--- a/setup.py	Thu May 30 09:14:41 2019 +0200
+++ b/setup.py	Thu May 23 02:05:32 2019 +0200
@@ -446,10 +446,12 @@
 
 class hgdist(Distribution):
     pure = False
+    rust = hgrustext is not None
     cffi = ispypy
 
     global_options = Distribution.global_options + [
         ('pure', None, "use pure (slow) Python code instead of C extensions"),
+        ('rust', None, "use Rust extensions additionally to C extensions"),
     ]
 
     def has_ext_modules(self):
@@ -460,18 +462,25 @@
 # This is ugly as a one-liner. So use a variable.
 buildextnegops = dict(getattr(build_ext, 'negative_options', {}))
 buildextnegops['no-zstd'] = 'zstd'
+buildextnegops['no-rust'] = 'rust'
 
 class hgbuildext(build_ext):
     user_options = build_ext.user_options + [
         ('zstd', None, 'compile zstd bindings [default]'),
         ('no-zstd', None, 'do not compile zstd bindings'),
+        ('rust', None,
+         'compile Rust extensions if they are in use '
+         '(requires Cargo) [default]'),
+        ('no-rust', None, 'do not compile Rust extensions'),
     ]
 
-    boolean_options = build_ext.boolean_options + ['zstd']
+    boolean_options = build_ext.boolean_options + ['zstd', 'rust']
     negative_opt = buildextnegops
 
     def initialize_options(self):
         self.zstd = True
+        self.rust = True
+
         return build_ext.initialize_options(self)
 
     def build_extensions(self):
@@ -484,14 +493,19 @@
             self.extensions = [e for e in self.extensions
                                if e.name != 'mercurial.zstd']
 
-        for rustext in ruststandalones:
-            rustext.build('' if self.inplace else self.build_lib)
+        # Build Rust standalon extensions if it'll be used
+        # and its build is not explictely disabled (for external build
+        # as Linux distributions would do)
+        if self.distribution.rust and self.rust and hgrustext != 'direct-ffi':
+            for rustext in ruststandalones:
+                rustext.build('' if self.inplace else self.build_lib)
 
         return build_ext.build_extensions(self)
 
     def build_extension(self, ext):
-        if isinstance(ext, RustExtension):
-            ext.rustbuild()
+        if (self.distribution.rust and self.rust
+            and isinstance(ext, RustExtension)):
+                ext.rustbuild()
         try:
             build_ext.build_extension(self, ext)
         except CCompilerError:
@@ -553,20 +567,14 @@
         basepath = os.path.join(self.build_lib, 'mercurial')
         self.mkpath(basepath)
 
+        rust = self.distribution.rust
         if self.distribution.pure:
             modulepolicy = 'py'
         elif self.build_lib == '.':
-            # in-place build should run without rebuilding C
-            # and Rust extensions
-            if hgrustext == 'cpython':
-                modulepolicy = 'rust+c-allow'
-            else:
-                modulepolicy = 'allow'
+            # in-place build should run without rebuilding and Rust extensions
+            modulepolicy = 'rust+c-allow' if rust else 'allow'
         else:
-            if hgrustext == 'cpython':
-                modulepolicy = 'rust+c'
-            else:
-                modulepolicy = 'c'
+            modulepolicy = 'rust+c' if rust else 'c'
 
         content = b''.join([
             b'# this file is autogenerated by setup.py\n',
@@ -1138,8 +1146,6 @@
     def __init__(self, mpath, sources, rustlibname, subcrate,
                  py3_features=None, **kw):
         Extension.__init__(self, mpath, sources, **kw)
-        if hgrustext is None:
-            return
         srcdir = self.rustsrcdir = os.path.join('rust', subcrate)
         self.py3_features = py3_features
 
@@ -1155,8 +1161,6 @@
                                 if os.path.splitext(fname)[1] == '.rs')
 
     def rustbuild(self):
-        if hgrustext is None:
-            return
         env = os.environ.copy()
         if 'HGTEST_RESTOREENV' in env:
             # Mercurial tests change HOME to a temporary directory,
@@ -1208,6 +1212,10 @@
         self.libraries.append(rustlibname)
         self.library_dirs.append(self.rusttargetdir)
 
+    def rustbuild(self):
+        if hgrustext == 'direct-ffi':
+            RustExtension.rustbuild(self)
+
 class RustStandaloneExtension(RustExtension):
 
     def __init__(self, pydottedname, rustcrate, dylibname, **kw):
@@ -1262,14 +1270,10 @@
         ]),
     Extension('hgext.fsmonitor.pywatchman.bser',
               ['hgext/fsmonitor/pywatchman/bser.c']),
+    RustStandaloneExtension('mercurial.rustext', 'hg-cpython', 'librusthg',
+                            py3_features='python3'),
     ]
 
-if hgrustext == 'cpython':
-    extmodules.append(
-        RustStandaloneExtension('mercurial.rustext', 'hg-cpython', 'librusthg',
-                                py3_features='python3')
-    )
-
 
 sys.path.insert(0, 'contrib/python-zstandard')
 import setup_zstd