Emit a SetuptoolsDeprecationWarning when setup_requires is used. Ref #…

…2823.

changelog.d/2823.change.rst

@@ -0,0 +1 @@
+Officially deprecated support for ``setup_requires``. Users are encouraged instead to migrate to PEP 518 ``build-system.requires`` in ``pyproject.toml``. Users reliant on ``setup_requires`` should consider pinning to this major version to avoid disruption.

pytest.ini

@@ -8,21 +8,24 @@ doctest_optionflags=ALLOW_UNICODE ELLIPSIS
 # workaround for warning pytest-dev/pytest#6178
 junit_family=xunit2
 filterwarnings=
-    # Fail on warnings
-    error
+	# Fail on warnings
+	error
 
-    ## upstream
+	## upstream
 	# Suppress deprecation warning in flake8
 	ignore:SelectableGroups dict interface is deprecated::flake8
 	# Suppress deprecation warning in pypa/packaging#433
 	ignore:The distutils package is deprecated::packaging.tags
 	## end upstream
 
-    # https://github.com/pypa/setuptools/issues/1823
-    ignore:bdist_wininst command is deprecated
-    # Suppress this error; unimportant for CI tests
-    ignore:Extraction path is writable by group/others:UserWarning
-    # Suppress weird RuntimeWarning.
-    ignore:Parent module 'setuptools' not found while handling absolute import:RuntimeWarning
-    # Suppress use of bytes for filenames on Windows until fixed #2016
-    ignore:The Windows bytes API has been deprecated:DeprecationWarning
+	# https://github.com/pypa/setuptools/issues/1823
+	ignore:bdist_wininst command is deprecated
+	# Suppress this error; unimportant for CI tests
+	ignore:Extraction path is writable by group/others:UserWarning
+	# Suppress weird RuntimeWarning.
+	ignore:Parent module 'setuptools' not found while handling absolute import:RuntimeWarning
+	# Suppress use of bytes for filenames on Windows until fixed #2016
+	ignore:The Windows bytes API has been deprecated:DeprecationWarning
+
+	# https://github.com/pypa/setuptools/issues/2823
+	ignore:setup_requires is deprecated.

setuptools/__init__.py

@@ -4,6 +4,7 @@
 import functools
 import os
 import re
+import warnings
 
 import _distutils_hack.override  # noqa: F401
 
@@ -144,6 +145,11 @@ def finalize_options(self):
     # Honor setup.cfg's options.
     dist.parse_config_files(ignore_option_errors=True)
     if dist.setup_requires:
+        warnings.warn(
+            "setup_requires is deprecated. Supply build "
+            "dependencies using PEP 517 pyproject.toml build-requires.",
+            SetuptoolsDeprecationWarning,
+        )
         dist.fetch_build_eggs(dist.setup_requires)