From e0304ef73abf76400347aa21dabd50d125791cd5 Mon Sep 17 00:00:00 2001
From: bitloi <raphaelaloi.eth@gmail.com>
Date: Thu, 26 Mar 2026 09:10:09 +0100
Subject: [PATCH] [`refurb`] Parenthesize generator arguments in FURB142 fixer
 (#21098)

Preserve semantics when rewriting `set.add(...)` loops by parenthesizing unparenthesized generator arguments and add snapshot-backed regressions for both parenthesized and unparenthesized generator cases.
---
 .../resources/test/fixtures/refurb/FURB142.py |  8 ++++
 .../refurb/rules/for_loop_set_mutations.rs    | 22 ++++++----
 ...es__refurb__tests__FURB142_FURB142.py.snap | 42 +++++++++++++++++++
 3 files changed, 65 insertions(+), 7 deletions(-)

diff --git a/crates/ruff_linter/resources/test/fixtures/refurb/FURB142.py b/crates/ruff_linter/resources/test/fixtures/refurb/FURB142.py
index cd39b2f9831c0..45acc2345239c 100644
--- a/crates/ruff_linter/resources/test/fixtures/refurb/FURB142.py
+++ b/crates/ruff_linter/resources/test/fixtures/refurb/FURB142.py
@@ -99,3 +99,11 @@ def g():
 
 for x in (1,) if True else (2,):
     s.add(x)
+
+# https://github.com/astral-sh/ruff/issues/21098
+for x in ("abc", "def"):
+    s.add(c for c in x)
+
+# don't add extra parens for already parenthesized generators
+for x in ("abc", "def"):
+    s.add((c for c in x))
diff --git a/crates/ruff_linter/src/rules/refurb/rules/for_loop_set_mutations.rs b/crates/ruff_linter/src/rules/refurb/rules/for_loop_set_mutations.rs
index 202f5095578e7..e55aca2d5e248 100644
--- a/crates/ruff_linter/src/rules/refurb/rules/for_loop_set_mutations.rs
+++ b/crates/ruff_linter/src/rules/refurb/rules/for_loop_set_mutations.rs
@@ -111,13 +111,21 @@ pub(crate) fn for_loop_set_mutations(checker: &Checker, for_stmt: &StmtFor) {
                 parenthesize_loop_iter_if_necessary(for_stmt, checker, IterLocation::Call),
             )
         }
-        (for_target, arg) => format!(
-            "{}.{batch_method_name}({} for {} in {})",
-            set.id,
-            locator.slice(arg),
-            locator.slice(for_target),
-            parenthesize_loop_iter_if_necessary(for_stmt, checker, IterLocation::Comprehension),
-        ),
+        (for_target, arg) => {
+            let arg_content = match arg {
+                Expr::Generator(generator) if !generator.parenthesized => {
+                    format!("({})", locator.slice(arg))
+                }
+                _ => locator.slice(arg).to_string(),
+            };
+            format!(
+                "{}.{batch_method_name}({} for {} in {})",
+                set.id,
+                arg_content,
+                locator.slice(for_target),
+                parenthesize_loop_iter_if_necessary(for_stmt, checker, IterLocation::Comprehension),
+            )
+        }
     };
 
     let applicability = if checker.comment_ranges().intersects(for_stmt.range) {
diff --git a/crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB142_FURB142.py.snap b/crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB142_FURB142.py.snap
index 4133f63c729a3..820a3a3d1a8e8 100644
--- a/crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB142_FURB142.py.snap
+++ b/crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB142_FURB142.py.snap
@@ -386,6 +386,8 @@ FURB142 [*] Use of `set.add()` in a for loop
 100 | / for x in (1,) if True else (2,):
 101 | |     s.add(x)
     | |____________^
+102 |
+103 |   # https://github.com/astral-sh/ruff/issues/21098
     |
 help: Replace with `.update()`
 97  | for x in lambda: 0:
@@ -394,3 +396,43 @@ help: Replace with `.update()`
     - for x in (1,) if True else (2,):
     -     s.add(x)
 100 + s.update((1,) if True else (2,))
+101 | 
+102 | # https://github.com/astral-sh/ruff/issues/21098
+103 | for x in ("abc", "def"):
+
+FURB142 [*] Use of `set.add()` in a for loop
+   --> FURB142.py:104:1
+    |
+103 |   # https://github.com/astral-sh/ruff/issues/21098
+104 | / for x in ("abc", "def"):
+105 | |     s.add(c for c in x)
+    | |_______________________^
+106 |
+107 |   # don't add extra parens for already parenthesized generators
+    |
+help: Replace with `.update()`
+101 |     s.add(x)
+102 | 
+103 | # https://github.com/astral-sh/ruff/issues/21098
+    - for x in ("abc", "def"):
+    -     s.add(c for c in x)
+104 + s.update((c for c in x) for x in ("abc", "def"))
+105 | 
+106 | # don't add extra parens for already parenthesized generators
+107 | for x in ("abc", "def"):
+
+FURB142 [*] Use of `set.add()` in a for loop
+   --> FURB142.py:108:1
+    |
+107 |   # don't add extra parens for already parenthesized generators
+108 | / for x in ("abc", "def"):
+109 | |     s.add((c for c in x))
+    | |_________________________^
+    |
+help: Replace with `.update()`
+105 |     s.add(c for c in x)
+106 | 
+107 | # don't add extra parens for already parenthesized generators
+    - for x in ("abc", "def"):
+    -     s.add((c for c in x))
+108 + s.update((c for c in x) for x in ("abc", "def"))