check for external references to the for loop variable

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

@@ -156,3 +156,15 @@ def f():
     result.append(1)
     for i in range(10):
         result.append(i*2) # PERF401
+
+def f():
+    result = []
+    result += [1]
+    for i in range(10):
+        result.append(i*2) # PERF401
+
+def f():
+    result = []
+    for val in range(5):
+        result.append(val * 2) # Ok
+    print(val)

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

@@ -218,6 +218,34 @@ pub(crate) fn manual_list_comprehension(checker: &mut Checker, for_stmt: &ast::S
         return;
     }
 
+    // Avoid if the for-loop target is used outside of the for loop, e.g.,
+    //
+    // ```python
+    // for x in y:
+    //     filtered.append(x)
+    // print(x)
+    // ```
+    //
+    // If this were a comprehension, x would no longer have the correct scope:
+    //
+    // ```python
+    // filtered = [x for x in y]
+    // print(x)
+    // ```
+    let for_loop_target = checker
+        .semantic()
+        .lookup_symbol(id.as_str())
+        .map(|id| checker.semantic().binding(id))
+        .expect("for loop target must exist");
+    // TODO: this currently does not properly find usages outside the for loop; figure out why
+    if for_loop_target
+        .references()
+        .map(|id| checker.semantic().reference(id))
+        .any(|reference| !for_stmt.range.contains_range(reference.range()))
+    {
+        return;
+    }
+
     let binding_stmt = binding
         .statement(checker.semantic())
         .and_then(|stmt| stmt.as_assign_stmt());
@@ -261,7 +289,7 @@ pub(crate) fn manual_list_comprehension(checker: &mut Checker, for_stmt: &ast::S
         // 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);

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

@@ -121,5 +121,17 @@ PERF401.py:158:9: PERF401 Use `list.extend` to create a transformed list
 157 |     for i in range(10):
 158 |         result.append(i*2) # PERF401
     |         ^^^^^^^^^^^^^^^^^^ PERF401
+159 | 
+160 | def f():
     |
     = help: Replace for loop with list.extend
+
+PERF401.py:169:9: PERF401 Use a list comprehension to create a transformed list
+    |
+167 |     result = []
+168 |     for val in range(5):
+169 |         result.append(val * 2) # Ok
+    |         ^^^^^^^^^^^^^^^^^^^^^^ PERF401
+170 |     print(val)
+    |
+    = help: Replace for loop with list comprehension

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

@@ -279,6 +279,8 @@ PERF401.py:158:9: PERF401 [*] Use `list.extend` to create a transformed list
 157 |     for i in range(10):
 158 |         result.append(i*2) # PERF401
     |         ^^^^^^^^^^^^^^^^^^ PERF401
+159 | 
+160 | def f():
     |
     = help: Replace for loop with list.extend
 
@@ -289,3 +291,26 @@ PERF401.py:158:9: PERF401 [*] Use `list.extend` to create a transformed list
 157     |-    for i in range(10):
 158     |-        result.append(i*2) # PERF401
     157 |+    result.extend(i*2 for i in range(10)) # PERF401
+159 158 | 
+160 159 | def f():
+161 160 |     result = []
+
+PERF401.py:169:9: PERF401 [*] Use a list comprehension to create a transformed list
+    |
+167 |     result = []
+168 |     for val in range(5):
+169 |         result.append(val * 2) # Ok
+    |         ^^^^^^^^^^^^^^^^^^^^^^ PERF401
+170 |     print(val)
+    |
+    = help: Replace for loop with list comprehension
+
+ℹ Unsafe fix
+164 164 |         result.append(i*2) # PERF401
+165 165 | 
+166 166 | def f():
+167     |-    result = []
+168     |-    for val in range(5):
+169     |-        result.append(val * 2) # Ok
+    167 |+    result = [val * 2 for val in range(5)] # Ok
+170 168 |     print(val)