rust: add a README

rust: add a README

In particular to explain how to build any of the rust. It's neither
obvious, nor easy to find out, nor easy to determine if you did it
right without some documentation.

Differential Revision: https://phab.mercurial-scm.org/D7952

--- a/rust/README.rst	Mon Jan 20 17:44:03 2020 -0500
+++ b/rust/README.rst	Mon Jan 20 18:28:46 2020 -0500
@@ -3,17 +3,50 @@
 ===================
 
 This directory contains various Rust code for the Mercurial project.
+Rust is not required to use (or build) Mercurial, but using it
+improves performance in some areas.
 
-The top-level ``Cargo.toml`` file defines a workspace containing
-all primary Mercurial crates.
+There are currently three independent rust projects:
+- chg. An implementation of chg, in rust instead of C.
+- hgcli. A experiment for starting hg in rust rather than in python,
+  by linking with the python runtime. Probably meant to be replaced by
+  PyOxidizer at some point.
+- hg-core (and hg-cpython/hg-directffi): implementation of some
+  functionality of mercurial in rust, e.g. ancestry computations in
+  revision graphs or pull discovery. The top-level ``Cargo.toml`` file
+  defines a workspace containing these crates.
+
+Using hg-core
+=============
 
-Building
-========
+Local use (you need to clean previous build artifacts if you have
+built without rust previously)::
 
-To build the Rust components::
+  $ HGWITHRUSTEXT=cpython make local # to use ./hg
+  $ HGWITHRUSTEXT=cpython make tests # to run all tests
+  $ (cd tests; HGWITHRUSTEXT=cpython ./run-tests.py) # only the .t
+  $ ./hg debuginstall | grep rust # to validate rust is in use
+  checking module policy (rust+c-allow)
 
-   $ cargo build
+Setting ``HGWITHRUSTEXT`` to other values like ``true`` is deprecated
+and enables only a fraction of the rust code.
 
-If you prefer a non-debug / release configuration::
+Developing hg-core
+==================
+
+Simply run::
 
    $ cargo build --release
+
+It is possible to build without ``--release``, but it is not
+recommended if performance is of any interest: there can be an order
+of magnitude of degradation when removing ``--release``.
+
+For faster builds, you may want to skip code generation::
+
+  $ cargo check
+
+You can run only the rust-specific tests (as opposed to tests of
+mercurial as a whole) with::
+
+  $ cargo test --all

--- a/setup.py	Mon Jan 20 17:44:03 2020 -0500
+++ b/setup.py	Mon Jan 20 18:28:46 2020 -0500
@@ -1382,9 +1382,9 @@
 class RustEnhancedExtension(RustExtension):
     """A C Extension, conditionally enhanced with Rust code.
 
-    If the HGRUSTEXT environment variable is set to something else
-    than 'cpython', the Rust sources get compiled and linked within the
-    C target shared library object.
+    If the HGWITHRUSTEXT environment variable is set to something else
+    than 'cpython', the Rust sources get compiled and linked within
+    the C target shared library object.
     """
 
     def __init__(self, mpath, sources, rustlibname, subcrate, **kw):