Fix and expand PEP 660 detection

Since there is no 'setup.py develop' fallback anymore,
this check needs to be updated, and also needs to run
when not using build isolation.

Author: sbidoul

src/pip/_internal/distributions/sdist.py

@@ -46,16 +46,20 @@ def prepare_distribution_metadata(
             # Setup an isolated environment and install the build backend static
             # requirements in it.
             self._prepare_build_backend(build_env_installer)
-            # Check that if the requirement is editable, it either supports PEP 660 or
-            # has a setup.py or a setup.cfg. This cannot be done earlier because we need
+            # Check that if the build backend supports PEP 660.
+            # This cannot be done earlier because we need
             # to setup the build backend to verify it supports build_editable, nor can
             # it be done later, because we want to avoid installing build requirements
             # needlessly. Doing it here also works around setuptools generating
             # UNKNOWN.egg-info when running get_requires_for_build_wheel on a directory
             # without setup.py nor setup.cfg.
-            self.req.isolated_editable_sanity_check()
+            self.req.editable_sanity_check()
             # Install the dynamic build requirements.
             self._install_build_reqs(build_env_installer)
+        else:
+            # When not using build isolation, we still need to check that
+            # the build backend supports PEP 660.
+            self.req.editable_sanity_check()
         # Check if the current environment provides build dependencies
         if check_build_deps:
             pyproject_requires = self.req.pyproject_requires

src/pip/_internal/req/req_install.py

@@ -464,13 +464,6 @@ def setup_py_path(self) -> str:
 
         return setup_py
 
-    @property
-    def setup_cfg_path(self) -> str:
-        assert self.source_dir, f"No source dir for {self}"
-        setup_cfg = os.path.join(self.unpacked_source_directory, "setup.cfg")
-
-        return setup_cfg
-
     @property
     def pyproject_toml_path(self) -> str:
         assert self.source_dir, f"No source dir for {self}"
@@ -496,21 +489,15 @@ def load_pyproject_toml(self) -> None:
             backend_path=backend_path,
         )
 
-    def isolated_editable_sanity_check(self) -> None:
+    def editable_sanity_check(self) -> None:
         """Check that an editable requirement if valid for use with PEP 517/518.
 
-        This verifies that an editable that has a pyproject.toml either supports PEP 660
-        or as a setup.py or a setup.cfg
+        This verifies that an editable has a build backend that supports PEP 660.
         """
-        if (
-            self.editable
-            and not self.supports_pyproject_editable
-            and not os.path.isfile(self.setup_py_path)
-            and not os.path.isfile(self.setup_cfg_path)
-        ):
+        if self.editable and not self.supports_pyproject_editable:
             raise InstallationError(
-                f"Project {self} has a 'pyproject.toml' and its build "
-                f"backend is missing the 'build_editable' hook, so "
+                f"Project {self} uses a build backend"
+                f"that is missing the 'build_editable' hook, so "
                 f"it cannot be installed in editable mode. "
                 f"Consider using a build backend that supports PEP 660."
             )