Introduce CharSequenceRules Refaster rule collection

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/CharSequenceRules.java

@@ -0,0 +1,30 @@
+package tech.picnic.errorprone.refasterrules;
+
+import com.google.errorprone.refaster.Refaster;
+import com.google.errorprone.refaster.annotation.AfterTemplate;
+import com.google.errorprone.refaster.annotation.AlsoNegation;
+import com.google.errorprone.refaster.annotation.BeforeTemplate;
+import tech.picnic.errorprone.refaster.annotation.OnlineDocumentation;
+
+/** Refaster rules related to expressions dealing with {@link CharSequence}s. */
+@OnlineDocumentation
+final class CharSequenceRules {
+  private CharSequenceRules() {}
+
+  /**
+   * Prefer {@link CharSequence#isEmpty()} over alternatives that consult the char sequence's
+   * length.
+   */
+  static final class CharSequenceIsEmpty {
+    @BeforeTemplate
+    boolean before(CharSequence ch) {
+      return Refaster.anyOf(ch.length() == 0, ch.length() <= 0, ch.length() < 1);
+    }
+
+    @AfterTemplate
+    @AlsoNegation
+    boolean after(CharSequence ch) {
+      return ch.isEmpty();
+    }
+  }
+}

error-prone-contrib/src/test/java/tech/picnic/errorprone/refasterrules/RefasterRulesTest.java

@@ -36,6 +36,7 @@ final class RefasterRulesTest {
           AssortedRules.class,
           BigDecimalRules.class,
           BugCheckerRules.class,
+          CharSequenceRules.class,
           ClassRules.class,
           CollectionRules.class,
           ComparatorRules.class,

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/CharSequenceRulesTestInput.java

@@ -0,0 +1,16 @@
+package tech.picnic.errorprone.refasterrules;
+
+import com.google.common.collect.ImmutableSet;
+import tech.picnic.errorprone.refaster.test.RefasterRuleCollectionTestCase;
+
+final class CharSequenceRulesTest implements RefasterRuleCollectionTestCase {
+  ImmutableSet<Boolean> testCharSequenceIsEmpty() {
+    return ImmutableSet.of(
+        ((CharSequence) "foo").length() == 0,
+        ((CharSequence) "bar").length() <= 0,
+        ((CharSequence) "baz").length() < 1,
+        ((CharSequence) "foo").length() != 0,
+        ((CharSequence) "bar").length() > 0,
+        ((CharSequence) "baz").length() >= 1);
+  }
+}

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/CharSequenceRulesTestOutput.java

@@ -0,0 +1,16 @@
+package tech.picnic.errorprone.refasterrules;
+
+import com.google.common.collect.ImmutableSet;
+import tech.picnic.errorprone.refaster.test.RefasterRuleCollectionTestCase;
+
+final class CharSequenceRulesTest implements RefasterRuleCollectionTestCase {
+  ImmutableSet<Boolean> testCharSequenceIsEmpty() {
+    return ImmutableSet.of(
+        ((CharSequence) "foo").isEmpty(),
+        ((CharSequence) "bar").isEmpty(),
+        ((CharSequence) "baz").isEmpty(),
+        !((CharSequence) "foo").isEmpty(),
+        !((CharSequence) "bar").isEmpty(),
+        !((CharSequence) "baz").isEmpty());
+  }
+}