(
         Arrays.asList(