Deprecate setup.py install fallback when wheel package is absent

Author: sbidoul

news/8559.removal.rst

@@ -0,0 +1,2 @@
+Deprecate installation with 'setup.py install' when the 'wheel' package is absent for
+source distributions without 'pyproject.toml'.

src/pip/_internal/req/req_install.py

@@ -811,6 +811,18 @@ def install(
         install_options = list(install_options) + self.install_options
 
         try:
+            if self.legacy_install_reason == 8559:
+                deprecated(
+                    reason=(
+                        f"{self.name} is being installed using the legacy "
+                        f"'setup.py install' method, because it does not have a "
+                        f"'pyproject.toml' and the 'wheel' package "
+                        f"is not installed."
+                    ),
+                    replacement="to enable the '--use-pep517' option",
+                    gone_in=None,
+                    issue=8559,
+                )
             success = install_legacy(
                 install_options=install_options,
                 global_options=global_options,

src/pip/_internal/wheel_builder.py

@@ -86,11 +86,7 @@ def _should_build(
 
     if not is_wheel_installed():
         # we don't build legacy requirements if wheel is not installed
-        logger.info(
-            "Using legacy 'setup.py install' for %s, "
-            "since package 'wheel' is not installed.",
-            req.name,
-        )
+        req.legacy_install_reason = 8559
         return False
 
     return True