Add arch test for preview API usage

pom.xml

@@ -410,6 +410,12 @@
       <artifactId>commons-lang3</artifactId>
       <version>3.9</version>
     </dependency>
+    <dependency>
+      <groupId>com.tngtech.archunit</groupId>
+      <artifactId>archunit</artifactId>
+      <version>0.14.1</version>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.hamcrest</groupId>
       <artifactId>hamcrest</artifactId>

src/test/java/org/kohsuke/github/ArchTests.java

@@ -0,0 +1,56 @@
+package org.kohsuke.github;
+
+import com.tngtech.archunit.core.domain.JavaClass;
+import com.tngtech.archunit.core.domain.JavaClasses;
+import com.tngtech.archunit.core.domain.properties.HasAnnotations;
+import com.tngtech.archunit.core.domain.properties.HasName.AndFullName;
+import com.tngtech.archunit.core.importer.ClassFileImporter;
+import com.tngtech.archunit.core.importer.ImportOption;
+import com.tngtech.archunit.lang.ArchCondition;
+import com.tngtech.archunit.lang.ArchRule;
+import com.tngtech.archunit.lang.ConditionEvents;
+import com.tngtech.archunit.lang.SimpleConditionEvent;
+import org.junit.Test;
+
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes;
+
+public class ArchTests {
+
+    private final JavaClasses classFiles = new ClassFileImporter()
+            .withImportOption(new ImportOption.DoNotIncludeTests())
+            .withImportOption(new ImportOption.DoNotIncludeJars())
+            .importPackages("org.kohsuke.github");
+
+    @Test
+    public void testPreviewsAreFlaggedAsDeprecated() {
+
+        String description = "annotate all preview APIs as @Deprecated until they are promoted to stable";
+
+        ArchRule rule = classes().should(new ArchCondition<JavaClass>(description) {
+
+            @Override
+            public void check(final JavaClass targetClazz, final ConditionEvents events) {
+                checkForPreviewAnnotation(targetClazz, events);
+                targetClazz.getAllMethods().forEach(method -> {
+                    checkForPreviewAnnotation(method, events);
+                });
+            }
+
+            <T extends HasAnnotations<T> & AndFullName> void checkForPreviewAnnotation(T codeTarget,
+                    ConditionEvents events) {
+
+                if (codeTarget.tryGetAnnotationOfType(Preview.class).isPresent()
+                        && !codeTarget.tryGetAnnotationOfType(Deprecated.class).isPresent()) {
+
+                    String message = codeTarget.getFullName()
+                            + " uses a preview API and is missing the '@Deprecated' annotation.";
+
+                    events.add(new SimpleConditionEvent(codeTarget, false, message));
+                }
+            }
+        });
+
+        rule.check(classFiles);
+
+    }
+}