Re: [PATCH 0/7] RFC: Accelerate xdiff and begin its rustification

[Patrick Steinhardt <ps@pks.im>]

On Sat, Jul 19, 2025 at 02:48:39AM +0200, Haelwenn (lanodan) Monnier wrote:
> [2025-07-18 17:25:01-0400] Eli Schwartz:
> > On 7/18/25 9:34 AM, Phillip Wood wrote:
> > > Hi Ezekiel
> > > 
> > > Thanks for working on this
> > > 
> > > On 17/07/2025 21:32, Ezekiel Newren via GitGitGadget wrote:
> > > 
> > > > So...
> > > > 
> > > > This obviously raises the question of whether we are ready to accept a
> > > > hard
> > > > dependency on Rust. Previous discussions on the mailing list and at Git
> > > > Merge 2024 have not answered that question. If not now, will we be
> > > > willing
> > > > to accept such a hard dependency later? And what route do we want to
> > > > take to
> > > > get there?
> > > 
> > > As far as git goes I think introducing a hard dependency on rust is
> > > fine. It is widely supported, the only issue I'm aware of is the lack of
> > > support on NonStop and I don't think it is reasonable for such a
> > > minority platform to hold the rest of the project to ransom. There is a
> > > question about the other users of the xdiff code though. libgit2 carries
> > > a copy as do other projects like neovim. I've cc'd the libgit2
> > > maintainer and posted a link to this thread in neovim github [1]
> > 
> > 
> > A hard dependency on rust for Gentoo amd64 would potentially require
> > building https://github.com/thepowersgang/mrustc followed by building 13
> > and counting versions of rustc in order to get to the latest version.
> > What is the minimum supported version in this series, by the way?
> > 
> > bin packages for rust do exist but not everyone wants to use non-distro
> > provided binaries, sometimes for auditability reasons.
> > 
> > 
> > For Gentoo HPPA, Alpha, m68k it will simply mean the removal (or end of
> > life and staying forever on 2.50, perhaps) of Git. There is no rust
> > compiler there.
> > 
> > Even s390 support for rust is limited to a precompiled version not
> > everyone is willing to use.
> 
> Also in other distro concerns, if it trickles down to libgit2,
> extra care should be taken to avoid creating circular dependencies
> due to cargo depending on libgit2 (via git2 crate).
> 
> For example with making sure it can reasonably be built via meson's
> Rust support rather than through cargo.

I think it's unlikely that this eventually trickles down into libgit2.
The bundled versions of xdiff have already diverged for a long time, and
unfortunately libgit2 is mostly in maintenance mode nowadays. So I guess
that this change here just means that things will diverge even further
in the future, which is probably okay-ish. After all, the whole xdiff
library didn't really evolve in a fast pace over the last years.

That being said, there is an xdiff fork located at [1] that libgit2
maintains nowadays. So if the Rust dependency ever became a problem for
any of the downstream users I think we could simply redirect them to
that fork and make it the canonical upstream for C-only xdiff.

Patrick

[1]: https://github.com/libgit2/xdiff