rust-cpython: management of shared libray suffix

rust-cpython: management of shared libray suffix

Before this changeset, the shared library objects suffixes
were both (rustc output and Python input) hardcoded to '.so',
which is wrong for Python3 and non Linux targets.

--- a/setup.py	Mon May 27 16:55:46 2019 -0400
+++ b/setup.py	Fri Jun 14 10:57:07 2019 +0100
@@ -32,6 +32,7 @@
     ])
 
 import sys, platform
+import sysconfig
 if sys.version_info[0] >= 3:
     printf = eval('print')
     libdir_escape = 'unicode_escape'
@@ -104,6 +105,12 @@
         printf(error, file=sys.stderr)
         sys.exit(1)
 
+if sys.version_info[0] >= 3:
+    DYLIB_SUFFIX = sysconfig.get_config_vars()['EXT_SUFFIX']
+else:
+    # deprecated in Python 3
+    DYLIB_SUFFIX = sysconfig.get_config_vars()['SO']
+
 # Solaris Python packaging brain damage
 try:
     import hashlib
@@ -1160,6 +1167,19 @@
                                 for fname in fnames
                                 if os.path.splitext(fname)[1] == '.rs')
 
+    @staticmethod
+    def rustdylibsuffix():
+        """Return the suffix for shared libraries produced by rustc.
+
+        See also: https://doc.rust-lang.org/reference/linkage.html
+        """
+        if sys.platform == 'darwin':
+            return '.dylib'
+        elif os.name == 'nt':
+            return '.dll'
+        else:
+            return '.so'
+
     def rustbuild(self):
         env = os.environ.copy()
         if 'HGTEST_RESTOREENV' in env:
@@ -1227,9 +1247,9 @@
         self.rustbuild()
         target = [target_dir]
         target.extend(self.name.split('.'))
-        ext = '.so'  # TODO Unix only
-        target[-1] += ext
-        shutil.copy2(os.path.join(self.rusttargetdir, self.dylibname + ext),
+        target[-1] += DYLIB_SUFFIX
+        shutil.copy2(os.path.join(self.rusttargetdir,
+                                  self.dylibname + self.rustdylibsuffix()),
                      os.path.join(*target))