D7952: rust: add a README

valentin.gatienbaron (Valentin Gatien-Baron) phabricator at mercurial-scm.org
Mon Jan 20 23:36:27 UTC 2020


valentin.gatienbaron created this revision.
Herald added subscribers: mercurial-devel, mjpieters.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  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.

REPOSITORY
  rHG Mercurial

BRANCH
  default

REVISION DETAIL
  https://phab.mercurial-scm.org/D7952

AFFECTED FILES
  rust/README.rst
  setup.py

CHANGE DETAILS

diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -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):
diff --git a/rust/README.rst b/rust/README.rst
--- a/rust/README.rst
+++ b/rust/README.rst
@@ -3,17 +3,41 @@
 ===================
 
 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``.



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