Short-circuit get_requires hooks when there are no extra build dependencies

The PEP 517 get_requires_for_build_(wheel|sdist) hooks function by
running the egg_info command with a patched Distribution that raises a
special exception when build dependencies (from setup_requires) are
about to be installed.

This works great at minimizing the overhead of the requires hooks...
except when there are no dynamic build dependencies. In such a
situation, the injected install_build_eggs() shim that raises
SetupRequirementsError is never invoked, thus the entire egg_info
command will execute. This often results in substantial overhead
for modern projects that use [build-system].requires and not
setup_requires.

Author: ichard26

newsfragments/4973.feature.rst

@@ -0,0 +1,2 @@
+PEP 517 ``get_requires_for_build_sdist`` and ``get_requires_for_build_wheel`` hooks
+now terminate as soon as it is determined there are no extra build dependencies.

setuptools/__init__.py

@@ -85,8 +85,11 @@ def finalize_options(self):
 
     # Honor setup.cfg's options.
     dist.parse_config_files(ignore_option_errors=True)
-    if dist.setup_requires:
-        _fetch_build_eggs(dist)
+    # NOTE: fetch_build_eggs is called so even if there are no build
+    # requirements, get_requires_for_build_(sdist/wheel) will still
+    # short-circuit once build dependencies are known (avoiding the
+    # costly full execution of setup.py).
+    _fetch_build_eggs(dist)
 
 
 def _fetch_build_eggs(dist: Distribution):