Don't setup.py develop if there is no setup.{py,cfg}

src/pip/_internal/req/req_install.py

@@ -752,6 +752,18 @@ def install(
 
         global_options = global_options if global_options is not None else []
         if self.editable and not self.is_wheel:
+            if not os.path.isfile(self.setup_py_path) and not os.path.isfile(
+                self.setup_cfg_path
+            ):
+                # We may reach this point if there is a pyproject.toml using the
+                # __legacy__ build backend and no setup.py nor setup.cfg. Since
+                # the legacy build backend accepts to generate metadata in such
+                # a situation we can't fail earlier.
+                raise InstallationError(
+                    f"Refusing to do a legacy editable install of {self.source_dir} "
+                    f"which has no 'setup.py' nor 'setup.cfg'. Consider using a build "
+                    f"backend that supports PEP 660."
+                )
             install_editable_legacy(
                 install_options,
                 global_options,