Make `upb_proto_aspect` reexport all `deps` when `srcs` is empty.

This special `proto_library` behavior is known as an *alias library*:
https://bazel.build/reference/be/protocol-buffer#proto_library_args

Without this CL, the compilation of the added test fails with:

```
test_import_empty_srcs.upb.c:12:10: error:
  module :test_import_empty_srcs_proto.upb does not depend on a
  module exporting 'test.upb.h'
```

PiperOrigin-RevId: 545153380

bazel/upb_proto_library.bzl

@@ -291,52 +291,65 @@ def _upb_proto_rule_impl(ctx):
     ]
 
 def _upb_proto_aspect_impl(target, ctx, generator, cc_provider, file_provider, provide_cc_shared_library_hints = True):
-    proto_info = target[ProtoInfo]
-    files = _compile_upb_protos(ctx, generator, proto_info, proto_info.direct_sources)
-    deps = ctx.rule.attr.deps + getattr(ctx.attr, "_" + generator)
-    dep_ccinfos = [dep[CcInfo] for dep in deps if CcInfo in dep]
-    dep_ccinfos += [dep[UpbWrappedCcInfo].cc_info for dep in deps if UpbWrappedCcInfo in dep]
-    dep_ccinfos += [dep[_UpbDefsWrappedCcInfo].cc_info for dep in deps if _UpbDefsWrappedCcInfo in dep]
-    if generator == "upbdefs":
-        if UpbWrappedCcInfo not in target:
-            fail("Target should have UpbWrappedCcInfo provider")
-        dep_ccinfos.append(target[UpbWrappedCcInfo].cc_info)
     name = ctx.rule.attr.name + "." + generator
     owners = [ctx.label.relative(name)]
-    cc_info = _cc_library_func(
-        ctx = ctx,
-        name = name,
-        hdrs = files.hdrs,
-        srcs = files.srcs,
-        includes = files.includes,
-        copts = ctx.attr._copts[UpbProtoLibraryCoptsInfo].copts,
-        dep_ccinfos = dep_ccinfos,
-    )
 
-    if files.thunks:
-        name_thunks = ctx.rule.attr.name + "." + generator + ".thunks"
-        owners.append(ctx.label.relative(name_thunks))
-        cc_info_with_thunks = _cc_library_func(
+    providers = []
+    if not getattr(ctx.rule.attr, "srcs", []):
+        # This target doesn't declare any sources, reexport all its deps instead.
+        deps = ctx.rule.attr.deps
+        providers += [cc_provider(cc_info = dep[CcInfo]) for dep in deps if CcInfo in dep]
+        providers += [dep[UpbWrappedCcInfo] for dep in deps if UpbWrappedCcInfo in dep]
+        providers += [dep[_UpbDefsWrappedCcInfo] for dep in deps if _UpbDefsWrappedCcInfo in dep]
+        providers += [dep[_UpbWrappedGeneratedSrcsInfo] for dep in deps if _UpbWrappedGeneratedSrcsInfo in dep]
+        providers += [dep[_WrappedDefsGeneratedSrcsInfo] for dep in deps if _WrappedDefsGeneratedSrcsInfo in dep]
+    else:
+        proto_info = target[ProtoInfo]
+        files = _compile_upb_protos(ctx, generator, proto_info, proto_info.direct_sources)
+        deps = ctx.rule.attr.deps + getattr(ctx.attr, "_" + generator)
+        dep_ccinfos = [dep[CcInfo] for dep in deps if CcInfo in dep]
+        dep_ccinfos += [dep[UpbWrappedCcInfo].cc_info for dep in deps if UpbWrappedCcInfo in dep]
+        dep_ccinfos += [dep[_UpbDefsWrappedCcInfo].cc_info for dep in deps if _UpbDefsWrappedCcInfo in dep]
+        if generator == "upbdefs":
+            if UpbWrappedCcInfo not in target:
+                fail("Target should have UpbWrappedCcInfo provider")
+            dep_ccinfos.append(target[UpbWrappedCcInfo].cc_info)
+
+        cc_info = _cc_library_func(
             ctx = ctx,
-            name = name_thunks,
-            hdrs = [],
-            srcs = files.thunks,
+            name = name,
+            hdrs = files.hdrs,
+            srcs = files.srcs,
             includes = files.includes,
             copts = ctx.attr._copts[UpbProtoLibraryCoptsInfo].copts,
-            dep_ccinfos = dep_ccinfos + [cc_info],
-        )
-        wrapped_cc_info = cc_provider(
-            cc_info = cc_info,
-            cc_info_with_thunks = cc_info_with_thunks,
+            dep_ccinfos = dep_ccinfos,
         )
-    else:
-        wrapped_cc_info = cc_provider(
-            cc_info = cc_info,
-        )
-    providers = [
-        wrapped_cc_info,
-        file_provider(srcs = files),
-    ]
+
+        if files.thunks:
+            name_thunks = ctx.rule.attr.name + "." + generator + ".thunks"
+            owners.append(ctx.label.relative(name_thunks))
+            cc_info_with_thunks = _cc_library_func(
+                ctx = ctx,
+                name = name_thunks,
+                hdrs = [],
+                srcs = files.thunks,
+                includes = files.includes,
+                copts = ctx.attr._copts[UpbProtoLibraryCoptsInfo].copts,
+                dep_ccinfos = dep_ccinfos + [cc_info],
+            )
+            wrapped_cc_info = cc_provider(
+                cc_info = cc_info,
+                cc_info_with_thunks = cc_info_with_thunks,
+            )
+        else:
+            wrapped_cc_info = cc_provider(
+                cc_info = cc_info,
+            )
+        providers += [
+            wrapped_cc_info,
+            file_provider(srcs = files),
+        ]
+
     if provide_cc_shared_library_hints:
         if hasattr(cc_common, "CcSharedLibraryHintInfo"):
             providers.append(cc_common.CcSharedLibraryHintInfo(owners = owners))

upb/test/BUILD

@@ -209,3 +209,32 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+proto_library(
+    name = "empty_srcs_proto",
+    testonly = 1,
+    srcs = [],
+    deps = [":test_proto"],
+)
+
+proto_library(
+    name = "test_import_empty_srcs_proto",
+    testonly = 1,
+    srcs = ["test_import_empty_srcs.proto"],
+    deps = [":empty_srcs_proto"],
+)
+
+upb_proto_library(
+    name = "test_import_empty_srcs_upb_proto",
+    testonly = 1,
+    deps = [":test_import_empty_srcs_proto"],
+)
+
+cc_test(
+    name = "test_import_empty_srcs",
+    srcs = ["test_import_empty_srcs.cc"],
+    deps = [
+        ":test_import_empty_srcs_upb_proto",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

upb/test/test_import_empty_srcs.cc

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2009-2021, Google LLC
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of Google LLC nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL Google LLC BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "gtest/gtest.h"
+#include "upb/test/test_import_empty_srcs.upb.h"
+
+TEST(Test, Reexport) {
+  // This test really just ensures that compilation succeeds.
+  ASSERT_GT(sizeof(upb_test_ContainsImported_msg_init), 0);
+}

upb/test/test_import_empty_srcs.proto

@@ -0,0 +1,9 @@
+syntax = "proto2";
+
+package upb_test;
+
+import "upb/test/test.proto";
+
+message ContainsImported {
+  optional MessageName message_name = 1;
+}