From: Patrick Steinhardt Date: Thu, 02 Oct 2025 07:29:25 GMT Subject: [PATCH v8 0/9] Introduce Rust and announce that it will become mandatory Message-ID: <20251002-b4-pks-rust-breaking-change-v8-0-3a89fd5b1ce7@pks.im> In-Reply-To: <20250904-b4-pks-rust-breaking-change-v1-0-3af1d25e0be9@pks.im> Hi, this small patch series introduces Rust into the core of Git. This patch series is designed as a test balloon, similar to how we introduced test balloons for C99 features in the past. The goal is threefold: - Give us some time to experiment with Rust and introduce proper build infrastructure. - Give distributors time to ease into the new toolchain requirements. Introducing Rust is impossible for some platforms and hard for others. - Announce that Git 3.0 will make Rust a mandatory part of our build infrastructure. The test balloon itself is quite uninteresting: I've chosen to convert the "varint.c" subsystem, mostly because it is trivial and does not have any dependencies. But it does allow us to verify that C to Rust interop works as expected, and to play around with tooling. All tests pass with the "varint.rs" implementation. For now, the series only contains support for Meson. If we agree to go down this route I'll also introduce support for Rust into our Makefiles at a later point in time. Furthermore missing is additional tooling: - At least one CI job to verify that Rust builds and works as expected. - Tooling and CI jobs to ensure that we have consistent formatting via `cargo format`. And probably lots more. As said, the entire goal is for us to have an easy playground that we can experiment on and develop the infrastructure incrementally without yet having to commit to anything. I'm mostly splitting out the topic of introducing Rust from the larger series that introduce it into xdiff so that we can focus more on the actual process of introducing Rust into Git and less on the potential features that we want to build on top of it. Changes in v2: - Introduce support for building the Rust library via our Makefile. - Introduce a '-DWITH_RUST' define. This define is used to print whether or not Git is built with Rust via `git version --build-options`. - Adjust Meson to not depend on v1.9.0 and newer anymore. - Introduce a roadmap into our BreakingChanges document to explain how we'll iterate towards mandatory Rust support. - Rework the Fedora job to do a full compile-and-test run with Meson and breaking changes enabled. - Adapt our breaking-changes jobs to enable Rust support. - Link to v1: https://lore.kernel.org/r/20250904-b4-pks-rust-breaking-change-v1-0-3af1d25e0be9@pks.im Changes in v3: - Reorder all uses of `WITH_RUST` after the include of "config.mak". - Add a test to verify overflow behaviour in Rust and explicitly use `add_wrapping()`. - Use explicit dependencies for the Rust library in our Makefile. - Fix Alma Linux CI job. - Stop tying maintenance of our LTS release to the availability of gcc-rs. - Add a fallback to Meson to use cargo directly. - I've fixed the Rust edition to 2018 for now. This is intentionally conservative so that we might be able to use Rust 1.49. For now, we don't have any reason to use a newer edition, either. So let's take the oldest version we can live with for now and then bump it as required. - Link to v2: https://lore.kernel.org/r/20250905-b4-pks-rust-breaking-change-v2-0-6939cbf4a0b8@pks.im Changes in v4: - Convert "varint.c" to use explicit integer width so that we don't need to use C types in Rust. - Adapt Meson to unconditionally use Cargo. - Don't use the unstable `--out-dir` option in Cargo. Instead, we resort to a wrapper script in Meson. - Shorten the timeline a bit to drop the extra step that ties Rust support to `-Dbreaking_changes=true`. This accelerates the timeline until distros are made forcibly aware of the upcoming changes in Rust. - Link to v3: https://lore.kernel.org/r/20250908-b4-pks-rust-breaking-change-v3-0-1cd7189fed3b@pks.im Changes