cc_proto_library runtime not linked with --incompatible_enable_proto_toolchain_resolution + dynamic_deps

[nigelgbanks]

Summary

When using --incompatible_enable_proto_toolchain_resolution with a cc_binary that uses
dynamic_deps, the protobuf runtime library is not linked - even if the proto is in the
static deps and the shared library is completely unrelated to protobuf.

Minimal Reproduction

Prerequisites

• Bazel 8.x

Steps

bazel build //:main_dynamic

Expected Result

Build succeeds.

Actual Result

Linker fails with undefined references to protobuf and abseil symbols:

undefined reference to 'google::protobuf::Message::kDescriptorMethods'
undefined reference to 'google::protobuf::internal::TcParser::GenericFallback(...)'
undefined reference to 'absl::lts_20250127::log_internal::MakeCheckOpString<...>(...)'

Key Findings

The issue requires both conditions:

1. --incompatible_enable_proto_toolchain_resolution enabled
2. dynamic_deps attribute used (even with an unrelated shared library)

The shared library does NOT need to depend on protobuf. Simply having any dynamic_deps
with the toolchain resolution flag causes the protobuf runtime to be excluded from linking.

Target	dynamic_deps	Result
//:main	No	✓ Works
//:main_dynamic	Yes (unrelated)	✗ Fails

Analysis

When --incompatible_enable_proto_toolchain_resolution is enabled, cc_proto_library
obtains the protobuf runtime via toolchain resolution (proto_lang_toolchain.runtime)
rather than as a direct attribute dependency.

This toolchain-provided runtime is not properly propagated to the linker when the
binary uses dynamic_deps - even when the shared library has no relationship to protobuf.

Workaround

Disable the flag:

bazel build //target --noincompatible_enable_proto_toolchain_resolution

Or explicitly include @protobuf as a dependency.

Environment

• Bazel: 8.5
• Protobuf: 33.4
• rules_cc: 0.2.11
• OS: Linux

Files

issue.zip

File	Description
MODULE.bazel	Bazel module dependencies
.bazelrc	Enables --incompatible_enable_proto_toolchain_resolution
BUILD.bazel	Two targets: main (works) and main_dynamic (fails)
example.proto	Simple proto with a Person message
main.cc	C++ binary that uses the proto
unrelated_lib.cc	Unrelated library used by the shared library