From f69040f888db1e9e7a311ab6744a88f0c945d687 Mon Sep 17 00:00:00 2001
From: Charlie Marsh <charlie.r.marsh@gmail.com>
Date: Mon, 31 Jul 2023 23:47:58 -0400
Subject: [PATCH] Avoid PERF401 false positive on list access in loop

---
 crates/ruff/resources/test/fixtures/perflint/PERF401.py   | 7 +++++++
 .../src/rules/perflint/rules/manual_list_comprehension.rs | 8 ++++++++
 crates/ruff/src/rules/perflint/rules/manual_list_copy.rs  | 5 ++---
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/crates/ruff/resources/test/fixtures/perflint/PERF401.py b/crates/ruff/resources/test/fixtures/perflint/PERF401.py
index 2c4434c8ed433c..f50cf49e066ec7 100644
--- a/crates/ruff/resources/test/fixtures/perflint/PERF401.py
+++ b/crates/ruff/resources/test/fixtures/perflint/PERF401.py
@@ -45,3 +45,10 @@ def f():
     for i in items:
         if i not in result:
             result.append(i)  # OK
+
+
+def f():
+    fibonacci = [0, 1]
+    for i in range(20):
+        fibonacci.append(sum(fibonacci[-2:]))  # OK
+    print(fibonacci)
diff --git a/crates/ruff/src/rules/perflint/rules/manual_list_comprehension.rs b/crates/ruff/src/rules/perflint/rules/manual_list_comprehension.rs
index 2881cd70bb1d8a..1f9f287edfcd96 100644
--- a/crates/ruff/src/rules/perflint/rules/manual_list_comprehension.rs
+++ b/crates/ruff/src/rules/perflint/rules/manual_list_comprehension.rs
@@ -2,6 +2,7 @@ use ruff_python_ast::{self as ast, Expr, Stmt};
 
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::comparable::ComparableExpr;
 use ruff_python_ast::helpers::any_over_expr;
 
 use crate::checkers::ast::Checker;
@@ -129,6 +130,13 @@ pub(crate) fn manual_list_comprehension(checker: &mut Checker, target: &Expr, bo
         return;
     }
 
+    // Avoid, e.g., `for x in y: filtered.append(filtered[-1] * 2)`.
+    if any_over_expr(arg, &|expr| {
+        ComparableExpr::from(expr) == ComparableExpr::from(value)
+    }) {
+        return;
+    }
+
     // Avoid if the value is used in the conditional test, e.g.,
     //
     // ```python
diff --git a/crates/ruff/src/rules/perflint/rules/manual_list_copy.rs b/crates/ruff/src/rules/perflint/rules/manual_list_copy.rs
index 051b508adab86b..902fb2138b42fe 100644
--- a/crates/ruff/src/rules/perflint/rules/manual_list_copy.rs
+++ b/crates/ruff/src/rules/perflint/rules/manual_list_copy.rs
@@ -1,8 +1,7 @@
-use ruff_python_ast::{self as ast, Expr, Stmt};
-
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::helpers::any_over_expr;
+use ruff_python_ast::{self as ast, Expr, Stmt};
 
 use crate::checkers::ast::Checker;
 
@@ -89,7 +88,7 @@ pub(crate) fn manual_list_copy(checker: &mut Checker, target: &Expr, body: &[Stm
         return;
     }
 
-    // Avoid, e.g., `for x in y: filtered[x].append(x * x)`.
+    // Avoid, e.g., `for x in y: filtered[x].append(x)`.
     if any_over_expr(value, &|expr| {
         expr.as_name_expr().map_or(false, |expr| expr.id == *id)
     }) {