[flake8-simplify] Implement enumerate-for-loop (SIM113) (#7777)

Implements SIM113 from #998

Added tests
Limitations 
   - No fix yet
   - Only flag cases where index variable immediately precede `for` loop

@charliermarsh please review and let me know any improvements

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM113.py

@@ -0,0 +1,168 @@
+def func():
+    # SIM113
+    idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        h(x)
+
+
+def func():
+    # SIM113
+    sum = 0
+    idx = 0
+    for x in range(5):
+        if g(x):
+            break
+        idx += 1
+        sum += h(x, idx)
+
+
+def func():
+    # SIM113
+    idx = 0
+    for x, y in zip(range(5), range(5)):
+        g(x)
+        h(x, y)
+        idx += 1
+
+
+def func():
+    # SIM113
+    idx = 0
+    sum = 0
+    for x in range(5):
+        sum += h(x, idx)
+        idx += 1
+
+
+def func():
+    # SIM113
+    sum, idx = 0, 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        h(x)
+
+
+def func():
+    # OK (index doesn't start at 0
+    idx = 10
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        h(x)
+
+
+def func():
+    # OK (incremented by more than 1)
+    idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 2
+        h(x)
+
+
+def func():
+    # OK (incremented in `if`)
+    idx = 0
+    for x in range(5):
+        if g(x):
+            idx += 1
+        h(x)
+
+
+def func():
+    # OK (`continue` in match-case)
+    idx = 0
+    for x in range(5):
+        match g(x):
+            case 1:
+                h(x)
+            case 2:
+                continue
+            case _:
+                h(idx)
+        idx += 1
+
+
+def func():
+    # OK (`continue` inside `with` clause)
+    idx = 0
+    for x in range(5):
+        with context as c:
+            if g(x):
+                continue
+            h(x, idx, c)
+        idx += 1
+
+
+def func():
+    # OK (incremented in `try` block)
+    idx = 0
+    for x in range(5):
+        try:
+            g(x, idx)
+        except:
+            h(x)
+            continue
+        idx += 1
+
+
+def func(idx: 0):
+    # OK (index is an argument)
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+
+
+def func(x: int):
+    # OK (index _may_ be non-zero)
+    if x > 0:
+        idx = 1
+    else:
+        idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        h(x)
+
+
+def func():
+    # OK
+    idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        idx += 1
+        h(x)
+
+
+def func():
+    # OK
+    idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        if idx > 10:
+            idx *= 2
+        h(x)
+
+
+def func():
+    # OK (index used after loop)
+    idx = 0
+    for x in range(5):
+        g(x, idx)
+        idx += 1
+        h(x)
+    print(idx)
+
+
+def func():
+    # OK (index within nested loop)
+    idx = 0
+    for x in range(5):
+        for y in range(5):
+            g(x, idx)
+            idx += 1

crates/ruff_linter/src/checkers/ast/analyze/deferred_for_loops.rs

@@ -2,7 +2,7 @@ use ruff_python_ast::Stmt;
 
 use crate::checkers::ast::Checker;
 use crate::codes::Rule;
-use crate::rules::{flake8_bugbear, perflint, pyupgrade, refurb};
+use crate::rules::{flake8_bugbear, flake8_simplify, perflint, pyupgrade, refurb};
 
 /// Run lint rules over all deferred for-loops in the [`SemanticModel`].
 pub(crate) fn deferred_for_loops(checker: &mut Checker) {
@@ -27,6 +27,9 @@ pub(crate) fn deferred_for_loops(checker: &mut Checker) {
             if checker.enabled(Rule::UnnecessaryEnumerate) {
                 refurb::rules::unnecessary_enumerate(checker, stmt_for);
             }
+            if checker.enabled(Rule::EnumerateForLoop) {
+                flake8_simplify::rules::enumerate_for_loop(checker, stmt_for);
+            }
         }
     }
 }

