[F812] Warn when list comprehension redefines a name

This lint rule is probably in Flake8 to prevent elements from leaking
out of the comprehension scope, but the behavior was fixed in Python3.
I worry such a strict lint rule will cause more problems than it solves,
considering we use Python 3.10 at Meta. Perhaps we should consider
disabling this rule by default.

ghstack-source-id: 0f7f3b64d39e518b4b64bd1ca55aa3bd6b934eb2
Pull Request resolved: #276

src/fixit/rules/pyflakes/list_comp_must_use_unique_name.py

@@ -0,0 +1,104 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+from typing import Optional, Set
+
+import libcst as cst
+
+from fixit import CstLintRule, InvalidTestCase as Invalid, ValidTestCase as Valid
+
+
+class ListCompMustUseUniqueNameRule(CstLintRule):
+    """
+    A list comprehension shouldn't use the same name as another variable
+    defined in the module.
+
+    Autofix: N/A
+    """
+
+    CUSTOM_MESSAGE = ("The name {name} is already defined in the module."
+                      "Although list comprehensions have their own "
+                      "scope, it is best practice to use a unique name.")
+
+    METADATA_DEPENDENCIES = (cst.metadata.ScopeProvider,)
+
+    def __init__(self) -> None:
+        super().__init__()
+
+    def visit_ListComp(self, node: cst.ListComp) -> None:
+        names: Set[str] = set()
+        for_in: Optional[cst.CompFor] = node.for_in
+        while for_in:
+            assert isinstance(for_in.target, cst.Name)
+            names.add(for_in.target.value)
+            for_in = for_in.inner_for_in
+        metadata = self.get_metadata(cst.metadata.ScopeProvider, node)
+        assert isinstance(metadata, cst.metadata.Scope)
+        for name in names:
+            if metadata._contains_in_self_or_parent(name):
+                self.report(node, self.CUSTOM_MESSAGE.format(name=name))
+
+    VALID = [
+        Valid(
+            """
+            n = 1
+            squares = [i ** 2 for i in range(10)]
+            """
+        ),
+        Valid(
+            """
+            doubles = [i * 2 for i in range(10)]
+            squares = [i ** 2 for i in range(10)]
+            """
+        ),
+        Valid(
+            """
+            tags = ["a", "b", "c", "d"]
+            entries = [["a", "b"],["c", "d"]]
+            results = [lst for tag in tags for lst in entries if tag in lst]
+            """
+        ),
+    ]
+
+    INVALID = [
+        Invalid(
+            """
+            def fn():
+                return [i ** 2 for i in range(10)]
+
+            i = 1
+            """
+        ),
+        Invalid(
+            """
+            i = 1
+            squares = [i ** 2 for i in range(10)]
+            """
+        ),
+        Invalid(
+            """
+            i = 1
+
+            def fn():
+                return [i ** 2 for i in range(10)]
+            """
+        ),
+        Invalid(
+            """
+            tags = ["a", "b", "c", "d"]
+            entries = [["a", "b"],["c", "d"]]
+            tag = "a"
+            results = [lst for tag in tags for lst in entries if tag in lst]
+            """
+        ),
+        Invalid(
+            """
+            tags = ["a", "b", "c", "d"]
+            entries = [["a", "b"],["c", "d"]]
+            lst = ["a", "b"]
+            results = [lst for tag in tags for lst in entries if tag in lst]
+            """
+        ),
+    ]