in v5: - Fix indentation in the BreakingChanges document. - Fix a commit message typo. - Include "Cargo.lock" in the `make clean` target again. - Link to v4: https://lore.kernel.org/r/20250910-b4-pks-rust-breaking-change-v4-0-4a63fc69278d@pks.im Changes in v6: - Give attribution to Ezekiel for kickstarting the Rust adoption again. I'm happy to change how I do the attribution. - Fix "varint.rs" to use `u64` instead of `usize`. Issues like these will eventually be catched by cbindgen. - Adapt the breaking changes document to mention that we already have Rust in our tree starting with Git 2.49. - Mention that we won't blindly make Rust mandatory, but consider the impact on downstream distributions. - Slightly reword how we'll handle LTS maintainership. This probably still is an ongoing discussion. - Link to v5: https://lore.kernel.org/r/20250915-b4-pks-rust-breaking-change-v5-0-dc3a32fbb216@pks.im Changes in v7: - Rename "git" crate to "gitcore". - Some word smithing for the breaking changes doc. - Punt on the exact details of how we hand over maintenance of the LTS release to the community. This is something we can decide on in the future. - Link to v6: https://lore.kernel.org/r/20250923-b4-pks-rust-breaking-change-v6-0-59076fee486a@pks.im Changes in v8: - Some final typo fixes. - Link to v7: https://lore.kernel.org/r/20250925-b4-pks-rust-breaking-change-v7-0-4e49dcb904d5@pks.im Thanks! Patrick --- Patrick Steinhardt (9): meson: add infrastructure to build internal Rust library Makefile: reorder sources after includes Makefile: introduce infrastructure to build internal Rust library help: report on whether or not Rust is enabled varint: use explicit width for integers varint: reimplement as test balloon for Rust BreakingChanges: announce Rust becoming mandatory ci: convert "pedantic" job into full build with breaking changes ci: enable Rust for breaking-changes jobs .github/workflows/main.yml | 4 +- .gitignore | 2 + .gitlab-ci.yml | 4 +- Cargo.toml | 9 ++ Documentation/BreakingChanges.adoc | 45 ++++++++ Makefile | 214 ++++++++++++++++++++++--------------- ci/install-dependencies.sh | 8 +- ci/run-build-and-tests.sh | 31 ++---- dir.c | 18 ++-- help.c | 6 ++ meson.build | 15 ++- meson_options.txt | 2 + read-cache.c | 6 +- shared.mak | 1 + src/cargo-meson.sh | 32 ++++++ src/lib.rs | 1 + src/meson.build | 41 +++++++ src/varint.rs | 92 ++++++++++++++++ varint.c | 6 +- varint.h | 4 +- 20 files changed, 410 insertions(+), 131 deletions(-) Range-diff versus v7: 1: 3f916bebd4 = 1: bf7b33291d meson: add infrastructure to build internal Rust library 2: ed849dcfed = 2: 59e7879c63 Makefile: reorder sources after includes 3: 955f262ef5 = 3: 635cebc0a6 Makefile: introduce infrastructure to build internal Rust library 4: 7a90192b5a = 4: 43b50563cc help: report on whether or not Rust is enabled 5: 9365a78efd ! 5: 37d03d7774 varint: use explicit width for integers @@ Metadata ## Commit message ## varint: use explicit width for integers - The varint subsystem currently uses implcit widths for integers. On the + The varint subsystem currently uses implicit widths for integers. On the one hand we use `uintmax_t` for the actual value. On the other hand, we use `int` for the length of the encoded varint. - Both of these have known maximum vaules, as we only support at most 16 + Both of these have known maximum values, as we only support at most 16 bytes when encoding varints. Thus, we know that we won't ever exceed `uint64_t` for the actual value and `uint8_t` for the prefix length. 6: e7e0621b68 = 6: 0d265f9675 varint: reimplement as test balloon for Rust 7: 8d8e9cb8a8 = 7: a6e0d668f0 BreakingChanges: announce Rust becoming mandatory 8: 07dc8171ac = 8: 79470835fd ci: convert "pedantic" job into full build with breaking changes 9: 708a0d3c67 = 9: 67f8dea13f ci: enable Rust for breaking-changes jobs --- base-commit: 2462961280690837670d997bde64bd4ebf8ae66d change-id: 20250904-b4-pks-rust-breaking-change-7167d9d3e37d