Disable PEP 658 for the legacy resolver

news/12156.bugfix.rst

@@ -0,0 +1 @@
+Disable PEP 658 metadata fetching with the legacy resolver.

src/pip/_internal/cli/req_command.py

@@ -287,6 +287,7 @@ def make_requirement_preparer(
         """
         temp_build_dir_path = temp_build_dir.path
         assert temp_build_dir_path is not None
+        legacy_resolver = False
 
         resolver_variant = cls.determine_resolver_variant(options)
         if resolver_variant == "2020-resolver":
@@ -300,6 +301,7 @@ def make_requirement_preparer(
                     "production."
                 )
         else:
+            legacy_resolver = True
             lazy_wheel = False
             if "fast-deps" in options.features_enabled:
                 logger.warning(
@@ -320,6 +322,7 @@ def make_requirement_preparer(
             use_user_site=use_user_site,
             lazy_wheel=lazy_wheel,
             verbosity=verbosity,
+            legacy_resolver=legacy_resolver,
         )
 
     @classmethod

src/pip/_internal/operations/prepare.py

@@ -226,6 +226,7 @@ def __init__(
         use_user_site: bool,
         lazy_wheel: bool,
         verbosity: int,
+        legacy_resolver: bool,
     ) -> None:
         super().__init__()
 
@@ -259,6 +260,9 @@ def __init__(
         # How verbose should underlying tooling be?
         self.verbosity = verbosity
 
+        # Are we using the legacy resolver?
+        self.legacy_resolver = legacy_resolver
+
         # Memoized downloaded files, as mapping of url: path.
         self._downloaded: Dict[str, str] = {}
 
@@ -365,6 +369,11 @@ def _fetch_metadata_only(
         self,
         req: InstallRequirement,
     ) -> Optional[BaseDistribution]:
+        if self.legacy_resolver:
+            logger.debug(
+                "Metadata-only fetching is not used in the legacy resolver",
+            )
+            return None
         if self.require_hashes:
             logger.debug(
                 "Metadata-only fetching is not used as hash checking is required",

tests/unit/test_req.py

@@ -105,6 +105,7 @@ def _basic_resolver(
                 use_user_site=False,
                 lazy_wheel=False,
                 verbosity=0,
+                legacy_resolver=True,
             )
             yield Resolver(
                 preparer=preparer,