CONTRIBUTING

rust: new rust options in setup.py

The --rust global option turns on usage (and by default compilation)
of the rust-cpython based mercurial.rustext.

Similarly to what's previously done for zstd, there is a --no-rust
option for the build_ext subcommand in order not to build
mercurial.rustext, allowing for an OS distribution to prebuild it.

The HGWITHRUSTEXT environment variable is still honored, and has
the same effect as before, but now it works mostly by making
the --rust global option defaulting to True, with some special
cases for the direct-ffi case (see more about that below)

Coincidentally, the --rust flag can also be passed from the make
commands, like actually all global options, in the PURE variable

   make local PURE=--rust

This feels inappropriate, though, and we should follow up with
a proper make variable for that case.

Although the direct-ffi bindings aren't directly useful any more, we
keep them at this stage because

- they provide a short prototyping path for experiments in which a C extension
  module has to call into a Rust extension. The proper way of doing that would
  be to use capsules, and it's best to wait for our pull request onto
  rust-cpython for that: https://github.com/dgrunwald/rust-cpython/pull/169
- Build support for capsules defined in Rust will probably need to reuse
  some of what's currently in use for direct-ffi.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).