Latest Protobuf release now uses nodiscard, which causes compiler warnings

[Sir-Ace-git]

Description

comet:/usr/src/mumble/build# make
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[ 25%] Building CXX object src/CMakeFiles/shared.dir/MumbleProtocol.cpp.o
/usr/src/mumble/src/MumbleProtocol.cpp: In function ‘std::size_t Mumble::Protocol::encodeProtobuf(const google::protobuf::Message&, std::vector&, std::size_t, std::size_t, bool)’:
/usr/src/mumble/src/MumbleProtocol.cpp:105:48: error: ignoring return value of ‘bool google::protobuf::MessageLite::SerializePartialToArray(void*, int) const’, declared with attribute ‘nodiscard’ [-Werror=unused-result]
105 | message.SerializePartialToArray(buffer.data() + offset, static_cast< int >(serializedSize));
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/google/protobuf/generated_message_tctable_decl.h:24,
from /usr/src/mumble/build/src/MumbleUDP.pb.h:24,
from /usr/src/mumble/src/MumbleProtocol.h:9,
from /usr/src/mumble/src/MumbleProtocol.cpp:6:
/usr/include/google/protobuf/message_lite.h:770:44: note: declared here
770 | PROTOBUF_FUTURE_ADD_EARLY_NODISCARD bool SerializePartialToArray(
| ^~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[2]: *** [src/CMakeFiles/shared.dir/build.make:247: src/CMakeFiles/shared.dir/MumbleProtocol.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:3626: src/CMakeFiles/shared.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

Steps to reproduce

cmake .. -DPostgreSQL_INCLUDE_DIR=/usr/local/pgsql/include -DPostgreSQL_LIBRARY=/usr/local/pgsql/lib/libpq.so
make -j 16

Mumble version

v1.6.870

Mumble component

Both

OS

Linux

Reproducible?

Yes

Additional information

/usr/src/mumble/src/MumbleProtocol.cpp:105:48: error: ignoring return value of ‘bool google::protobuf::MessageLite::SerializePartialToArray(void*, int) const’, declared with attribute ‘nodiscard’ [-Werror=unused-result]
105 | message.SerializePartialToArray(buffer.data() + offset, static_cast< int >(serializedSize));

/usr/include/google/protobuf/message_lite.h:770:44: note: declared here
770 | PROTOBUF_FUTURE_ADD_EARLY_NODISCARD bool SerializePartialToArray(

Relevant log output

Screenshots

No response

[Krzmbrzl]

What compiler are you using?

[Sir-Ace-git]

comet:/usr/src/mumble/build# gcc -v
Reading specs from /usr/lib64/gcc/x86_64-slackware-linux/15.2.0/specs
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-slackware-linux/15.2.0/lto-wrapper
Target: x86_64-slackware-linux
Configured with: ../configure --prefix=/usr --libdir=/usr/lib64 --mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap --enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust,cobol --enable-threads=posix --enable-checking=release --with-system-zlib --enable-libstdcxx-dual-abi --with-default-libstdcxx-abi=new --disable-libstdcxx-pch --disable-libunwind-exceptions --enable-__cxa_atexit --disable-libssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-plugin --enable-lto --disable-install-libiberty --disable-werror --with-gnu-ld --with-isl --verbose --with-arch-directory=amd64 --disable-gtktest --enable-clocale=gnu --with-arch=x86-64 --enable-multilib --target=x86_64-slackware-linux --build=x86_64-slackware-linux --host=x86_64-slackware-linux
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.2.0 (GCC)

[Krzmbrzl]

Ah, actually a newer Protobuf update added the nodiscard specifier to that function, which is why you are getting this warning…
More specifically, this is the commit that introduced the attribute: protocolbuffers/protobuf@a70115f, which is part of the latest Protobuf release v34 (https://github.com/protocolbuffers/protobuf/releases/tag/v34.0)

For the time being, you can use -Dwarnings-as-errors=OFF as a wworkaround

[Sir-Ace-git]

That did the trick. Thank you for your quick reply!
That was the last step for a native build package for Slackware, so the last thing I need to do is bundle everything up and get them finalized for release.