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -1258,9 +1258,10 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             },
         ) => {
             if checker.any_enabled(&[
-                Rule::UnusedLoopControlVariable,
+                Rule::EnumerateForLoop,
                 Rule::IncorrectDictIterator,
                 Rule::UnnecessaryEnumerate,
+                Rule::UnusedLoopControlVariable,
                 Rule::YieldInForLoop,
             ]) {
                 checker.deferred.for_loops.push(checker.semantic.snapshot());

crates/ruff_linter/src/codes.rs

@@ -454,6 +454,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Simplify, "109") => (RuleGroup::Stable, rules::flake8_simplify::rules::CompareWithTuple),
         (Flake8Simplify, "110") => (RuleGroup::Stable, rules::flake8_simplify::rules::ReimplementedBuiltin),
         (Flake8Simplify, "112") => (RuleGroup::Stable, rules::flake8_simplify::rules::UncapitalizedEnvironmentVariables),
+        (Flake8Simplify, "113") => (RuleGroup::Preview, rules::flake8_simplify::rules::EnumerateForLoop),
         (Flake8Simplify, "114") => (RuleGroup::Stable, rules::flake8_simplify::rules::IfWithSameArms),
         (Flake8Simplify, "115") => (RuleGroup::Stable, rules::flake8_simplify::rules::OpenFileWithContextHandler),
         (Flake8Simplify, "116") => (RuleGroup::Stable, rules::flake8_simplify::rules::IfElseBlockInsteadOfDictLookup),

crates/ruff_linter/src/rules/flake8_simplify/mod.rs

@@ -27,6 +27,7 @@ mod tests {
     #[test_case(Rule::ReimplementedBuiltin, Path::new("SIM110.py"))]
     #[test_case(Rule::ReimplementedBuiltin, Path::new("SIM111.py"))]
     #[test_case(Rule::UncapitalizedEnvironmentVariables, Path::new("SIM112.py"))]
+    #[test_case(Rule::EnumerateForLoop, Path::new("SIM113.py"))]
     #[test_case(Rule::OpenFileWithContextHandler, Path::new("SIM115.py"))]
     #[test_case(Rule::MultipleWithStatements, Path::new("SIM117.py"))]
     #[test_case(Rule::InDictKeys, Path::new("SIM118.py"))]

crates/ruff_linter/src/rules/flake8_simplify/rules/ast_bool_op.rs

@@ -287,7 +287,7 @@ impl AlwaysFixableViolation for ExprAndFalse {
 }
 
 /// Return `true` if two `Expr` instances are equivalent names.
-fn is_same_expr<'a>(a: &'a Expr, b: &'a Expr) -> Option<&'a str> {
+pub(crate) fn is_same_expr<'a>(a: &'a Expr, b: &'a Expr) -> Option<&'a str> {
     if let (Expr::Name(ast::ExprName { id: a, .. }), Expr::Name(ast::ExprName { id: b, .. })) =
         (&a, &b)
     {

