Allow `rust_proto_library` aspect to handle targets with empty `srcs`

There is an old convention that a `proto_library` target with empty `srcs`
should re-export its dependencies. This is sometimes useful as a way of
creating something like an alias target, except that it can point to multiple
dependencies rather than just one.

The `rust_proto_library` aspect currently cannot handle this pattern and will
raise an error if it encounters a `proto_library` without `srcs`. This CL fixes
that problem by updating the `RustProtoInfo` provider generated by the aspect
to include a list of `DepVariantInfo` instead of just one. In the typical case
the provider will have just one `DepVariantInfo`, but this gives us the ability
to return more than one in the case where we're exporting all the dependencies
of a `src`-less `proto_library`.

One thing this CL specifically does not attempt to do is add support for a
`rust_proto_library` directly wrapping a `proto_library` without `srcs`. The
reason for that is that it appears difficult to do in a way that would respect
the layering check. It's also not obvious that we would want to encourage this
pattern anyway. It's still valuable to support `src`-less `proto_library`
targets when they're somewhere else in the transitive dependencies. This way
you can freely create a `rust_proto_library` without having to fix up all your
transitive deps first.

PiperOrigin-RevId: 687344284

rust/aspects.bzl

@@ -24,8 +24,8 @@ CrateMappingInfo = provider(
 RustProtoInfo = provider(
     doc = "Rust protobuf provider info",
     fields = {
-        "dep_variant_info": "DepVariantInfo for the compiled Rust gencode (also covers its " +
-                            "transitive dependencies)",
+        "dep_variant_infos": "List of DepVariantInfo for the compiled Rust " +
+                             "gencode (also covers its transitive dependencies)",
         "crate_mapping": "depset(CrateMappingInfo) containing mappings of all transitive " +
                          "dependencies of the current proto_library.",
     },
@@ -344,25 +344,33 @@ def _rust_proto_aspect_common(target, ctx, is_upb):
     )
     dep_variant_info_for_native_gencode = DepVariantInfo(cc_info = thunks_cc_info)
 
-    dep_variant_info = _compile_rust(
-        ctx = ctx,
-        attr = ctx.rule.attr,
-        src = gencode[0],
-        extra_srcs = gencode[1:],
-        deps = [dep_variant_info_for_runtime, dep_variant_info_for_native_gencode] + (
-            [d[RustProtoInfo].dep_variant_info for d in proto_deps]
-        ),
-    )
-    return [RustProtoInfo(
-        dep_variant_info = dep_variant_info,
-        crate_mapping = depset(
-            direct = [CrateMappingInfo(
-                crate_name = label_to_crate_name(ctx, target.label, toolchain),
-                import_paths = tuple([get_import_path(f) for f in proto_srcs]),
-            )],
-            transitive = transitive_crate_mappings,
-        ),
-    )]
+    dep_variant_infos = []
+    for info in [d[RustProtoInfo].dep_variant_infos for d in proto_deps]:
+        dep_variant_infos += info
+
+    if proto_srcs:
+        dep_variant_info = _compile_rust(
+            ctx = ctx,
+            attr = ctx.rule.attr,
+            src = gencode[0],
+            extra_srcs = gencode[1:],
+            deps = [dep_variant_info_for_runtime, dep_variant_info_for_native_gencode] + dep_variant_infos,
+        )
+        return [RustProtoInfo(
+            dep_variant_infos = [dep_variant_info],
+            crate_mapping = depset(
+                direct = [CrateMappingInfo(
+                    crate_name = label_to_crate_name(ctx, target.label, toolchain),
+                    import_paths = tuple([get_import_path(f) for f in proto_srcs]),
+                )],
+                transitive = transitive_crate_mappings,
+            ),
+        )]
+    else:
+        return [RustProtoInfo(
+            dep_variant_infos = dep_variant_infos,
+            crate_mapping = depset(transitive = transitive_crate_mappings),
+        )]
 
 def _make_proto_library_aspect(is_upb):
     return aspect(

rust/defs.bzl

@@ -81,7 +81,12 @@ def _rust_proto_library_impl(ctx):
     dep = deps[0]
     rust_proto_info = dep[RustProtoInfo]
 
-    dep_variant_info = rust_proto_info.dep_variant_info
+    if len(rust_proto_info.dep_variant_infos) != 1:
+        fail(
+            "{}: rust_proto_library does not support src-less proto_library targets."
+                .format(_user_visible_label(ctx)),
+        )
+    dep_variant_info = rust_proto_info.dep_variant_infos[0]
     crate_info = dep_variant_info.crate_info
 
     # Change the crate name from the hame of the proto_library to the name of the rust_proto_library.

rust/test/rust_proto_library_unit_test/rust_proto_library_unit_test.bzl

@@ -78,7 +78,7 @@ def _relevant_linker_inputs(ltl):
     )
 
 def _find_linker_input(rust_proto_info, basename_substring):
-    cc_info = rust_proto_info.dep_variant_info.cc_info
+    cc_info = rust_proto_info.dep_variant_infos[0].cc_info
     for linker_input in cc_info.linking_context.linker_inputs.to_list():
         for ltl in linker_input.libraries:
             for file in _relevant_linker_inputs(ltl):