Deprecate pip install --editable calling setup.py develop

Deprecates `pip install --editable` falling back to `setup.py develop`
when using a setuptools version that does not support PEP 660
(setuptools v63 and older).

See:
https://peps.python.org/pep-0660/
https://setuptools.pypa.io/en/latest/history.html#v64-0-0

Closes #11457.

Author: edmorley

news/11457.removal.rst

@@ -0,0 +1,3 @@
+Deprecate ``pip install --editable`` falling back to ``setup.py develop``
+when using a setuptools version that does not support :pep:`660`
+(setuptools v63 and older).

src/pip/_internal/req/req_install.py

@@ -825,6 +825,21 @@ def install(
         )
 
         if self.editable and not self.is_wheel:
+            deprecated(
+                reason=(
+                    f"Legacy editable install of {self} (setup.py develop) "
+                    "is deprecated."
+                ),
+                replacement=(
+                    "to add a pyproject.toml or enable --use-pep517, "
+                    "and use setuptools >= 64. "
+                    "If the resulting installation is not behaving as expected, "
+                    "try using --config-settings editable_mode=compat. "
+                    "Please consult the setuptools documentation for more information"
+                ),
+                gone_in="25.0",
+                issue=11457,
+            )
             if self.config_settings:
                 logger.warning(
                     "--config-settings ignored for legacy editable install of %s. "

tests/functional/test_install.py

@@ -1350,7 +1350,7 @@ def test_install_package_with_prefix(
 
 def _test_install_editable_with_prefix(
     script: PipTestEnvironment, files: Dict[str, str]
-) -> None:
+) -> TestPipResult:
     # make a dummy project
     pkga_path = script.scratch_path / "pkga"
     pkga_path.mkdir()
@@ -1378,6 +1378,8 @@ def _test_install_editable_with_prefix(
     install_path = script.scratch / site_packages / "pkga.egg-link"
     result.did_create(install_path)
 
+    return result
+
 
 @pytest.mark.network
 def test_install_editable_with_target(script: PipTestEnvironment) -> None:
@@ -1427,9 +1429,10 @@ def test_install_editable_legacy_with_prefix_setup_cfg(
 requires = ["setuptools<64", "wheel"]
 build-backend = "setuptools.build_meta"
 """
-    _test_install_editable_with_prefix(
+    result = _test_install_editable_with_prefix(
         script, {"setup.cfg": setup_cfg, "pyproject.toml": pyproject_toml}
     )
+    assert "(setup.py develop) is deprecated" in result.stderr
 
 
 def test_install_package_conflict_prefix_and_user(