crates/ruff_linter/src/rules/flake8_simplify/rules/enumerate_for_loop.rs

@@ -0,0 +1,193 @@
+use ruff_diagnostics::{Diagnostic, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast::statement_visitor::{walk_stmt, StatementVisitor};
+use ruff_python_ast::{self as ast, Expr, Int, Number, Operator, Stmt};
+use ruff_python_semantic::analyze::typing;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+
+/// ## What it does
+/// Checks for `for` loops with explicit loop-index variables that can be replaced
+/// with `enumerate()`.
+///
+/// ## Why is this bad?
+/// When iterating over a sequence, it's often desirable to keep track of the
+/// index of each element alongside the element itself. Prefer the `enumerate`
+/// builtin over manually incrementing a counter variable within the loop, as
+/// `enumerate` is more concise and idiomatic.
+///
+/// ## Example
+/// ```python
+/// fruits = ["apple", "banana", "cherry"]
+/// for fruit in fruits:
+///     print(f"{i + 1}. {fruit}")
+///     i += 1
+/// ```
+///
+/// Use instead:
+/// ```python
+/// fruits = ["apple", "banana", "cherry"]
+/// for i, fruit in enumerate(fruits):
+///     print(f"{i + 1}. {fruit}")
+/// ```
+///
+/// ## References
+/// - [Python documentation: `enumerate`](https://docs.python.org/3/library/functions.html#enumerate)
+#[violation]
+pub struct EnumerateForLoop {
+    index: String,
+}
+
+impl Violation for EnumerateForLoop {
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        let EnumerateForLoop { index } = self;
+        format!("Use `enumerate()` for index variable `{index}` in `for` loop")
+    }
+}
+
+/// SIM113
+pub(crate) fn enumerate_for_loop(checker: &mut Checker, for_stmt: &ast::StmtFor) {
+    // If the loop contains a `continue`, abort.
+    let mut visitor = LoopControlFlowVisitor::default();
+    visitor.visit_body(&for_stmt.body);
+    if visitor.has_continue {
+        return;
+    }
+
+    for stmt in &for_stmt.body {
+        // Find the augmented assignment expression (e.g., `i += 1`).
+        if let Some(index) = match_index_increment(stmt) {
+            // Find the binding corresponding to the initialization (e.g., `i = 1`).
+            let Some(id) = checker.semantic().resolve_name(index) else {
+                continue;
+            };
+
+            // If it's not an assignment (e.g., it's a function argument), ignore it.
+            let binding = checker.semantic().binding(id);
+            if !binding.kind.is_assignment() {
+                continue;
+            }
+
+            // If the variable is global or nonlocal, ignore it.
+            if binding.is_global() || binding.is_nonlocal() {
+                continue;
+            }
+
+            // Ensure that the index variable was initialized to 0.
+            let Some(value) = typing::find_binding_value(&index.id, binding, checker.semantic())
+            else {
+                continue;
+            };
+            if !matches!(
+                value,
+                Expr::NumberLiteral(ast::ExprNumberLiteral {
+                    value: Number::Int(Int::ZERO),
+                    ..
+                })
+            ) {
+                continue;
+            }
+
+            // If the binding is not at the same level as the `for` loop (e.g., it's in an `if`),
+            // ignore it.
+            let Some(for_loop_id) = checker.semantic().current_statement_id() else {
+                continue;
+            };
+            let Some(assignment_id) = binding.source else {
+                continue;
+            };
+            if checker.semantic().parent_statement_id(for_loop_id)
+                != checker.semantic().parent_statement_id(assignment_id)
+            {
+                continue;
+            }
+
+            // Identify the binding created by the augmented assignment.
+            // TODO(charlie): There should be a way to go from `ExprName` to `BindingId` (like
+            // `resolve_name`, but for bindings rather than references).
+            let binding = {
+                let mut bindings = checker
+                    .semantic()
+                    .current_scope()
+                    .get_all(&index.id)
+                    .map(|id| checker.semantic().binding(id))
+                    .filter(|binding| for_stmt.range().contains_range(binding.range()));
+
+                let Some(binding) = bindings.next() else {
+                    continue;
+                };
+
+                // If there are multiple assignments to this variable _within_ the loop, ignore it.
+                if bindings.next().is_some() {
+                    continue;
+                }
+
+                binding
+            };
+
+            // If the variable is used _after_ the loop, ignore it.
+            // Find the binding for the augmented assignment.
+            if binding.references.iter().any(|id| {
+                let reference = checker.semantic().reference(*id);
+                reference.start() > for_stmt.end()
+            }) {
+                continue;
+            }
+
+            let diagnostic = Diagnostic::new(
+                EnumerateForLoop {
+                    index: index.id.to_string(),
+                },
+                stmt.range(),
+            );
+            checker.diagnostics.push(diagnostic);
+        }
+    }
+}
+
+/// If the statement is an index increment statement (e.g., `i += 1`), return
+/// the name of the index variable.
+fn match_index_increment(stmt: &Stmt) -> Option<&ast::ExprName> {
+    let Stmt::AugAssign(ast::StmtAugAssign {
+        target,
+        op: Operator::Add,
+        value,
+        ..
+    }) = stmt
+    else {
+        return None;
+    };
+
+    let name = target.as_name_expr()?;
+
+    if matches!(
+        value.as_ref(),
+        Expr::NumberLiteral(ast::ExprNumberLiteral {
+            value: Number::Int(Int::ONE),
+            ..
+        })
+    ) {
+        return Some(name);
+    }
+
+    None
+}
+
+#[derive(Debug, Default)]
+struct LoopControlFlowVisitor {
+    has_continue: bool,
+}
+
+impl StatementVisitor<'_> for LoopControlFlowVisitor {
+    fn visit_stmt(&mut self, stmt: &Stmt) {
+        match stmt {
+            Stmt::Continue(_) => self.has_continue = true,
+            Stmt::FunctionDef(_) | Stmt::ClassDef(_) => {
+                // Don't recurse.
+            }
+            _ => walk_stmt(self, stmt),
+        }
+    }
+}

