docs: add documentation about Rust

This makes the features more discoverable for users.

Differential Revision: https://phab.mercurial-scm.org/D11808

mercurial/helptext/rust.txt

@@ -0,0 +1,94 b''
+Mercurial can be augmented with Rust extensions for speeding up certain
+operations.
+
+Compatibility
+=============
+
+Though the Rust extensions are only tested by the project under Linux, users of
+MacOS, FreeBSD and other UNIX-likes have been using the Rust extensions. Your
+mileage may vary, but by all means do give us feedback or signal your interest
+for better support.
+
+No Rust extensions are available for Windows at this time.
+
+Features
+========
+
+The following operations are sped up when using Rust:
+    - discovery of differences between repositories (pull/push)
+    - nodemap (see :hg:`help config.format.use-persistent-nodemap`)
+    - all commands using the dirstate (status, commit, diff, add, update, etc.)
+    - dirstate-v2 (see :hg:`help config.format.exp-rc-dirstate-v2`)
+    - iteration over ancestors in a graph
+
+More features are in the works, and improvements on the above listed are still
+in progress. For more experimental work see the "rhg" section.
+
+Checking for Rust
+=================
+
+You may already have the Rust extensions depending on how you install Mercurial.
+
+  $ hg debuginstall | grep -i rust
+  checking Rust extensions (installed)
+  checking module policy (rust+c-allow)
+
+If those lines don't even exist, you're using an old version of `hg` which does
+not have any Rust extensions yet.
+
+Installing
+==========
+
+You will need `cargo` to be in your `$PATH`. See the "MSRV" section for which
+version to use.
+
+Using pip
+---------
+
+Users of `pip` can install the Rust extensions with the following command:
+
+  $ pip install mercurial --global-option --rust --no-use-pep517
+
+`--no-use-pep517` is here to tell `pip` to preserve backwards compatibility with
+the legacy `setup.py` system. Mercurial has not yet migrated its complex setup
+to the new system, so we still need this to add compiled extensions.
+
+This might take a couple of minutes because you're compiling everything.
+
+See the "Checking for Rust" section to see if the install succeeded.
+
+From your distribution
+----------------------
+
+Some distributions are shipping Mercurial with Rust extensions enabled and
+pre-compiled (meaning you won't have to install `cargo`), or allow you to
+specify an install flag. Check with your specific distribution for how to do
+that, or ask their team to add support for hg+Rust!
+
+From source
+-----------
+
+Please refer to the `rust/README.rst` file in the Mercurial repository for
+instructions on how to install from source.
+
+MSRV
+====
+
+The minimum supported Rust version is currently 1.48.0. The project's policy is
+to follow the version from Debian stable, to make the distributions' job easier.
+
+rhg
+===
+
+There exists an experimental pure-Rust version of Mercurial called `rhg` with a
+fallback mechanism for unsupported invocations. It allows for much faster
+execution of certain commands while adding no discernable overhead for the rest.
+
+The only way of trying it out is by building it from source. Please refer to
+`rust/README.rst` in the Mercurial repository.
+
+Contributing
+============
+
+If you would like to help the Rust endeavor, please refer to `rust/README.rst`
+in the Mercurial repository.

mercurial/help.py

@@ -484,6 +484,15 b' helptable = sorted('
             TOPIC_CATEGORY_IDS,
         ),
         (
+            [
+                b'rust',
+                b'rustext',
+            ],
+            _(b'Rust in Mercurial'),
+            loaddoc(b'rust'),
+            TOPIC_CATEGORY_CONFIG,
+        ),
+        (
             [b'filesets', b'fileset'],
             _(b"Specifying File Sets"),
             loaddoc(b'filesets'),

tests/test-globalopts.t

@@ -414,6 +414,7 b' Testing -h/--help:'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   
@@ -548,6 +549,7 b' Testing -h/--help:'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   

tests/test-help-hide.t

@@ -113,6 +113,7 b' Test hiding some commands (which also ha'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   
@@ -251,6 +252,7 b' Test hiding some topics.'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   

tests/test-help.t

@@ -165,6 +165,7 b' the extension is unknown.'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   
@@ -295,6 +296,7 b' the extension is unknown.'
    hgweb         Configuring hgweb
    merge-tools   Merge Tools
    pager         Pager Support
+   rust          Rust in Mercurial
   
   Concepts:
   
@@ -1583,6 +1585,8 b' Separate sections from subsections'
   
       "usefncache"
   
+      "exp-rc-dirstate-v2"
+  
       "use-persistent-nodemap"
   
       "use-share-safe"
@@ -2413,6 +2417,13 b' Dish up an empty repo; serve it cold.'
   Specifying Revisions
   </td></tr>
   <tr><td>
+  <a href="/help/rust">
+  rust
+  </a>
+  </td><td>
+  Rust in Mercurial
+  </td></tr>
+  <tr><td>
   <a href="/help/scripting">
   scripting
   </a>

tests/test-hgweb-json.t

@@ -2324,6 +2324,10 b' help/ shows help topics'
         "topic": "revisions"
       },
       {
+        "summary": "Rust in Mercurial",
+        "topic": "rust"
+      },
+      {
         "summary": "Using Mercurial from scripts and automation",
         "topic": "scripting"
       },