## Summary Stacked on #2941. After the rustc bump, `snap-build` gets past `maturin` and fails on `protobuf`: ``` ModuleNotFoundError: No module named 'pkg_resources' ERROR: Failed to build 'protobuf' when getting requirements to build wheel ``` The snap builds everything from source (`PIP_NO_BINARY: ":all:"`), so pip also creates protobuf's isolated build environment. protobuf 3.20.3's `setup.py` imports `pkg_resources`, which setuptools removed in 81; current setuptools is 84, so that sdist cannot build. pip also ignores constraints files for build-isolation, so the build environment cannot be pinned from the repo (verified: `--constraint setuptools<81` still installed setuptools 84.0.0 into the isolated env). protobuf 3.20.3 -> 4.25.8 is API compatible: the 4.x line of that era is a re-versioning of the 3.20 code, and `macaroonbakery` (the only consumer) just requires `protobuf >=3.20.0`. Verified locally that the 4.25.8 sdist builds fully on Python 3.10 with setuptools 84. The pure-python `protobuf-3.20.3-py2.py3-none-any.whl` could be pulled in with `PIP_ONLY_BINARY: protobuf` to keep the old version, but that ships the slower pure-Python implementation to avoid a version bump, so it was rejected. Leaving 3.20.3 was not an option either, since it cannot be built from source at all now, and the snap has no wheel for it on armhf, ppc64el, riscv64 or s390x. ## Test plan CI: `snap-build` completes the `charmcraft` part with `protobuf` compiled from source, which unblocks `snap-tests`. > [!WARNING] > Generated by an automated agent, not a human. 🤖 **BEEP BOOP I'M A TOASTER** _Model:_ `step-5-preview-free` _Harness:_ opencode CLI