BreakingChanges: announce Rust becoming mandatory

BreakingChanges: announce Rust becoming mandatory

Over the last couple of years the appetite for bringing Rust into the
codebase has grown significantly across the developer base. Introducing
Rust is a major change though and has ramifications for the whole
ecosystem:

  - Some platforms have a Rust toolchain available, but have not yet
    integrated it into their build infrastructure.

  - Some platforms don't have any support for Rust at all.

  - Some platforms may have to figure out how to fit Rust into their
    bootstrapping sequence.

Due to this, and given that Git is a critical piece of infrastructure
for the whole industry, we cannot just introduce such a heavyweight
dependency without doing our due diligence.

Instead, preceding commits have introduced a test balloon into our build
infrastructure that convert one tiny subsystem to use Rust. For now,
using Rust to build that subsystem is entirely optional -- if no Rust
support is available, we continue to use the C implementation. This test
balloon has the intention to give distributions time and let them ease
into our adoption of Rust.

Having multiple implementations of the same subsystem is not sustainable
though, and the plan is to eventually be able to use Rust freely all
across our codebase. As such, there is the intent to make Rust become a
mandatory part of our build process.

Add an announcement to our breaking changes that Rust will become
mandatory in Git 3.0. A (very careful and non-binding) estimate might be
that this major release might be released in the second half of next
year, which should give distributors enough time to prepare for the
change.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>