Tweak SubjectUtils.

core/src/main/java/com/google/common/truth/IterableSubject.java

@@ -634,8 +634,8 @@ private static String keyToServeAsHeader(String label, DuplicateGroupedAndTyped
      * an associated value, so it won't factor into alignment.
      */
     String key = keyToGoWithElementsString(label, elements);
-    if (elements.homogeneousTypeToDisplay != null) {
-      key += " (" + elements.homogeneousTypeToDisplay + ")";
+    if (elements.getHomogeneousTypeToDisplay() != null) {
+      key += " (" + elements.getHomogeneousTypeToDisplay() + ")";
     }
     return key;
   }

core/src/main/java/com/google/common/truth/NullnessCasts.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2021 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.common.truth;
+
+import org.jspecify.annotations.Nullable;
+
+/** A utility method to perform unchecked casts to suppress errors produced by nullness analyses. */
+final class NullnessCasts {
+  /**
+   * Accepts a {@code @Nullable T} and returns a plain {@code T}, without performing any check that
+   * that conversion is safe.
+   *
+   * <p>This method is intended to help with usages of type parameters that have parametric
+   * nullness. If a type parameter instead ranges over only non-null types (or if the type is a
+   * non-variable type, like {@code String}), then code should almost never use this method,
+   * preferring instead to call {@code requireNonNull} so as to benefit from its runtime check.
+   *
+   * <p>An example use case for this method is in implementing an {@code Iterator<T>} whose {@code
+   * next} field is lazily initialized. The type of that field would be {@code @Nullable T}, and the
+   * code would be responsible for populating a "real" {@code T} (which might still be the value
+   * {@code null}!) before returning it to callers. Depending on how the code is structured, a
+   * nullness analysis might not understand that the field has been populated. To avoid that problem
+   * without having to add {@code @SuppressWarnings}, the code can call this method.
+   *
+   * <p>Why <i>not</i> just add {@code SuppressWarnings}? The problem is that this method is
+   * typically useful for {@code return} statements. That leaves the code with two options: Either
+   * add the suppression to the whole method (which turns off checking for a large section of code),
+   * or extract a variable, and put the suppression on that. However, a local variable typically
+   * doesn't work: Because nullness analyses typically infer the nullness of local variables,
+   * there's no way to assign a {@code @Nullable T} to a field {@code T foo;} and instruct the
+   * analysis that that means "plain {@code T}" rather than the inferred type {@code @Nullable T}.
+   * (Even if supported added {@code @NonNull}, that would not help, since the problem case
+   * addressed by this method is the case in which {@code T} has parametric nullness -- and thus its
+   * value may be legitimately {@code null}.)
+   */
+  @SuppressWarnings("nullness")
+  static <T extends @Nullable Object> T uncheckedCastNullableTToT(@Nullable T t) {
+    return t;
+  }
+
+  private NullnessCasts() {}
+}

core/src/main/java/com/google/common/truth/SubjectUtils.java

@@ -20,6 +20,7 @@
 import static com.google.common.collect.Iterables.isEmpty;
 import static com.google.common.collect.Iterables.transform;
 import static com.google.common.collect.Multisets.immutableEntry;
+import static com.google.common.truth.NullnessCasts.uncheckedCastNullableTToT;
 
 import com.google.common.base.Equivalence;
 import com.google.common.base.Equivalence.Wrapper;
@@ -49,15 +50,20 @@ private SubjectUtils() {}
 
   static final String HUMAN_UNDERSTANDABLE_EMPTY_STRING = "\"\" (empty String)";
 
-  static <T extends @Nullable Object> List<T> accumulate(T first, T second, T @Nullable ... rest) {
+  static <T extends @Nullable Object> List<T> accumulate(T first, T second, T @Nullable [] rest) {
     // rest should never be deliberately null, so assume that the caller passed null
     // in the third position but intended it to be the third element in the array of values.
     // Javac makes the opposite inference, so handle that here.
     List<T> items = new ArrayList<>(2 + ((rest == null) ? 1 : rest.length));
     items.add(first);
     items.add(second);
     if (rest == null) {
-      items.add((T) null);
+      /*
+       * This cast is probably not actually safe as used in IterableSubject.UsingCorrespondence. But
+       * that whole API is stuck being type-unsafe unless we re-generify IterableSubject:
+       * b/145689657#comment1.
+       */
+      items.add(uncheckedCastNullableTToT(null));
     } else {
       items.addAll(asList(rest));
     }
@@ -195,15 +201,19 @@ protected int doHash(Object o) {
    * elements and even to output different elements on different lines.
    */
   static final class DuplicateGroupedAndTyped {
-    final NonHashingMultiset<?> valuesAndMaybeTypes;
-    final @Nullable String homogeneousTypeToDisplay;
+    private final NonHashingMultiset<?> valuesAndMaybeTypes;
+    private final @Nullable String homogeneousTypeToDisplay;
 
     DuplicateGroupedAndTyped(
         NonHashingMultiset<?> valuesAndMaybeTypes, @Nullable String homogeneousTypeToDisplay) {
       this.valuesAndMaybeTypes = valuesAndMaybeTypes;
       this.homogeneousTypeToDisplay = homogeneousTypeToDisplay;
     }
 
+    @Nullable String getHomogeneousTypeToDisplay() {
+      return homogeneousTypeToDisplay;
+    }
+
     int totalCopies() {
       return valuesAndMaybeTypes.totalCopies();
     }
@@ -257,10 +267,10 @@ public String toString() {
    * <p>Example: {@code hasMatchingToStringPair([1L, 2L], [1]) == true}
    */
   static boolean hasMatchingToStringPair(Iterable<?> items1, Iterable<?> items2) {
-    if (isEmpty(items1) || isEmpty(items2)) {
-      return false; // Bail early to avoid calling hashCode() on the elements unnecessarily.
-    }
-    return !retainMatchingToString(items1, items2).isEmpty();
+    // Bail early for empty iterables to avoid calling hashCode() on the elements unnecessarily.
+    return !isEmpty(items1)
+        && !isEmpty(items2)
+        && !retainMatchingToString(items1, items2).isEmpty();
   }
 
   static String objectToTypeName(@Nullable Object item) {