rust: update README with new information about the Re2 path

Differential Revision: https://phab.mercurial-scm.org/D8452

rust/README.rst

@@ -1,119 +1,127 b''
 ===================
 Mercurial Rust Code
 ===================
 
 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.
 
 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): implementation of some
   functionality of mercurial in rust, e.g. ancestry computations in
   revision graphs, status or pull discovery. The top-level ``Cargo.toml`` file
   defines a workspace containing these crates.
 
 Using Rust code
 ===============
 
 Local use (you need to clean previous build artifacts if you have
 built without rust previously)::
 
   $ make PURE=--rust local # to use ./hg
   $ ./tests/run-tests.py --rust # to run all tests
   $ ./hg debuginstall | grep -i rust # to validate rust is in use
   checking Rust extensions (installed)
   checking module policy (rust+c-allow)
   checking "re2" regexp engine Rust bindings (installed)
 
 
 If the environment variable ``HGWITHRUSTEXT=cpython`` is set, the Rust
 extension will be used by default unless ``--no-rust``.
 
 One day we may use this environment variable to switch to new experimental
 binding crates like a hypothetical ``HGWITHRUSTEXT=hpy``.
 
 Using the fastest ``hg status``
 -------------------------------
 
 The code for ``hg status`` needs to conform to ``.hgignore`` rules, which are
 all translated into regex. 
 
 In the first version, for compatibility and ease of development reasons, the 
 Re2 regex engine was chosen until we figured out if the ``regex`` crate had
 similar enough behavior.
 
 Now that that work has been done, the default behavior is to use the ``regex``
 crate, that provides a significant performance boost compared to the standard 
 Python + C path in many commands such as ``status``, ``diff`` and ``commit``,
 
 However, the ``Re2`` path remains slightly faster for our use cases and remains
 a better option for getting the most speed out of your Mercurial. 
 
 If you want to use ``Re2``, you need to install ``Re2`` following Google's 
-guidelines: https://github.com/google/re2/wiki/Install
-Then, use ``HG_RUST_FEATURES=with-re2`` when building ``hg`` to use the full
-status code.
+guidelines: https://github.com/google/re2/wiki/Install.
+Then, use ``HG_RUST_FEATURES=with-re2`` and 
+``HG_RE2_PATH=system|<path to your re2 install>`` when building ``hg`` to 
+signal the use of Re2. Using the local path instead of the "system" RE2 links
+it statically.
+
+For example::
+
+  $ HG_RUST_FEATURES=with-re2 HG_RE2_PATH=system make PURE=--rust
+  $ # OR
+  $ HG_RUST_FEATURES=with-re2 HG_RE2_PATH=/path/to/re2 make PURE=--rust
 
 Developing Rust
 ===============
 
 The current version of Rust in use is ``1.34.2``, because it's what Debian
 stable has. You can use ``rustup override set 1.34.2`` at the root of the repo
 to make it easier on you.
 
 Go to the ``hg-cpython`` folder::
 
   $ cd rust/hg-cpython
 
 Or, only the ``hg-core`` folder. Be careful not to break compatibility::
 
   $ cd rust/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
 
 For even faster typing::
 
   $ cargo c
 
 You can run only the rust-specific tests (as opposed to tests of
 mercurial as a whole) with::
 
   $ cargo test --all
 
 Formatting the code
 -------------------
 
 We use ``rustfmt`` to keep the code formatted at all times. For now, we are
 using the nightly version because it has been stable enough and provides
 comment folding.
 
 To format the entire Rust workspace::
 
   $ cargo +nightly fmt
 
 This requires you to have the nightly toolchain installed.
 
 Additional features
 -------------------
 
 As mentioned in the section about ``hg status``, code paths using ``re2`` are
 opt-in.
 
 For example::
 
   $ cargo check --features with-re2