Deprecate legacy install when --no-binary is used

Author: sbidoul

news/9769.feature.rst

@@ -0,0 +1,9 @@
+Deprecate legacy setup.py install when --no-binary is used:
+
+- add a new feature flag ``--use-feature=always-install-via-wheel``, that is intended to become the default and only mechanism in the future
+- when ``--no-binary`` is used without the feature flag, emit a deprecation warning about the fallback to ``setup.py install``
+- when ``--no-binary`` is used with the feature flag, build a wheel from the sdist (via PEP 517 or ``setup.py bdist_wheel``) then install it
+- when ``--no-binary`` is used with the feature flag, the wheel that was built locally is cached (unless the cache is disabled)
+- since ``--install-option``, ``--build-option`` and ``--global-option`` imply ``--no-binary``, the deprecation warning will be emitted when these options are used without the feature flag
+- deprecate ``--install-option``
+- allow using ``--build-option`` in the ``install`` command, as well as in ``requirement.txt`` lines, as a transitory mechanism until the ecosystem supports PEP 517 config settings, which are meant to replace both ``--build-options`` and ``--global-options``

src/pip/_internal/req/req_install.py

@@ -800,6 +800,17 @@ def install(
             self.install_succeeded = True
             return
 
+        if self.legacy_install_reason == 9769:
+            deprecated(
+                reason=(
+                    "Installing {} using the legacy 'setup.py install' "
+                    "method, due to binaries being disabled for it.".format(self.name)
+                ),
+                replacement="--use-feature=always-install-via-wheel",
+                gone_in=None,
+                issue=9769,
+            )
+
         # TODO: Why don't we do this for editable installs?
 
         # Extend the list of global and install options passed on to

src/pip/_internal/wheel_builder.py

@@ -78,10 +78,7 @@ def _should_build(
         return True
 
     if not check_binary_allowed(req):
-        logger.info(
-            "Using legacy 'setup.py install' for %s, due to binaries "
-            "being disabled for it.", req.name,
-        )
+        req.legacy_install_reason = 9769
         return False
 
     if not is_wheel_installed():