Re: [PATCH RFC v2 0/7] Introduce Rust and announce that it will become mandatorty

> 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.

> 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
> 
> Thanks!
> 
> Patrick
> 
> ---
> Patrick Steinhardt (7):
>        meson: add infrastructure to build internal Rust library
>        Makefile: introduce infrastructure to build internal Rust library
>        help: report on whether or not Rust is enabled
>        rust: implement a test balloon via the "varint" subsystem
>        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 | 36 +++++++++++++++
>   Makefile                           | 47 ++++++++++++++++++-
>   ci/install-dependencies.sh         |  4 +-
>   ci/run-build-and-tests.sh          | 31 +++++--------
>   help.c                             |  6 +++
>   meson.build                        | 17 ++++++-
>   meson_options.txt                  |  2 +
>   src/lib.rs                         |  1 +
>   src/meson.build                    | 16 +++++++
>   src/varint.rs                      | 92 ++++++++++++++++++++++++++++++++++++++
>   14 files changed, 240 insertions(+), 31 deletions(-)
> 
> Range-diff versus v1:
> 
> 1:  4df400823c ! 1:  8f6e89bc7d meson: add infrastructure to build internal Rust library
>      @@ meson.build: version_def_h = custom_target(
>        
>       +libgit_libraries = [ ]
>       +
>      -+if meson.version().version_compare('>=1.9.0')
>      -+  rust_available = add_languages('rust', native: false, required: get_option('rust'))
>      -+else
>      -+  rust_available = false
>      -+endif
>      ++rust_available = add_languages('rust', native: false, required: get_option('rust'))
>       +rust_option = get_option('rust').disable_auto_if(not rust_available)
>      -+
>      -+if rust_option.allowed() and meson.version().version_compare('>=1.9.0')
>      ++if rust_option.allowed()
>       +  subdir('src')
>      ++  libgit_c_args += '-DWITH_RUST'
>       +endif
>       +
>        libgit = declare_dependency(
>      @@ src/lib.rs (new)
>       
>        ## src/meson.build (new) ##
>       @@
>      -+rustmod = import('rust')
>      -+
>       +libgit_rs = static_library('git_rs',
>       +  sources: [
>       +    'lib.rs',
>       +  ],
>      -+  rust_abi: 'c',
>      ++  rust_crate_type: 'staticlib',
>       +)
>      -+
>      -+rustmod.test('git-rs', libgit_rs)
>      -+
>       +libgit_libraries += libgit_rs
>      ++
>      ++# The 'rust' module was only introduced in Meson 1.0. Furthermore, the module
>      ++# does not seem to work on macOS as expected right now. As such, we only
>      ++# conditionally enable tests.
>      ++if meson.version().version_compare('>=1.0.0') and host_machine.system() != 'darwin'
>      ++  rustmod = import('rust')
>      ++  rustmod.test('rust', libgit_rs)
>      ++endif
> -:  ---------- > 2:  cd1d642d04 Makefile: introduce infrastructure to build internal Rust library
> -:  ---------- > 3:  e60a8353a4 help: report on whether or not Rust is enabled
> 2:  575f6de44d ! 4:  b27811aea4 rust: implement a test balloon via the "varint" subsystem
>      @@ Commit message
>       
>           Signed-off-by: Patrick Steinhardt <ps@pks.im>
>       
>      + ## Makefile ##
>      +@@ Makefile: LIB_OBJS += urlmatch.o
>      + LIB_OBJS += usage.o
>      + LIB_OBJS += userdiff.o
>      + LIB_OBJS += utf8.o
>      ++ifndef WITH_RUST
>      + LIB_OBJS += varint.o
>      ++endif
>      + LIB_OBJS += version.o
>      + LIB_OBJS += versioncmp.o
>      + LIB_OBJS += walker.o
>      +
>        ## meson.build ##
>       @@ meson.build: libgit_sources = [
>          'usage.c',
>      @@ meson.build: libgit_sources = [
>          'versioncmp.c',
>          'walker.c',
>       @@ meson.build: rust_option = get_option('rust').disable_auto_if(not rust_available)
>      -
>      - if rust_option.allowed() and meson.version().version_compare('>=1.9.0')
>      + if rust_option.allowed()
>          subdir('src')
>      +   libgit_c_args += '-DWITH_RUST'
>       +else
>       +  libgit_sources += [
>       +    'varint.c',
>      @@ src/lib.rs
>       +pub mod varint;
>       
>        ## src/meson.build ##
>      -@@ src/meson.build: rustmod = import('rust')
>      +@@
>        libgit_rs = static_library('git_rs',
>          sources: [
>            'lib.rs',
>       +    'varint.rs',
>          ],
>      -   rust_abi: 'c',
>      +   rust_crate_type: 'staticlib',
>        )
>       
>        ## src/varint.rs (new) ##
> 3:  e54393392a < -:  ---------- BreakingChanges: announce Rust becoming mandatory
> -:  ---------- > 5:  d5946e0114 BreakingChanges: announce Rust becoming mandatory
> -:  ---------- > 6:  0d367976de ci: convert "pedantic" job into full build with breaking changes
> -:  ---------- > 7:  82086a5328 ci: enable Rust for breaking-changes jobs
> 
> ---
> base-commit: 2462961280690837670d997bde64bd4ebf8ae66d
> change-id: 20250904-b4-pks-rust-breaking-change-7167d9d3e37d
> 
>