Percolate --use-feature from req file upwards

We explicitly propagate --use-feature options from req files upwards.
This is not strictly necessary for the option to be enabled, because
of the default value is a global list, but that implicit behaviour is
certainly accidental, so we make it explicit, with a test.

src/pip/_internal/req/req_file.py

@@ -225,12 +225,18 @@ def handle_option_line(
 ):
     # type:  (...) -> None
 
-    # percolate hash-checking option upward
-    if options and opts.require_hashes:
-        options.require_hashes = opts.require_hashes
+    if options:
+        # percolate options upward
+        if opts.require_hashes:
+            options.require_hashes = opts.require_hashes
+        if opts.features_enabled:
+            options.features_enabled.extend(
+                f for f in opts.features_enabled
+                if f not in options.features_enabled
+            )
 
     # set finder options
-    elif finder:
+    if finder:
         find_links = finder.find_links
         index_urls = finder.index_urls
         if opts.index_url:

tests/unit/test_req_file.py

@@ -47,6 +47,7 @@ def options(session):
         isolated_mode=False,
         index_url='default_url',
         format_control=FormatControl(set(), set()),
+        features_enabled=[],
     )
 
 
@@ -382,9 +383,12 @@ class TestProcessLine(object):
         line_processor("--pre", "file", 1, finder=finder)
         assert finder.allow_all_prereleases
 
-    def test_use_feature(self, line_processor):
+    def test_use_feature(self, line_processor, options):
         """--use-feature can be set in requirements files."""
-        line_processor("--use-feature=2020-resolver", "filename", 1)
+        line_processor(
+            "--use-feature=2020-resolver", "filename", 1, options=options
+        )
+        assert "2020-resolver" in options.features_enabled
 
     def test_relative_local_find_links(
         self, line_processor, finder, monkeypatch, tmpdir