Add always-install-via-wheel feature

When enabled this feature does the following:

- the presence of --build-option, --install-option, --global-option
  no longer imply --no-binary :all:
- --no-binary does not disable wheel building anymore, so a wheel
  is built during the install process and setup.py install is
  not used anymore in this case

news/9778.feature.rst

@@ -0,0 +1,12 @@
+Add an experimental ``always-install-via-wheel`` feature flag. When enabled, it has the
+following effects:
+
+- Installation of source distributions is always done by first building a wheel
+  then installing from it (assuming the ``wheel`` package is installed).
+- ``--no-binary`` now only means "do not download wheels", whereas before it also
+  implied using the legacy ``setup.py install`` and disabling the wheel cache.
+- The use of ``--install-option``, ``--build-option``, ``--global-option`` no longer
+  imply ``--no-binary :all:``.
+- ``--build-option`` and ``--global-option`` are passed to ``setup.py bdist_wheel``
+  during installation, as it was already done when using ``pip wheel``.
+- ``--install-option`` is ignored (because there is no ``setup.py install`` involved)

src/pip/_internal/cli/cmdoptions.py

@@ -74,6 +74,15 @@ def getname(n):
         # type: (str) -> Optional[Any]
         return getattr(check_options, n, None)
 
+    if "always-install-via-wheel" in options.features_enabled:
+        if getname("install_options"):
+            warnings.warn(
+                "Ignoring --install-option because 'always-install-via-wheel' "
+                "is enabled.",
+                stacklevel=2,
+            )
+        return
+
     names = ["build_options", "global_options", "install_options"]
     if any(map(getname, names)):
         control = options.format_control
@@ -965,7 +974,7 @@ def check_list_path_option(options):
     metavar="feature",
     action="append",
     default=[],
-    choices=["2020-resolver", "fast-deps", "in-tree-build"],
+    choices=["2020-resolver", "fast-deps", "in-tree-build", "always-install-via-wheel"],
     help="Enable new functionality, that may be backward incompatible.",
 )  # type: Callable[..., Option]
 

src/pip/_internal/commands/install.py

@@ -45,10 +45,12 @@
 logger = logging.getLogger(__name__)
 
 
-def get_check_binary_allowed(format_control):
-    # type: (FormatControl) -> BinaryAllowedPredicate
+def get_check_binary_allowed(format_control, features_enabled):
+    # type: (FormatControl, List[str]) -> BinaryAllowedPredicate
     def check_binary_allowed(req):
         # type: (InstallRequirement) -> bool
+        if "always-install-via-wheel" in features_enabled:
+            return True
         canonical_name = canonicalize_name(req.name or "")
         allowed_formats = format_control.get_allowed_formats(canonical_name)
         return "binary" in allowed_formats
@@ -330,7 +332,7 @@ def run(self, options, args):
             )
 
             check_binary_allowed = get_check_binary_allowed(
-                finder.format_control
+                finder.format_control, options.features_enabled
             )
 
             reqs_to_build = [

src/pip/_internal/wheel_builder.py

@@ -75,7 +75,7 @@ def _should_build(
 
     if not check_binary_allowed(req):
         logger.info(
-            "Skipping wheel build for %s, due to binaries "
+            "Using legacy 'setup.py install' for %s, due to binaries "
             "being disabled for it.", req.name,
         )
         return False

tests/functional/test_install.py

@@ -724,6 +724,17 @@ def test_install_global_option(script):
     assert not result.files_created
 
 
+def test_install_global_option_does_not_disable_wheel_building(
+    script, data, with_wheel
+):
+    res = script.pip(
+        'install', '--no-index', '--global-option=-v', '-f', data.find_links,
+        '--use-feature=always-install-via-wheel',
+        'upper', expect_stderr=True)
+    assert "Successfully installed upper-2.0" in str(res), str(res)
+    assert "Building wheel for upper" in str(res), str(res)
+
+
 def test_install_with_hacked_egg_info(script, data):
     """
     test installing a package which defines its own egg_info class
@@ -1439,6 +1450,17 @@ def test_install_no_binary_disables_building_wheels(script, data, with_wheel):
     assert "Running setup.py install for upper" in str(res), str(res)
 
 
+def test_install_no_binary_does_not_disable_building_wheels(
+    script, data, with_wheel
+):
+    res = script.pip(
+        'install', '--no-index', '--no-binary=upper', '-f', data.find_links,
+        '--use-feature=always-install-via-wheel',
+        'upper', expect_stderr=True)
+    assert "Successfully installed upper-2.0" in str(res), str(res)
+    assert "Building wheel for upper" in str(res), str(res)
+
+
 @pytest.mark.network
 def test_install_no_binary_builds_pep_517_wheel(script, data, with_wheel):
     to_install = data.packages.joinpath('pep517_setup_and_pyproject')
@@ -1484,6 +1506,29 @@ def test_install_no_binary_disables_cached_wheels(script, data, with_wheel):
     assert "Running setup.py install for upper" in str(res), str(res)
 
 
+def test_install_no_binary_does_not_disables_cached_wheels(script, data, with_wheel):
+    """When using always-install-via-wheel, --no-binary does not disable
+    the wheel cache"""
+    # Seed the cache
+    res = script.pip(
+        'install', '--no-index',
+        '--use-feature=always-install-via-wheel',
+        '--no-binary=:all:', '-f', data.find_links,
+        'upper')
+    # wheel building occured for upper
+    assert "Building wheel for upper" in str(res), str(res)
+    script.pip('uninstall', 'upper', '-y')
+    res = script.pip(
+        'install', '--no-index',
+        '--use-feature=always-install-via-wheel',
+        '--no-binary=:all:', '-f', data.find_links,
+        'upper', expect_stderr=True)
+    assert "Successfully installed upper-2.0" in str(res), str(res)
+    # Must have used cached wheel.
+    assert "Building wheel for upper" not in str(res), str(res)
+    assert "Using cached Upper" in str(res), str(res)
+
+
 def test_install_editable_with_wrong_egg_name(script, resolver_variant):
     script.scratch_path.joinpath("pkga").mkdir()
     pkga_path = script.scratch_path / 'pkga'