don't use a comprehension when the list is referenced

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

@@ -149,3 +149,10 @@ def f():
     result = [] # comment should be protected
     for i in range(10):
         result.append(i*2) # PERF401
+
+
+def f():
+    result = []
+    result.append(1)
+    for i in range(10):
+        result.append(i*2) # PERF401

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

@@ -249,6 +249,19 @@ pub(crate) fn manual_list_comprehension(checker: &mut Checker, for_stmt: &ast::S
         }
     };
 
+    // If the binding gets used in between the assignment and the for loop, a list comprehension is no longer safe
+    let binding_unused_between = binding_stmt.is_some_and(|binding_stmt| {
+        let from_assign_to_loop = binding_stmt.range.cover(for_stmt.range);
+        // count the number of references between the assignment and the for loop
+        let count = binding
+            .references()
+            .map(|ref_id| checker.semantic().reference(ref_id).range())
+            .filter(|text_range| from_assign_to_loop.contains_range(*text_range))
+            .count();
+        // if there's more than one, then it's been accessed in the middle somewhere, so it's not safe to change into a list comprehension
+        count < 2
+    });
+
     // If the binding has multiple statements on its line, it gets more complicated to fix, so we use an extend
     // TODO: make this work
     let binding_only_stmt_on_line = binding_stmt.is_some_and(|_binding_stmt| true);
@@ -258,6 +271,7 @@ pub(crate) fn manual_list_comprehension(checker: &mut Checker, for_stmt: &ast::S
         && assignment_in_same_statement
         && binding_has_one_target
         && binding_only_stmt_on_line
+        && binding_unused_between
     {
         ComprehensionType::ListComprehension
     } else {

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

@@ -114,3 +114,12 @@ PERF401.py:151:9: PERF401 Use a list comprehension to create a transformed list
     |         ^^^^^^^^^^^^^^^^^^ PERF401
     |
     = help: Replace for loop with list comprehension
+
+PERF401.py:158:9: PERF401 Use `list.extend` to create a transformed list
+    |
+156 |     result.append(1)
+157 |     for i in range(10):
+158 |         result.append(i*2) # PERF401
+    |         ^^^^^^^^^^^^^^^^^^ PERF401
+    |
+    = help: Replace for loop with list.extend

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

@@ -269,3 +269,23 @@ PERF401.py:151:9: PERF401 [*] Use a list comprehension to create a transformed l
 151     |-        result.append(i*2) # PERF401
     149 |+    # comment should be protected
     150 |+    result = [i*2 for i in range(10)] # PERF401
+152 151 | 
+153 152 | 
+154 153 | def f():
+
+PERF401.py:158:9: PERF401 [*] Use `list.extend` to create a transformed list
+    |
+156 |     result.append(1)
+157 |     for i in range(10):
+158 |         result.append(i*2) # PERF401
+    |         ^^^^^^^^^^^^^^^^^^ PERF401
+    |
+    = help: Replace for loop with list.extend
+
+ℹ Unsafe fix
+154 154 | def f():
+155 155 |     result = []
+156 156 |     result.append(1)
+157     |-    for i in range(10):
+158     |-        result.append(i*2) # PERF401
+    157 |+    result.extend(i*2 for i in range(10)) # PERF401