Hide feature flag if deprecated behaviour is gone

Author: pradyunsg

src/pip/_internal/utils/deprecation.py

@@ -97,7 +97,9 @@ def deprecated(
         ),
         (
             feature_flag,
-            "You can use the flag --use-feature={} to test the upcoming behaviour.",
+            "You can use the flag --use-feature={} to test the upcoming behaviour."
+            if not is_gone
+            else None,
         ),
         (
             issue,
@@ -108,7 +110,7 @@ def deprecated(
     message = " ".join(
         format_str.format(value)
         for value, format_str in message_parts
-        if value is not None
+        if format_str is not None and value is not None
     )
 
     # Raise as an error if this behaviour is deprecated.

tests/unit/test_utils.py

@@ -920,20 +920,22 @@ def test_deprecated_raises_error_if_too_old(replacement, issue, feature_flag):
 
     assert "DEPRECATION: Stop doing this!" in message
     assert "1.0" in message
+    assert str(feature_flag) not in message
     # Ensure non-None values are mentioned.
-    for item in [replacement, issue, feature_flag]:
+    for item in [replacement, issue]:
         if item is not None:
             assert str(item) in message
 
 
 @pytest.mark.usefixtures("patch_deprecation_check_version")
-def test_deprecated_message_reads_well():
+def test_deprecated_message_reads_well_past():
     with pytest.raises(PipDeprecationWarning) as exception:
         deprecated(
             reason="Stop doing this!",
             gone_in="1.0",  # this matches the patched version.
             replacement="to be nicer",
-            issue="100000",  # I hope we never reach this number.
+            feature_flag="magic-8-ball",
+            issue="100000",
         )
 
     message = exception.value.args[0]
@@ -946,6 +948,29 @@ def test_deprecated_message_reads_well():
     )
 
 
+@pytest.mark.usefixtures("patch_deprecation_check_version")
+def test_deprecated_message_reads_well_future():
+    with pytest.warns(PipDeprecationWarning) as record:
+        deprecated(
+            reason="Stop doing this!",
+            gone_in="2.0",  # this is greater than the patched version.
+            replacement="to be nicer",
+            feature_flag="crisis",
+            issue="100000",
+        )
+
+    assert len(record) == 1
+    message = record[0].message.args[0]
+
+    assert message == (
+        "DEPRECATION: Stop doing this! "
+        "pip 2.0 will enforce this behaviour change. "
+        "A possible replacement is to be nicer. "
+        "You can use the flag --use-feature=crisis to test the upcoming behaviour. "
+        "Discussion can be found at https://github.com/pypa/pip/issues/100000"
+    )
+
+
 def test_make_setuptools_shim_args():
     # Test all arguments at once, including the overall ordering.
     args = make_setuptools_shim_args(