Add simple conformance test that builds the old gencode against the current runtime.

PiperOrigin-RevId: 631486123

WORKSPACE

@@ -226,3 +226,18 @@ crates_repository(
 
 load("@crate_index//:defs.bzl", "crate_repositories")
 crate_repositories()
+
+# For testing runtime against old gencode from a previous major version.
+http_archive(
+    name = "com_google_protobuf_v25.0",
+    strip_prefix = "protobuf-25.0",
+    url = "https://github.com/protocolbuffers/protobuf/releases/download/v25.0/protobuf-25.0.tar.gz",
+)
+
+# Needed as a dependency of @com_google_protobuf_v25.x, which was before
+# utf8_range was merged in.
+http_archive(
+    name = "utf8_range",
+    strip_prefix = "utf8_range-d863bc33e15cba6d873c878dcca9e6fe52b2f8cb",
+    url = "https://github.com/protocolbuffers/utf8_range/archive/d863bc33e15cba6d873c878dcca9e6fe52b2f8cb.zip",
+)

compatibility/BUILD.bazel

@@ -0,0 +1,20 @@
+# Simple build tests for compatibility of gencode from previous major versions
+# with the current runtime.
+#
+# To add more test cases in Java, use java_runtime_conformance as below, and add
+# the corresponding http_archive in the WORKSPACE file for the version.
+
+load("//compatibility:runtime_conformance.bzl", "java_runtime_conformance")
+
+# main gencode builds with main runtime as a proof of concept.
+java_runtime_conformance(
+    name = "java_conformance_main",
+    gencode_version = "main",
+)
+
+# Generates a build_test named "conformance_v3.25.0"
+java_runtime_conformance(
+    name = "java_conformance_v3.25.0",
+    gencode_version = "3.25.0",
+    tags = ["manual"],
+)

compatibility/runtime_conformance.bzl

@@ -0,0 +1,53 @@
+"""Provides a rule to generate a conformance test for a given version of the gencode."""
+
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+
+def java_runtime_conformance(name, gencode_version, tags = []):
+    """Generates a conformance test for the given version of the runtime.
+
+    For example, runtime_conformance("3.19.4") will generate a build test named "conformance_v3.19.4"
+    that will fail if the runtime at that version fails to compile the unittest proto.
+
+    Args:
+        name: The name of the test.
+        gencode_version: The version of the runtime to test.
+        tags: Any tags to apply to the generated test.
+    """
+
+    if gencode_version == "main":
+        protoc = "//:protoc"
+    else:
+        minor = gencode_version[gencode_version.find(".") + 1:]
+        protoc = Label("@com_google_protobuf_v%s//:protoc" % minor)
+
+    gencode = [
+        "v%s/protobuf_unittest/UnittestProto.java" % gencode_version,
+        "v%s/com/google/protobuf/test/UnittestImport.java" % gencode_version,
+        "v%s/com/google/protobuf/test/UnittestImportPublic.java" % gencode_version,
+    ]
+    native.genrule(
+        name = "unittest_proto_gencode_v" + gencode_version,
+        srcs = [
+            "//src/google/protobuf:unittest_proto_srcs",
+        ],
+        outs = gencode,
+        cmd = "$(location %s) " % protoc +
+              "$(locations //src/google/protobuf:unittest_proto_srcs) " +
+              " -Isrc/ --java_out=$(@D)/v%s" % gencode_version,
+        tools = [protoc],
+    )
+
+    conformance_name = "conformance_v" + gencode_version
+    conformance_lib_name = conformance_name + "_lib"
+    native.java_library(
+        name = conformance_lib_name,
+        srcs = gencode,
+        deps = ["//java/core"],
+        tags = tags,
+    )
+
+    build_test(
+        name = name,
+        targets = [":" + conformance_lib_name],
+        tags = tags,
+    )

src/google/protobuf/BUILD.bazel

@@ -840,6 +840,16 @@ filegroup(
     visibility = ["//:__subpackages__"],
 )
 
+filegroup(
+    name = "unittest_proto_srcs",
+    srcs = [
+        "unittest.proto",
+        "unittest_import.proto",
+        "unittest_import_public.proto",
+    ],
+    visibility = ["//:__subpackages__"],
+)
+
 filegroup(
     name = "test_proto_editions_srcs",
     srcs = [