Add conditional end-to-end integration test for unused deps

Author: JonathanPerry651

src/test/shell/bazel/bazel_java_test.sh

@@ -2531,4 +2531,81 @@ EOF
   expect_log "foo.txt"
 }
 
+function test_unused_deps() {
+  if [[ -n "${RULES_JAVA_OVERRIDE_PATH:-}" ]]; then
+    cat >> MODULE.bazel <<EOF
+local_path_override(
+    module_name = "rules_java",
+    path = "${RULES_JAVA_OVERRIDE_PATH}",
+)
+EOF
+  fi
+
+  # Check if rules_java has the toolchain-driven unused_deps support
+  # We check this by querying the attributes of java_package_configuration
+  # or attempting to build a package configuration target with 'unused_deps'.
+  # If rules_java does not have it, we skip the test.
+
+  mkdir -p pkg
+  cat > pkg/BUILD <<EOF
+load("@rules_java//java:defs.bzl", "java_package_configuration")
+java_package_configuration(
+    name = "test_config_query",
+    package_specs = [":spec_query"],
+    unused_deps = "error",
+)
+package_group(
+    name = "spec_query",
+    packages = ["//pkg/..."],
+)
+EOF
+
+  if ! bazel query //pkg:test_config_query >/dev/null 2>&1; then
+    echo "Skipping test_unused_deps: rules_java does not support unused_deps configuration yet"
+    return 0
+  fi
+
+  # Write the actual integration test targets
+  cat << 'EOF' > pkg/BUILD
+load("@rules_java//java:java_library.bzl", "java_library")
+load("@rules_java//toolchains:default_java_toolchain.bzl", "default_java_toolchain")
+load("@rules_java//java:defs.bzl", "java_package_configuration")
+
+package_group(
+    name = "my_package_spec",
+    packages = ["//pkg/..."],
+)
+
+java_package_configuration(
+    name = "unused_deps_error_config",
+    package_specs = [":my_package_spec"],
+    unused_deps = "error",
+)
+
+default_java_toolchain(
+    name = "java_toolchain",
+    package_configuration = [":unused_deps_error_config"],
+)
+
+java_library(name = "a", srcs = ["A.java"], deps = [":b"])
+java_library(name = "b", srcs = ["B.java"])
+EOF
+
+  cat << 'EOF' > pkg/A.java
+public class A {
+  // Does not reference B
+}
+EOF
+
+  cat << 'EOF' > pkg/B.java
+public class B {}
+EOF
+
+  bazel build //pkg:a \
+    --extra_toolchains=//pkg:java_toolchain_definition \
+    >& $TEST_log && fail "build succeeded, but expected it to fail due to unused dependency"
+
+  expect_log "Target '//pkg:b' is declared as a direct dependency of '//pkg:a' but is unused"
+}
+
 run_suite "Java integration tests"