[nullsafe] Improve the test for checking `onDestroy` handling

Summary:
1. Let's make the intention of the test more visible, also let's provide an example
when the error does occur.
2. `onDestroy` silence "field not nullable" warnigs not only for `View`, but for any objects, so let's use `String` (as an example of a trivial object) instead.

Original diff that introduced the test: D10024458

Reviewed By: artempyanykh

Differential Revision: D17202839

fbshipit-source-id: 037d937e4

infer/tests/codetoanalyze/java/nullsafe-default/FieldNotNullable.java

@@ -10,7 +10,6 @@ package codetoanalyze.java.nullsafe_default;
 import android.app.Activity;
 import android.os.Bundle;
 import android.support.v4.app.Fragment;
-import android.view.View;
 import com.facebook.infer.annotation.Assertions;
 import com.facebook.infer.annotation.Cleanup;
 import com.facebook.infer.annotation.Initializer;
@@ -24,13 +23,28 @@ abstract class A {
   }
 }
 
-class FragmentExample extends Fragment {
+/**
+ * It is common in Android code to recycle the objects (e.g. views) by nullifying them in the
+ * onDestroy() or onDestroyView() methods. This allows the GC to recycle it not waiting the outer
+ * object to be freed. This is safe because the lifecycle of the object is over so those field are
+ * not going to be accessed. so it is not necessary to annotate those fields with @Nullable.
+ */
+class CanAssignNullInDestroyMethods extends Fragment {
 
-  View view;
+  String someObject = "";
 
   @Override
   public void onDestroyView() {
-    view = null;
+    someObject = null;
+  }
+
+  @Override
+  public void onDestroy() {
+    someObject = null;
+  }
+
+  public void assignNullDissalowedInAnyOtherMethod() {
+    someObject = null; // BAD: field not nullable
   }
 }
 

infer/tests/codetoanalyze/java/nullsafe-default/issues.exp

@@ -22,11 +22,11 @@ codetoanalyze/java/nullsafe-default/FieldNotInitialized.java, codetoanalyze.java
 codetoanalyze/java/nullsafe-default/FieldNotInitialized.java, codetoanalyze.java.nullsafe_default.FieldNotInitialized$Swap.<init>(codetoanalyze.java.nullsafe_default.FieldNotInitialized), 0, ERADICATE_FIELD_NOT_INITIALIZED, no_bucket, WARNING, [Field `FieldNotInitialized$Swap.o1` is not initialized in the constructor and is not declared `@Nullable`]
 codetoanalyze/java/nullsafe-default/FieldNotInitialized.java, codetoanalyze.java.nullsafe_default.FieldNotInitialized$WriteItself.<init>(codetoanalyze.java.nullsafe_default.FieldNotInitialized), 0, ERADICATE_FIELD_NOT_INITIALIZED, no_bucket, WARNING, [Field `FieldNotInitialized$WriteItself.o` is not initialized in the constructor and is not declared `@Nullable`]
 codetoanalyze/java/nullsafe-default/FieldNotInitialized.java, codetoanalyze.java.nullsafe_default.FieldNotInitialized.<init>(), 0, ERADICATE_FIELD_NOT_INITIALIZED, no_bucket, WARNING, [Field `FieldNotInitialized.a` is not initialized in the constructor and is not declared `@Nullable`]
-codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.<init>(java.lang.Integer), -25, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.static_s` can be null but is not declared `@Nullable`. (Origin: null constant at line 41)]
+codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.CanAssignNullInDestroyMethods.assignNullDissalowedInAnyOtherMethod():void, 1, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `CanAssignNullInDestroyMethods.someObject` can be null but is not declared `@Nullable`. (Origin: null constant at line 47)]
-codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.<init>(java.lang.String), -2, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.static_s` can be null but is not declared `@Nullable`. (Origin: null constant at line 41)]
+codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.<init>(java.lang.Integer), -25, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.static_s` can be null but is not declared `@Nullable`. (Origin: null constant at line 55)]
-codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.setYNull():void, 1, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.y` can be null but is not declared `@Nullable`. (Origin: null constant at line 59)]
+codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.<init>(java.lang.String), -2, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.static_s` can be null but is not declared `@Nullable`. (Origin: null constant at line 55)]
+codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.setYNull():void, 1, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.y` can be null but is not declared `@Nullable`. (Origin: null constant at line 73)]
 codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FieldNotNullable.setYNullable(java.lang.String):void, 1, ERADICATE_FIELD_NOT_NULLABLE, no_bucket, WARNING, [Field `FieldNotNullable.y` can be null but is not declared `@Nullable`. (Origin: method parameter s)]
-codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.FragmentExample.<init>(), 0, ERADICATE_FIELD_NOT_INITIALIZED, no_bucket, WARNING, [Field `FragmentExample.view` is not initialized in the constructor and is not declared `@Nullable`]
 codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.TestInitializerBuilder.build():codetoanalyze.java.nullsafe_default.TestInitializer, 2, ERADICATE_CONDITION_REDUNDANT, no_bucket, WARNING, [The condition TestInitializerBuilder.required1 is always true according to the existing annotations.]
 codetoanalyze/java/nullsafe-default/FieldNotNullable.java, codetoanalyze.java.nullsafe_default.TestInitializerBuilder.build():codetoanalyze.java.nullsafe_default.TestInitializer, 2, ERADICATE_CONDITION_REDUNDANT, no_bucket, WARNING, [The condition TestInitializerBuilder.required2 is always true according to the existing annotations.]
 codetoanalyze/java/nullsafe-default/InconsistentSubclassAnnotation.java, codetoanalyze.java.nullsafe_default.ExtendsExternalLibrary.externalMethod2(java.lang.Object):void, 0, ERADICATE_INCONSISTENT_SUBCLASS_PARAMETER_ANNOTATION, no_bucket, WARNING, [First parameter `object` of method `ExtendsExternalLibrary.externalMethod2(...)` is not `@Nullable` but is declared `@Nullable`in the parent class method `SomeExternalClass.externalMethod2(...)`.]