diff --git a/mercurial/__init__.py b/mercurial/__init__.py
--- a/mercurial/__init__.py
+++ b/mercurial/__init__.py
@@ -40,6 +40,10 @@
             # zstd is already dual-version clean, don't try and mangle it
             if fullname.startswith('mercurial.zstd'):
                 return None
+            # rustext is built for the right python version,
+            # don't try and mangle it
+            if fullname.startswith('mercurial.rustext'):
+                return None
             # pywatchman is already dual-version clean, don't try and mangle it
             if fullname.startswith('hgext.fsmonitor.pywatchman'):
                 return None
diff --git a/rust/Cargo.lock b/rust/Cargo.lock
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -19,6 +19,7 @@
  "libc 0.2.45 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.2.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "python27-sys 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "python3-sys 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
diff --git a/rust/hg-cpython/Cargo.toml b/rust/hg-cpython/Cargo.toml
--- a/rust/hg-cpython/Cargo.toml
+++ b/rust/hg-cpython/Cargo.toml
@@ -8,9 +8,14 @@
 crate-type = ["cdylib"]
 
 [features]
-default = ["python27", "python27-sys"]
+default = ["python27"]
 
-python27 = ["cpython/python27-sys", "cpython/extension-module-2-7"]
+python27 = ["cpython/python27-sys",
+            "cpython/extension-module-2-7",
+            "python27-sys",
+            ]
+
+python3 = ["python3-sys", "cpython/python3-sys", "cpython/extension-module"]
 
 [dependencies]
 hg-core = { path = "../hg-core" }
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -922,11 +922,13 @@
 
     rusttargetdir = os.path.join('rust', 'target', 'release')
 
-    def __init__(self, mpath, sources, rustlibname, subcrate, **kw):
+    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
 
         # adding Rust source and control files to depends so that the extension
         # gets rebuilt if they've changed
@@ -957,6 +959,9 @@
             env['HOME'] = pwd.getpwuid(os.getuid()).pw_dir
 
         cargocmd = ['cargo', 'build', '-vv', '--release']
+        if sys.version_info[0] == 3 and self.py3_features is not None:
+            cargocmd.extend(('--features', self.py3_features,
+                             '--no-default-features'))
         try:
             subprocess.check_call(cargocmd, env=env, cwd=self.rustsrcdir)
         except OSError as exc:
@@ -1047,7 +1052,8 @@
 
 if hgrustext == 'cpython':
     extmodules.append(
-        RustStandaloneExtension('mercurial.rustext', 'hg-cpython', 'librusthg')
+        RustStandaloneExtension('mercurial.rustext', 'hg-cpython', 'librusthg',
+                                py3_features='python3')
     )