Deprecate setup.py install fallback when wheel package is absent

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 +811,11 @@ def install(
         install_options = list(install_options) + self.install_options
 
         try:
+            if (
+                self.legacy_install_reason is not None
+                and self.legacy_install_reason.emit_before_install
+            ):
+                self.legacy_install_reason.emit_deprecation(self.name)
             success = install_legacy(
                 install_options=install_options,
                 global_options=global_options,

src/pip/_internal/utils/deprecation.py

@@ -159,3 +159,17 @@ def emit_deprecation(self, name: str) -> None:
     issue=8368,
     emit_after_success=True,
 )
+
+
+LegacyInstallReasonMissingWheelPackage = LegacyInstallReason(
+    reason=(
+        "{name} is being installed using the legacy "
+        "'setup.py install' method, because it does not have a "
+        "'pyproject.toml' and the 'wheel' package "
+        "is not installed."
+    ),
+    replacement="to enable the '--use-pep517' option",
+    gone_in=None,
+    issue=8559,
+    emit_before_install=True,
+)

src/pip/_internal/wheel_builder.py

@@ -19,6 +19,7 @@
 from pip._internal.operations.build.wheel_editable import build_wheel_editable
 from pip._internal.operations.build.wheel_legacy import build_wheel_legacy
 from pip._internal.req.req_install import InstallRequirement
+from pip._internal.utils.deprecation import LegacyInstallReasonMissingWheelPackage
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.misc import ensure_dir, hash_file, is_wheel_installed
 from pip._internal.utils.setuptools_build import make_setuptools_clean_args
@@ -86,11 +87,7 @@ 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 = LegacyInstallReasonMissingWheelPackage
         return False
 
     return True