doc/README

rust: switch hg-core and hg-cpython to rust 2018 edition

Many interesting changes have happened in Rust since the Oxidation Plan was
introduced, like the 2018 edition and procedural macros:

    - Opting in to the 2018 edition is a clear benefit in terms of future
      proofing, new (nice to have) syntactical sugar notwithstanding. It
      also has a new non-lexical, non-AST based borrow checker that has
      fewer bugs(!) and allows us to write correct code that in some cases
      would have been rejected by the old one.
    - Procedural macros allow us to use the PyO3 crate which maintainers have
      expressed the clear goal of compiling on stable, which would help in
      code maintainability compared to rust-cpython.

In this patch are the following changes:
    - Removing most `extern crate` uses
    - Updating `use` clauses (`crate` keyword, nested `use`)
    - Removing `mod.rs` in favor of an aptly named module file

Like discussed in the mailing list (
https://www.mercurial-scm.org/pipermail/mercurial-devel/2019-July/132316.html
), until Rust integration in Mercurial is considered to be out of the
experimental phase, the maximum version of Rust allowed is whatever the latest
version Debian packages.

Differential Revision: https://phab.mercurial-scm.org/D6597

Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:

  http://docutils.sourceforge.net/rst.html

It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:

  http://docutils.sourceforge.net/

Use the Makefile in this directory to generate the man and HTML pages.