Introduce @Contract annotation

Author: marcphilipp

gradle/plugins/common/src/main/kotlin/junitbuild.java-nullability-conventions.gradle.kts

@@ -54,6 +54,8 @@ tasks.withType<JavaCompile>().configureEach {
 				enable()
 			}
 			isJSpecifyMode = true
+			customContractAnnotations.add("org.junit.platform.commons.annotation.Contract")
+			checkContracts = true
 		}
 	}
 }

junit-platform-commons/src/main/java/org/junit/platform/commons/annotation/Contract.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2015-2025 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.platform.commons.annotation;
+
+import static org.apiguardian.api.API.Status.INTERNAL;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Target;
+
+import org.apiguardian.api.API;
+
+/**
+ * Specifies some aspects of the annotated method's behavior to be used by tools
+ * for data flow analysis.
+ *
+ * @since 6.0
+ * @see <a href="https://github.com/JetBrains/java-annotations/blob/2a28eab73042023559d2ec4cd00d6779213b6425/src/jvmMain/java/org/jetbrains/annotations/Contract.java">org.jetbrains.annotations.Contract</a>
+ * @see <a href="https://github.com/uber/NullAway/wiki/Configuration#custom-contract-annotations">NullAway custom contract annotations</a>
+ */
+@Documented
+@Target(ElementType.METHOD)
+@API(status = INTERNAL, since = "6.0")
+public @interface Contract {
+
+	/**
+	 * Contains the contract clauses describing causal relations between call
+	 * arguments and the returned value.
+	 */
+	String value();
+
+}