Deprecate rsync support

Fix pytest-dev#825

docs/index.rst

@@ -34,7 +34,7 @@ Features
 * ``--looponfail``: run your tests repeatedly in a subprocess.  After each run
   pytest waits until a file in your project changes and then re-runs
   the previously failing tests.  This is repeated until all tests pass
-  after which again a full run is performed.
+  after which again a full run is performed (DEPRECATED).
 
 * :ref:`Multi-Platform` coverage: you can specify different Python interpreters
   or different platforms and run tests in parallel on all of them.

docs/remote.rst

@@ -5,6 +5,17 @@
 Sending tests to remote SSH accounts
 ====================================
 
+.. deprecated:: 3.0
+
+.. warning::
+
+    This feature is deprecated because the support for ``rsync`` is faulty
+    in terms of reproducing the development environment in the remote
+    worker, and there is no clear solution moving forward.
+
+    This feature is scheduled to be removed in release 4.0, to let the team
+    focus on a smaller set of features.
+
 Suppose you have a package ``mypkg`` which contains some
 tests that you can successfully run locally. And you
 have a ssh-reachable machine ``myhost``.  Then

src/xdist/plugin.py

@@ -196,6 +196,13 @@ def pytest_configure(config):
         )
         config.issue_config_time_warning(warning, 2)
 
+    if config.getoption("rsyncdir", None) or config.getini("rsyncdirs"):
+        warning = DeprecationWarning(
+            "The --rsyncdir command line argument and rsyncdirs config variable are deprecated.\n"
+            "The rsync feature will be removed in pytest-xdist 4.0."
+        )
+        config.issue_config_time_warning(warning, 2)
+
     config_line = (
         "xdist_group: specify group for tests should run in same session."
         "in relation to one another. " + "Provided by pytest-xdist."