[`refurb`] Mark `single-item-membership-test` fix as always unsafe (`FURB171`) (#20279)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fixes #20255

Mark single-item-membership-test fixes as always unsafe

- Always set `Applicability::Unsafe` for FURB171 fixes
- Update “Fix safety” docs to reflect always-unsafe behavior
- Expand tests (not in, nested set/frozenset, commented args)

## Test Plan

<!-- How was it tested? -->

I have added new test cases to
`crates/ruff_linter/resources/test/fixtures/refurb/FURB171_0.py` and
`crates/ruff_linter/resources/test/fixtures/refurb/FURB171_1.py`.

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>

crates/ruff_linter/resources/test/fixtures/refurb/FURB171_0.py

@@ -117,3 +117,31 @@ def _():
         # After
     ] and \
         0 < 1: ...
+
+# https://github.com/astral-sh/ruff/issues/20255
+import math
+
+# NaN behavior differences
+if math.nan in [math.nan]:
+    print("This is True")
+
+if math.nan in (math.nan,):
+    print("This is True")
+
+if math.nan in {math.nan}:
+    print("This is True")
+
+# Potential type differences with custom __eq__ methods
+class CustomEq:
+    def __eq__(self, other):
+        return "custom"
+
+obj = CustomEq()
+if obj in [CustomEq()]:
+    pass
+
+if obj in (CustomEq(),):
+    pass
+
+if obj in {CustomEq()}:
+    pass

crates/ruff_linter/resources/test/fixtures/refurb/FURB171_1.py

@@ -51,3 +51,13 @@ if 1 in set(1,2):
 
 if 1 in set((x for x in range(2))):
     pass
+
+# https://github.com/astral-sh/ruff/issues/20255
+import math
+
+# set() and frozenset() with NaN
+if math.nan in set([math.nan]):
+    print("This should be marked unsafe")
+
+if math.nan in frozenset([math.nan]):
+    print("This should be marked unsafe")

crates/ruff_linter/src/rules/refurb/rules/single_item_membership_test.rs

@@ -6,7 +6,7 @@ use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
 use crate::fix::edits::pad;
-use crate::{Applicability, Edit, Fix, FixAvailability, Violation};
+use crate::{Edit, Fix, FixAvailability, Violation};
 
 /// ## What it does
 /// Checks for membership tests against single-item containers.
@@ -27,13 +27,16 @@ use crate::{Applicability, Edit, Fix, FixAvailability, Violation};
 /// ```
 ///
 /// ## Fix safety
+/// The fix is always marked as unsafe.
 ///
-/// When the right-hand side is a string, the fix is marked as unsafe.
+/// When the right-hand side is a string, this fix can change the behavior of your program.
-/// This is because `c in "a"` is true both when `c` is `"a"` and when `c` is the empty string,
+/// This is because `c in "a"` is true both when `c` is `"a"` and when `c` is the empty string.
-/// so the fix can change the behavior of your program in these cases.
 ///
-/// Additionally, if there are comments within the fix's range,
+/// Additionally, converting `in`/`not in` against a single-item container to `==`/`!=` can
-/// it will also be marked as unsafe.
+/// change runtime behavior: `in` may consider identity (e.g., `NaN`) and always
+/// yields a `bool`.
+///
+/// Comments within the replacement range will also be removed.
 ///
 /// ## References
 /// - [Python documentation: Comparisons](https://docs.python.org/3/reference/expressions.html#comparisons)
@@ -100,14 +103,8 @@ pub(crate) fn single_item_membership_test(
         expr.range(),
     );
 
-    let applicability =
+    // All supported cases can change runtime behavior; mark as unsafe.
-        if right.is_string_literal_expr() || checker.comment_ranges().intersects(expr.range()) {
+    let fix = Fix::unsafe_edit(edit);
-            Applicability::Unsafe
-        } else {
-            Applicability::Safe
-        };
-
-    let fix = Fix::applicable_edit(edit, applicability);
 
     checker
         .report_diagnostic(SingleItemMembershipTest { membership_test }, expr.range())

