Re: [PATCH v5 0/9] Introduce Rust and announce that it will become mandatory

>
> 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