[perflint] Parenthesize walrus expressions in autofix for `manual-l…

…ist-comprehension` (`PERF401`) (#15050)

crates/ruff_linter/resources/test/fixtures/perflint/PERF401.py

@@ -251,3 +251,12 @@ def f():
     for i in values:
         result.append(i + 1)  # Ok
     del i
+
+# The fix here must parenthesize the walrus operator
+# https://github.com/astral-sh/ruff/issues/15047
+def f():
+    items = []
+
+    for i in range(5):
+        if j := i:
+            items.append(j)

crates/ruff_linter/src/rules/perflint/rules/manual_list_comprehension.rs

@@ -46,6 +46,11 @@ use ruff_text_size::{Ranged, TextRange};
 /// original = list(range(10000))
 /// filtered.extend(x for x in original if x % 2)
 /// ```
+///
+/// Take care that if the original for-loop uses an assignment expression
+/// as a conditional, such as `if match:=re.match("\d+","123")`, then
+/// the corresponding comprehension must wrap the assignment
+/// expression in parentheses to avoid a syntax error.
 #[derive(ViolationMetadata)]
 pub(crate) struct ManualListComprehension {
     is_async: bool,
@@ -347,7 +352,21 @@ fn convert_to_list_extend(
     let semantic = checker.semantic();
     let locator = checker.locator();
     let if_str = match if_test {
-        Some(test) => format!(" if {}", locator.slice(test.range())),
+        Some(test) => {
+            // If the test is an assignment expression,
+            // we must parenthesize it when it appears
+            // inside the comprehension to avoid a syntax error.
+            //
+            // Notice that we do not need `any_over_expr` here,
+            // since if the assignment expression appears
+            // internally (e.g. as an operand in a boolean
+            // operation) then it will already be parenthesized.
+            if test.is_named_expr() {
+                format!(" if ({})", locator.slice(test.range()))
+            } else {
+                format!(" if {}", locator.slice(test.range()))
+            }
+        }
         None => String::new(),
     };
 

crates/ruff_linter/src/rules/perflint/snapshots/ruff_linter__rules__perflint__tests__PERF401_PERF401.py.snap

@@ -202,3 +202,12 @@ PERF401.py:245:13: PERF401 Use `list.extend` to create a transformed list
 246 |     result = []
     |
     = help: Replace for loop with list.extend
+
+PERF401.py:262:13: PERF401 Use a list comprehension to create a transformed list
+    |
+260 |     for i in range(5):
+261 |         if j := i:
+262 |             items.append(j)
+    |             ^^^^^^^^^^^^^^^ PERF401
+    |
+    = help: Replace for loop with list comprehension

crates/ruff_linter/src/rules/perflint/snapshots/ruff_linter__rules__perflint__tests__preview__PERF401_PERF401.py.snap

@@ -486,3 +486,23 @@ PERF401.py:245:13: PERF401 [*] Use `list.extend` to create a transformed list
 246 245 |     result = []
 247 246 | 
 248 247 | def f():
+
+PERF401.py:262:13: PERF401 [*] Use a list comprehension to create a transformed list
+    |
+260 |     for i in range(5):
+261 |         if j := i:
+262 |             items.append(j)
+    |             ^^^^^^^^^^^^^^^ PERF401
+    |
+    = help: Replace for loop with list comprehension
+
+ℹ Unsafe fix
+255 255 | # The fix here must parenthesize the walrus operator
+256 256 | # https://github.com/astral-sh/ruff/issues/15047
+257 257 | def f():
+258     |-    items = []
+259 258 | 
+260     |-    for i in range(5):
+261     |-        if j := i:
+262     |-            items.append(j)
+    259 |+    items = [j for i in range(5) if (j := i)]