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB171_FURB171_0.py.snap

@@ -18,6 +18,7 @@ help: Convert to equality test
 4 |     print("Single-element tuple")
 5 | 
 6 | if 1 in [1]:
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
  --> FURB171_0.py:6:4
@@ -37,6 +38,7 @@ help: Convert to equality test
 7 |     print("Single-element list")
 8 | 
 9 | if 1 in {1}:
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_0.py:9:4
@@ -56,6 +58,7 @@ help: Convert to equality test
 10 |     print("Single-element set")
 11 | 
 12 | if "a" in "a":
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_0.py:12:4
@@ -95,6 +98,7 @@ help: Convert to inequality test
 16 |     print("Check `not in` membership test")
 17 | 
 18 | if not 1 in (1,):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_0.py:18:8
@@ -114,6 +118,7 @@ help: Convert to equality test
 19 |     print("Check the negated membership test")
 20 | 
 21 | # Non-errors.
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_0.py:52:5
@@ -344,4 +349,122 @@ help: Convert to inequality test
     -     ] and \
 113 +     if foo != bar and \
 114 |         0 < 1: ...
+115 | 
+116 | # https://github.com/astral-sh/ruff/issues/20255
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:125:4
+    |
+124 | # NaN behavior differences
+125 | if math.nan in [math.nan]:
+    |    ^^^^^^^^^^^^^^^^^^^^^^
+126 |     print("This is True")
+    |
+help: Convert to equality test
+122 | import math
+123 | 
+124 | # NaN behavior differences
+    - if math.nan in [math.nan]:
+125 + if math.nan == math.nan:
+126 |     print("This is True")
+127 | 
+128 | if math.nan in (math.nan,):
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:128:4
+    |
+126 |     print("This is True")
+127 |
+128 | if math.nan in (math.nan,):
+    |    ^^^^^^^^^^^^^^^^^^^^^^^
+129 |     print("This is True")
+    |
+help: Convert to equality test
+125 | if math.nan in [math.nan]:
+126 |     print("This is True")
+127 | 
+    - if math.nan in (math.nan,):
+128 + if math.nan == math.nan:
+129 |     print("This is True")
+130 | 
+131 | if math.nan in {math.nan}:
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:131:4
+    |
+129 |     print("This is True")
+130 |
+131 | if math.nan in {math.nan}:
+    |    ^^^^^^^^^^^^^^^^^^^^^^
+132 |     print("This is True")
+    |
+help: Convert to equality test
+128 | if math.nan in (math.nan,):
+129 |     print("This is True")
+130 | 
+    - if math.nan in {math.nan}:
+131 + if math.nan == math.nan:
+132 |     print("This is True")
+133 | 
+134 | # Potential type differences with custom __eq__ methods
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:140:4
+    |
+139 | obj = CustomEq()
+140 | if obj in [CustomEq()]:
+    |    ^^^^^^^^^^^^^^^^^^^
+141 |     pass
+    |
+help: Convert to equality test
+137 |         return "custom"
+138 | 
+139 | obj = CustomEq()
+    - if obj in [CustomEq()]:
+140 + if obj == CustomEq():
+141 |     pass
+142 | 
+143 | if obj in (CustomEq(),):
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:143:4
+    |
+141 |     pass
+142 |
+143 | if obj in (CustomEq(),):
+    |    ^^^^^^^^^^^^^^^^^^^^
+144 |     pass
+    |
+help: Convert to equality test
+140 | if obj in [CustomEq()]:
+141 |     pass
+142 | 
+    - if obj in (CustomEq(),):
+143 + if obj == CustomEq():
+144 |     pass
+145 | 
+146 | if obj in {CustomEq()}:
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+   --> FURB171_0.py:146:4
+    |
+144 |     pass
+145 |
+146 | if obj in {CustomEq()}:
+    |    ^^^^^^^^^^^^^^^^^^^
+147 |     pass
+    |
+help: Convert to equality test
+143 | if obj in (CustomEq(),):
+144 |     pass
+145 | 
+    - if obj in {CustomEq()}:
+146 + if obj == CustomEq():
+147 |     pass
 note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB171_FURB171_1.py.snap

@@ -18,6 +18,7 @@ help: Convert to equality test
 4 |     print("Single-element set")
 5 | 
 6 | if 1 in set((1,)):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
  --> FURB171_1.py:6:4
@@ -37,6 +38,7 @@ help: Convert to equality test
 7 |     print("Single-element set")
 8 | 
 9 | if 1 in set({1}):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_1.py:9:4
@@ -56,6 +58,7 @@ help: Convert to equality test
 10 |     print("Single-element set")
 11 | 
 12 | if 1 in frozenset([1]):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_1.py:12:4
@@ -75,6 +78,7 @@ help: Convert to equality test
 13 |     print("Single-element set")
 14 | 
 15 | if 1 in frozenset((1,)):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_1.py:15:4
@@ -94,6 +98,7 @@ help: Convert to equality test
 16 |     print("Single-element set")
 17 | 
 18 | if 1 in frozenset({1}):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_1.py:18:4
@@ -113,6 +118,7 @@ help: Convert to equality test
 19 |     print("Single-element set")
 20 | 
 21 | if 1 in set(set([1])):
+note: This is an unsafe fix and may change runtime behavior
 
 FURB171 [*] Membership test against single-item container
   --> FURB171_1.py:21:4
@@ -132,3 +138,41 @@ help: Convert to equality test
 22 |     print('Recursive solution')
 23 | 
 24 | 
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+  --> FURB171_1.py:59:4
+   |
+58 | # set() and frozenset() with NaN
+59 | if math.nan in set([math.nan]):
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+60 |     print("This should be marked unsafe")
+   |
+help: Convert to equality test
+56 | import math
+57 | 
+58 | # set() and frozenset() with NaN
+   - if math.nan in set([math.nan]):
+59 + if math.nan == math.nan:
+60 |     print("This should be marked unsafe")
+61 | 
+62 | if math.nan in frozenset([math.nan]):
+note: This is an unsafe fix and may change runtime behavior
+
+FURB171 [*] Membership test against single-item container
+  --> FURB171_1.py:62:4
+   |
+60 |     print("This should be marked unsafe")
+61 |
+62 | if math.nan in frozenset([math.nan]):
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+63 |     print("This should be marked unsafe")
+   |
+help: Convert to equality test
+59 | if math.nan in set([math.nan]):
+60 |     print("This should be marked unsafe")
+61 | 
+   - if math.nan in frozenset([math.nan]):
+62 + if math.nan == math.nan:
+63 |     print("This should be marked unsafe")
+note: This is an unsafe fix and may change runtime behavior