Add --unstable-feature=resolver

This introduces a new general option --unstable-feature that can be used
to opt into "preview" features in pip not enabled by default. Currently
the only available feature is "resolver".

A stub resolver interface (which would fail on invocation) is provided
to respond to the flag.

The --unstable-feature option is hidden from --help since the resolver
does not yet work. This suppression should be removed when we release
the resolver for general/public testing.

src/pip/_internal/cli/cmdoptions.py

@@ -915,6 +915,19 @@ def check_list_path_option(options):
 )  # type: Callable[..., Option]
 
 
+unstable_feature = partial(
+    Option,
+    '--unstable-feature',
+    dest='unstable_features',
+    metavar='feature',
+    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.',
+)  # type: Callable[..., Option]
+
+
 ##########
 # groups #
 ##########
@@ -943,6 +956,7 @@ def check_list_path_option(options):
         disable_pip_version_check,
         no_color,
         no_python_version_warning,
+        unstable_feature,
     ]
 }  # type: Dict[str, Any]
 

src/pip/_internal/cli/req_command.py

@@ -26,7 +26,6 @@
 )
 from pip._internal.req.req_file import parse_requirements
 from pip._internal.req.req_set import RequirementSet
-from pip._internal.resolution.legacy.resolver import Resolver
 from pip._internal.self_outdated_check import (
     make_link_collector,
     pip_self_version_check,
@@ -42,6 +41,7 @@
     from pip._internal.models.target_python import TargetPython
     from pip._internal.req.req_install import InstallRequirement
     from pip._internal.req.req_tracker import RequirementTracker
+    from pip._internal.resolution.base import BaseResolver
     from pip._internal.utils.temp_dir import (
         TempDirectory,
         TempDirectoryTypeRegistry,
@@ -248,7 +248,7 @@ def make_resolver(
         use_pep517=None,                     # type: Optional[bool]
         py_version_info=None            # type: Optional[Tuple[int, ...]]
     ):
-        # type: (...) -> Resolver
+        # type: (...) -> BaseResolver
         """
         Create a Resolver instance for the given parameters.
         """
@@ -258,7 +258,25 @@ def make_resolver(
             wheel_cache=wheel_cache,
             use_pep517=use_pep517,
         )
-        return 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:
+            import pip._internal.resolution.resolvelib.resolver
+            return pip._internal.resolution.resolvelib.resolver.Resolver(
+                preparer=preparer,
+                finder=finder,
+                make_install_req=make_install_req,
+                use_user_site=use_user_site,
+                ignore_dependencies=options.ignore_dependencies,
+                ignore_installed=ignore_installed,
+                ignore_requires_python=ignore_requires_python,
+                force_reinstall=force_reinstall,
+                upgrade_strategy=upgrade_strategy,
+                py_version_info=py_version_info,
+            )
+        import pip._internal.resolution.legacy.resolver
+        return pip._internal.resolution.legacy.resolver.Resolver(
             preparer=preparer,
             finder=finder,
             make_install_req=make_install_req,

src/pip/_internal/resolution/base.py

@@ -0,0 +1,20 @@
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import Callable, List
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.req.req_set import RequirementSet
+
+    InstallRequirementProvider = Callable[
+        [str, InstallRequirement], InstallRequirement
+    ]
+
+
+class BaseResolver(object):
+    def resolve(self, root_reqs, check_supported_wheels):
+        # type: (List[InstallRequirement], bool) -> RequirementSet
+        raise NotImplementedError()
+
+    def get_installation_order(self, req_set):
+        # type: (RequirementSet) -> List[InstallRequirement]
+        raise NotImplementedError()

src/pip/_internal/resolution/legacy/resolver.py

@@ -29,6 +29,7 @@
     UnsupportedPythonVersion,
 )
 from pip._internal.req.req_set import RequirementSet
+from pip._internal.resolution.base import BaseResolver
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.misc import dist_in_usersite, normalize_version_info
 from pip._internal.utils.packaging import (
@@ -38,17 +39,15 @@
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
-    from typing import Callable, DefaultDict, List, Optional, Set, Tuple
+    from typing import DefaultDict, List, Optional, Set, Tuple
     from pip._vendor import pkg_resources
 
     from pip._internal.distributions import AbstractDistribution
     from pip._internal.index.package_finder import PackageFinder
     from pip._internal.operations.prepare import RequirementPreparer
     from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.resolution.base import InstallRequirementProvider
 
-    InstallRequirementProvider = Callable[
-        [str, InstallRequirement], InstallRequirement
-    ]
     DiscoveredDependencies = DefaultDict[str, List[InstallRequirement]]
 
 logger = logging.getLogger(__name__)
@@ -102,7 +101,7 @@ def _check_dist_requires_python(
         ))
 
 
-class Resolver(object):
+class Resolver(BaseResolver):
     """Resolves which packages need to be installed/uninstalled to perform \
     the requested operation without breaking the requirements of any package.
     """

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -0,0 +1,36 @@
+from pip._internal.resolution.base import BaseResolver
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import List, Optional, Tuple
+
+    from pip._internal.index.package_finder import PackageFinder
+    from pip._internal.operations.prepare import RequirementPreparer
+    from pip._internal.req.req_install import InstallRequirement
+    from pip._internal.req.req_set import RequirementSet
+    from pip._internal.resolution.base import InstallRequirementProvider
+
+
+class Resolver(BaseResolver):
+    def __init__(
+        self,
+        preparer,  # type: RequirementPreparer
+        finder,  # type: PackageFinder
+        make_install_req,  # type: InstallRequirementProvider
+        use_user_site,  # type: bool
+        ignore_dependencies,  # type: bool
+        ignore_installed,  # type: bool
+        ignore_requires_python,  # type: bool
+        force_reinstall,  # type: bool
+        upgrade_strategy,  # type: str
+        py_version_info=None,  # type: Optional[Tuple[int, ...]]
+    ):
+        super(Resolver, self).__init__()
+
+    def resolve(self, root_reqs, check_supported_wheels):
+        # type: (List[InstallRequirement], bool) -> RequirementSet
+        raise NotImplementedError()
+
+    def get_installation_order(self, req_set):
+        # type: (RequirementSet) -> List[InstallRequirement]
+        raise NotImplementedError()