Enable RequireExplicitNullMarking

As part of this change, the assertions on the configuration of Error
Prone have been split up. This was necessary as the ordering of the
checks map is undefined which results in the toString() output of
ErrorProneOptions that's used in the tests' build.gradle script being
non-deterministic.

Closes gh-23

Author: wilkinsona

src/main/java/io/spring/gradle/nullability/NullabilityOptions.java

@@ -74,7 +74,7 @@ private Map<String, String> checkOptions(Checking checking) {
 
 	private Map<String, CheckSeverity> checks(Checking checking) {
 		if (checking != Checking.DISABLED) {
-			return Map.of("NullAway", CheckSeverity.ERROR);
+			return Map.of("NullAway", CheckSeverity.ERROR, "RequireExplicitNullMarking", CheckSeverity.ERROR);
 		}
 		return Collections.emptyMap();
 	}

src/test/java/io/spring/gradle/nullability/NullabilityPluginIntegrationTests.java

@@ -67,10 +67,12 @@ void usesCustomNullAwayVersion() {
 	@Test
 	void configuresErrorProneOnCompileJava() {
 		BuildResult result = this.gradleBuild.build("checkCompileJava");
-		assertThat(result.getOutput())
-			.contains("-XepDisableAllChecks -Xep:NullAway:ERROR -XepOpt:NullAway:OnlyNullMarked=true "
-					+ "-XepOpt:NullAway:CustomContractAnnotations=org.springframework.lang.Contract "
-					+ "-XepOpt:NullAway:JSpecifyMode=true");
+		assertThat(result.getOutput()).contains("-XepDisableAllChecks")
+			.contains("-Xep:NullAway:ERROR")
+			.contains("-Xep:RequireExplicitNullMarking:ERROR")
+			.contains("-XepOpt:NullAway:OnlyNullMarked=true")
+			.contains("-XepOpt:NullAway:CustomContractAnnotations=org.springframework.lang.Contract")
+			.contains("-XepOpt:NullAway:JSpecifyMode=true");
 	}
 
 	@Test
@@ -82,10 +84,15 @@ void disablesErrorProneOnCompileTestJavaByDefault() {
 	@Test
 	void configuresErrorProneOnCompileTestJavaWhenEnabled() {
 		BuildResult result = this.gradleBuild.build("checkCompileTestJava");
-		assertThat(result.getOutput()).contains(
-				"-XepDisableAllChecks -XepCompilingTestOnlyCode -Xep:NullAway:ERROR -XepOpt:NullAway:OnlyNullMarked=true "
-						+ "-XepOpt:NullAway:CustomContractAnnotations=org.springframework.lang.Contract,org.assertj.core.internal.annotation.Contract "
-						+ "-XepOpt:NullAway:JSpecifyMode=true -XepOpt:NullAway:HandleTestAssertionLibraries=true");
+		assertThat(result.getOutput()).contains("-XepDisableAllChecks")
+			.contains("-XepCompilingTestOnlyCode")
+			.contains("-Xep:NullAway:ERROR")
+			.contains("-Xep:RequireExplicitNullMarking:ERROR")
+			.contains("-XepOpt:NullAway:OnlyNullMarked=true")
+			.contains(
+					"-XepOpt:NullAway:CustomContractAnnotations=org.springframework.lang.Contract,org.assertj.core.internal.annotation.Contract")
+			.contains("-XepOpt:NullAway:JSpecifyMode=true")
+			.contains("-XepOpt:NullAway:HandleTestAssertionLibraries=true");
 	}
 
 	@Test
@@ -109,17 +116,48 @@ void compileFailsForNullabilityViolationInTestCodeWhenCheckingIsEnabled() throws
 		assertThat(result.getOutput()).contains("[NullAway] assigning @Nullable expression to @NonNull field");
 	}
 
-	private void writeSource(String sourceSetName) {
+	@Test
+	void compileFailsForCodeThatIsNotNullMarked() throws IOException {
+		Path pkg = createSrcDirectories("main");
+		writeExampleClass(pkg);
+		BuildResult result = this.gradleBuild.prepareRunner("compileJava").buildAndFail();
+		assertThat(result.getOutput()).contains("[RequireExplicitNullMarking]");
+	}
+
+	private Path createSrcDirectories(String sourceSetName) {
 		Path projectDir = this.gradleBuild.getProjectDir().toPath();
 		Path pkg = projectDir.resolve("src/%s/java/com/example".formatted(sourceSetName));
 		try {
 			Files.createDirectories(pkg);
+		}
+		catch (IOException ex) {
+			throw new UncheckedIOException(ex);
+		}
+		return pkg;
+	}
+
+	private void writeSource(String sourceSetName) {
+		Path pkg = createSrcDirectories(sourceSetName);
+		writePackageInfo(pkg);
+		writeExampleClass(pkg);
+	}
+
+	private void writePackageInfo(Path pkg) {
+		try {
 			Files.writeString(pkg.resolve("package-info.java"), """
 					@NullMarked
 					package com.example;
 
 					import org.jspecify.annotations.NullMarked;
 					""");
+		}
+		catch (IOException ex) {
+			throw new UncheckedIOException(ex);
+		}
+	}
+
+	private void writeExampleClass(Path pkg) {
+		try {
 			Files.writeString(pkg.resolve("Example.java"), """
 					package com.example;