rust-threads: force Rayon to respect the worker count in config stable

rust-threads: force Rayon to respect the worker count in config

As per the inline comment, this is a workaround because Rust code does not yet
know how to read config files.

Differential Revision: https://phab.mercurial-scm.org/D7610

--- a/mercurial/dirstate.py	Mon Dec 09 12:29:46 2019 +0100
+++ b/mercurial/dirstate.py	Sun Nov 10 07:30:14 2019 -0800
@@ -1116,6 +1116,17 @@
             use_rust = False
 
         if use_rust:
+            # Force Rayon (Rust parallelism library) to respect the number of
+            # workers. This is a temporary workaround until Rust code knows
+            # how to read the config file.
+            numcpus = self._ui.configint("worker", "numcpus")
+            if numcpus is not None:
+                encoding.environ.setdefault(b'RAYON_NUM_THREADS', b'%d' % numcpus)
+
+            workers_enabled = self._ui.configbool("worker", "enabled", True)
+            if not workers_enabled:
+                encoding.environ[b"RAYON_NUM_THREADS"] = b"1"
+
             (
                 lookup,
                 modified,