Stop falling back to setup.py install by default

src/pip/_internal/cli/cmdoptions.py

@@ -969,6 +969,7 @@ def check_list_path_option(options: Values) -> None:
         "out-of-tree-build",
         "backtrack-on-build-failures",
         "html5lib",
+        "legacy-setup-py-install",
     ],
     help=("Enable deprecated functionality, that will be removed in the future."),
 )

src/pip/_internal/commands/install.py

@@ -243,6 +243,9 @@ def run(self, options: Values, args: List[str]) -> int:
             raise CommandError("Can not combine '--user' and '--target'")
 
         cmdoptions.check_install_build_global(options)
+        should_fallback_to_legacy_install = (
+            "legacy-setup-py-install" in options.deprecated_features_enabled
+        )
         upgrade_strategy = "to-satisfy-only"
         if options.upgrade:
             upgrade_strategy = options.upgrade_strategy
@@ -366,25 +369,25 @@ def run(self, options: Values, args: List[str]) -> int:
                 global_options=[],
             )
 
-            # If we're using PEP 517, we cannot do a legacy setup.py install
-            # so we fail here.
-            pep517_build_failure_names: List[str] = [
-                r.name for r in build_failures if r.use_pep517  # type: ignore
-            ]
-            if pep517_build_failure_names:
-                raise InstallationError(
-                    "Could not build wheels for {}, which is required to "
-                    "install pyproject.toml-based projects".format(
-                        ", ".join(pep517_build_failure_names)
+            if build_failures:
+                if not should_fallback_to_legacy_install:
+                    raise InstallationError(
+                        "Could not build wheels for {}, which is required to "
+                        "install pyproject.toml-based projects".format(
+                            ", ".join([r.name for r in build_failures])  # type: ignore
+                        )
                     )
-                )
 
-            # For now, we just warn about failures building legacy
-            # requirements, as we'll fall through to a setup.py install for
-            # those.
-            for r in build_failures:
-                if not r.use_pep517:
-                    r.legacy_install_reason = 8368
+                pep517_build_failures = [r for r in build_failures if r.use_pep517]
+                if pep517_build_failures:
+                    raise InstallationError(
+                        "Could not build wheels for {}, which is required to "
+                        "install pyproject.toml-based projects".format(
+                            ", ".join(
+                                [r.name for r in pep517_build_failures]  # type: ignore
+                            )
+                        )
+                    )
 
             to_install = resolver.get_installation_order(requirement_set)
 

src/pip/_internal/req/req_install.py

@@ -91,7 +91,6 @@ def __init__(
         self.constraint = constraint
         self.editable = editable
         self.permit_editable_wheels = permit_editable_wheels
-        self.legacy_install_reason: Optional[int] = None
 
         # source_dir is the local directory where the linked requirement is
         # located, or unpacked. In case unpacking is needed, creating and
@@ -788,6 +787,14 @@ def install(
         global_options = list(global_options) + self.global_options
         install_options = list(install_options) + self.install_options
 
+        deprecated(
+            reason=(
+                "Using 'setup.py install' for performing a package installation "
+                "is deprecated."
+            ),
+            replacement="correctly building a wheel for this package",
+            gone_in="23.0",
+        )
         try:
             success = install_legacy(
                 install_options=install_options,
@@ -814,19 +821,6 @@ def install(
 
         self.install_succeeded = success
 
-        if success and self.legacy_install_reason == 8368:
-            deprecated(
-                reason=(
-                    "{} was installed using the legacy 'setup.py install' "
-                    "method, because a wheel could not be built for it.".format(
-                        self.name
-                    )
-                ),
-                replacement="to fix the wheel build issue reported above",
-                gone_in=None,
-                issue=8368,
-            )
-
 
 def check_invalid_constraint_type(req: InstallRequirement) -> str: