Rollout flags (--use-feature, --use-deprecated)

src/pip/_internal/cli/base_command.py

@@ -204,6 +204,13 @@ def _main(self, args):
                 issue=8333,
             )
 
+        if 'resolver' in options.unstable_features:
+            logger.critical(
+                "--unstable-feature=resolver is no longer supported, and "
+                "has been replaced with --use-feature=2020-resolver instead."
+            )
+            sys.exit(ERROR)
+
         try:
             status = self.run(options, args)
             assert isinstance(status, int)

src/pip/_internal/cli/cmdoptions.py

@@ -906,8 +906,31 @@ def check_list_path_option(options):
     action='append',
     default=[],
     choices=['resolver'],
-    help=SUPPRESS_HELP,  # TODO: Enable this when the resolver actually works.
-    # help='Enable unstable feature(s) that may be backward incompatible.',
+    help=SUPPRESS_HELP,  # TODO: drop this in pip 20.3
+)  # type: Callable[..., Option]
+
+use_new_feature = partial(
+    Option,
+    '--use-feature',
+    dest='features_enabled',
+    metavar='feature',
+    action='append',
+    default=[],
+    choices=['2020-resolver'],
+    help='Enable new functionality, that may be backward incompatible.',
+)  # type: Callable[..., Option]
+
+use_deprecated_feature = partial(
+    Option,
+    '--deprecated-feature',
+    dest='deprecated_features_enabled',
+    metavar='feature',
+    action='append',
+    default=[],
+    choices=[],
+    help=(
+        'Enable deprecated functionality, that will be removed in the future.'
+    ),
 )  # type: Callable[..., Option]
 
 
@@ -939,6 +962,8 @@ def check_list_path_option(options):
         no_color,
         no_python_version_warning,
         unstable_feature,
+        use_new_feature,
+        use_deprecated_feature,
     ]
 }  # type: Dict[str, Any]
 

src/pip/_internal/cli/req_command.py

@@ -259,7 +259,7 @@ def make_resolver(
         # The long import name and duplicated invocation is needed to convince
         # Mypy into correctly typechecking. Otherwise it would complain the
         # "Resolver" class being redefined.
-        if 'resolver' in options.unstable_features:
+        if '2020-resolver' in options.features_enabled:
             import pip._internal.resolution.resolvelib.resolver
             return pip._internal.resolution.resolvelib.resolver.Resolver(
                 preparer=preparer,

tests/functional/test_install.py

@@ -405,7 +405,7 @@ def test_vcs_url_urlquote_normalization(script, tmpdir):
     )
 
 
-@pytest.mark.parametrize("resolver", ["", "--unstable-feature=resolver"])
+@pytest.mark.parametrize("resolver", ["", "--use-feature=2020-resolver"])
 def test_basic_install_from_local_directory(script, data, resolver):
     """
     Test installing from a local directory.