Add MoreTypes.isConversionFromObjectUnchecked. This method tells, for…

… a given type, whether casting Object to that type will elicit an "unchecked" warning from the compiler.

RELNOTES=Added MoreTypes.isConversionFromObjectUnchecked to test whether casting to a type will elicit an "unchecked" compiler warning.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=288896286

common/src/main/java/com/google/auto/common/MoreTypes.java

@@ -949,5 +949,71 @@ protected T defaultAction(TypeMirror e, Void v) {
     }
   }
 
+  /**
+   * Returns true if casting {@code Object} to the given type will elicit an unchecked warning from
+   * the compiler. Only type variables and parameterized types such as {@code List<String>} produce
+   * such warnings. There will be no warning if the type's only type parameters are simple
+   * wildcards, as in {@code Map<?, ?>}.
+   */
+  public static boolean isConversionFromObjectUnchecked(TypeMirror type) {
+    return new CastingUncheckedVisitor().visit(type, null);
+  }
+
+  /**
+   * Visitor that tells whether a type is erased, in the sense of {@link #castIsUnchecked}. Each
+   * visitX method returns true if its input parameter is true or if the type being visited is
+   * erased.
+   */
+  private static class CastingUncheckedVisitor extends SimpleTypeVisitor8<Boolean, Void> {
+    CastingUncheckedVisitor() {
+      super(false);
+    }
+
+    @Override
+    public Boolean visitUnknown(TypeMirror t, Void p) {
+      // We don't know whether casting is unchecked for this mysterious type but assume it is,
+      // so we will insert a possibly unnecessary @SuppressWarnings("unchecked").
+      return true;
+    }
+
+    @Override
+    public Boolean visitArray(ArrayType t, Void p) {
+      return visit(t.getComponentType(), p);
+    }
+
+    @Override
+    public Boolean visitDeclared(DeclaredType t, Void p) {
+      return t.getTypeArguments().stream().anyMatch(CastingUncheckedVisitor::uncheckedTypeArgument);
+    }
+
+    @Override
+    public Boolean visitTypeVariable(TypeVariable t, Void p) {
+      return true;
+    }
+
+    // If a type has a type argument, then casting to the type is unchecked, except if the argument
+    // is <?> or <? extends Object>. The same applies to all type arguments, so casting to Map<?, ?>
+    // does not produce an unchecked warning for example.
+    private static boolean uncheckedTypeArgument(TypeMirror arg) {
+      if (arg.getKind().equals(TypeKind.WILDCARD)) {
+        WildcardType wildcard = asWildcard(arg);
+        if (wildcard.getExtendsBound() == null || isJavaLangObject(wildcard.getExtendsBound())) {
+          // This is <?>, unless there's a super bound, in which case it is <? super Foo> and
+          // is erased.
+          return (wildcard.getSuperBound() != null);
+        }
+      }
+      return true;
+    }
+
+    private static boolean isJavaLangObject(TypeMirror type) {
+      if (type.getKind() != TypeKind.DECLARED) {
+        return false;
+      }
+      TypeElement typeElement = asTypeElement(type);
+      return typeElement.getQualifiedName().contentEquals("java.lang.Object");
+    }
+  }
+
   private MoreTypes() {}
 }

common/src/test/java/com/google/auto/common/MoreTypesTest.java

@@ -28,6 +28,7 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import com.google.common.testing.EquivalenceTester;
+import com.google.common.truth.Expect;
 import com.google.testing.compile.CompilationRule;
 import java.lang.annotation.Annotation;
 import java.util.List;
@@ -55,7 +56,8 @@
 
 @RunWith(JUnit4.class)
 public class MoreTypesTest {
-  @Rule public CompilationRule compilationRule = new CompilationRule();
+  @Rule public final CompilationRule compilationRule = new CompilationRule();
+  @Rule public final Expect expect = Expect.create();
 
   @Test
   public void equivalence() {
@@ -442,4 +444,58 @@ public List<? extends AnnotationMirror> getAnnotationMirrors() {
       return null;
     }
   };
+
+  @Test
+  public void testIsConversionFromObjectUnchecked_yes() {
+    Elements elements = compilationRule.getElements();
+    TypeElement unchecked = elements.getTypeElement(Unchecked.class.getCanonicalName());
+    for (VariableElement field : ElementFilter.fieldsIn(unchecked.getEnclosedElements())) {
+      TypeMirror type = field.asType();
+      expect
+          .withMessage("Casting to %s is unchecked", type)
+          .that(MoreTypes.isConversionFromObjectUnchecked(type))
+          .isTrue();
+    }
+  }
+
+  @Test
+  public void testIsConversionFromObjectUnchecked_no() {
+    Elements elements = compilationRule.getElements();
+    TypeElement notUnchecked = elements.getTypeElement(NotUnchecked.class.getCanonicalName());
+    for (VariableElement field : ElementFilter.fieldsIn(notUnchecked.getEnclosedElements())) {
+      TypeMirror type = field.asType();
+      expect
+          .withMessage("Casting to %s is not unchecked", type)
+          .that(MoreTypes.isConversionFromObjectUnchecked(type))
+          .isFalse();
+    }
+  }
+
+  // The type of every field here is such that casting to it provokes an "unchecked" warning.
+  @SuppressWarnings("unused")
+  private static class Unchecked<T> {
+    private List<String> listOfString;
+    private List<? extends CharSequence> listOfExtendsCharSequence;
+    private List<? super CharSequence> listOfSuperCharSequence;
+    private List<T> listOfT;
+    private List<T[]> listOfArrayOfT;
+    private T t;
+    private T[] arrayOfT;
+    private List<T>[] arrayOfListOfT;
+    private Map<?, String> mapWildcardToString;
+    private Map<String, ?> mapStringToWildcard;
+  }
+
+  // The type of every field here is such that casting to it doesn't provoke an "unchecked" warning.
+  @SuppressWarnings("unused")
+  private static class NotUnchecked {
+    private String string;
+    private int integer;
+    private String[] arrayOfString;
+    private int[] arrayOfInt;
+    private Thread.State threadStateEnum;
+    private List<?> listOfWildcard;
+    private List<? extends Object> listOfWildcardExtendsObject;
+    private Map<?, ?> mapWildcardToWildcard;
+  }
 }