Opt-in lazy wheel as a --use-feature

By invoking pip with --use-feature=lazy-wheel, the new resolver
will try to obtain dependency information by lazily download wheels
by HTTP range requests.

Author: McSinyx

news/8532.feature

@@ -0,0 +1,3 @@
+Allow the new resolver to obtain dependency information through wheels
+lazily downloaded using HTTP range requests.  To enable this feature,
+invoke ``pip`` with ``--use-feature=lazy-wheel``.

src/pip/_internal/cli/cmdoptions.py

@@ -916,7 +916,7 @@ def check_list_path_option(options):
     metavar='feature',
     action='append',
     default=[],
-    choices=['2020-resolver'],
+    choices=['2020-resolver', 'lazy-wheel'],
     help='Enable new functionality, that may be backward incompatible.',
 )  # type: Callable[..., Option]
 

src/pip/_internal/cli/req_command.py

@@ -271,6 +271,7 @@ def make_resolver(
                 force_reinstall=force_reinstall,
                 upgrade_strategy=upgrade_strategy,
                 py_version_info=py_version_info,
+                lazy_wheel='lazy-wheel' in options.features_enabled,
             )
         import pip._internal.resolution.legacy.resolver
         return pip._internal.resolution.legacy.resolver.Resolver(

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

@@ -304,12 +304,11 @@ def __init__(
     def iter_dependencies(self):
         # type: () -> Iterable[Optional[Requirement]]
         dist = None  # type: Optional[Distribution]
-        preparer, req = self._factory.preparer, self._ireq
+        preparer, lazy_wheel = self._factory.preparer, self._factory.lazy_wheel
         remote_wheel = self._link.is_wheel and not self._link.is_file
-        # TODO: Opt-in as unstable feature.
-        if remote_wheel and not preparer.require_hashes:
+        if lazy_wheel and remote_wheel and not preparer.require_hashes:
             assert self._name is not None
-            logger.info('Collecting %s', req.req or req)
+            logger.info('Collecting %s', self._ireq.req or self._ireq)
             # If RuntimeError is raised, fallback to self.dist.
             with indent_log(), suppress(RuntimeError):
                 logger.info(

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

@@ -82,6 +82,7 @@ def __init__(
         ignore_installed,  # type: bool
         ignore_requires_python,  # type: bool
         py_version_info=None,  # type: Optional[Tuple[int, ...]]
+        lazy_wheel=False,  # type: bool
     ):
         # type: (...) -> None
         self._finder = finder
@@ -92,6 +93,7 @@ def __init__(
         self._use_user_site = use_user_site
         self._force_reinstall = force_reinstall
         self._ignore_requires_python = ignore_requires_python
+        self.lazy_wheel = lazy_wheel
 
         self._link_candidate_cache = {}  # type: Cache[LinkCandidate]
         self._editable_candidate_cache = {}  # type: Cache[EditableCandidate]

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

@@ -49,6 +49,7 @@ def __init__(
         force_reinstall,  # type: bool
         upgrade_strategy,  # type: str
         py_version_info=None,  # type: Optional[Tuple[int, ...]]
+        lazy_wheel=False,  # type: bool
     ):
         super(Resolver, self).__init__()
 
@@ -64,6 +65,7 @@ def __init__(
             ignore_installed=ignore_installed,
             ignore_requires_python=ignore_requires_python,
             py_version_info=py_version_info,
+            lazy_wheel=lazy_wheel,
         )
         self.ignore_dependencies = ignore_dependencies
         self.upgrade_strategy = upgrade_strategy