crates/ruff_linter/src/rules/flake8_simplify/rules/mod.rs

@@ -4,6 +4,7 @@ pub(crate) use ast_ifexp::*;
 pub(crate) use ast_unary_op::*;
 pub(crate) use ast_with::*;
 pub(crate) use collapsible_if::*;
+pub(crate) use enumerate_for_loop::*;
 pub(crate) use if_else_block_instead_of_dict_get::*;
 pub(crate) use if_else_block_instead_of_dict_lookup::*;
 pub(crate) use if_else_block_instead_of_if_exp::*;
@@ -23,6 +24,7 @@ mod ast_ifexp;
 mod ast_unary_op;
 mod ast_with;
 mod collapsible_if;
+mod enumerate_for_loop;
 mod fix_with;
 mod if_else_block_instead_of_dict_get;
 mod if_else_block_instead_of_dict_lookup;

crates/ruff_linter/src/rules/flake8_simplify/snapshots/ruff_linter__rules__flake8_simplify__tests__SIM113_SIM113.py.snap

@@ -0,0 +1,47 @@
+---
+source: crates/ruff_linter/src/rules/flake8_simplify/mod.rs
+---
+SIM113.py:6:9: SIM113 Use `enumerate()` for index variable `idx` in `for` loop
+  |
+4 |     for x in range(5):
+5 |         g(x, idx)
+6 |         idx += 1
+  |         ^^^^^^^^ SIM113
+7 |         h(x)
+  |
+
+SIM113.py:17:9: SIM113 Use `enumerate()` for index variable `idx` in `for` loop
+   |
+15 |         if g(x):
+16 |             break
+17 |         idx += 1
+   |         ^^^^^^^^ SIM113
+18 |         sum += h(x, idx)
+   |
+
+SIM113.py:27:9: SIM113 Use `enumerate()` for index variable `idx` in `for` loop
+   |
+25 |         g(x)
+26 |         h(x, y)
+27 |         idx += 1
+   |         ^^^^^^^^ SIM113
+   |
+
+SIM113.py:36:9: SIM113 Use `enumerate()` for index variable `idx` in `for` loop
+   |
+34 |     for x in range(5):
+35 |         sum += h(x, idx)
+36 |         idx += 1
+   |         ^^^^^^^^ SIM113
+   |
+
+SIM113.py:44:9: SIM113 Use `enumerate()` for index variable `idx` in `for` loop
+   |
+42 |     for x in range(5):
+43 |         g(x, idx)
+44 |         idx += 1
+   |         ^^^^^^^^ SIM113
+45 |         h(x)
+   |
+
+

crates/ruff_linter/src/rules/flake8_trio/rules/zero_sleep_call.rs

@@ -1,6 +1,6 @@
 use ruff_diagnostics::{AlwaysFixableViolation, Diagnostic, Edit, Fix};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_ast::{self as ast, Expr, ExprCall, Int};
+use ruff_python_ast::{self as ast, Expr, ExprCall, Int, Number};
 use ruff_python_semantic::analyze::typing::find_assigned_value;
 use ruff_text_size::Ranged;
 
@@ -65,8 +65,7 @@ pub(crate) fn zero_sleep_call(checker: &mut Checker, call: &ExprCall) {
 
     match arg {
         Expr::NumberLiteral(ast::ExprNumberLiteral { value, .. }) => {
-            let Some(int) = value.as_int() else { return };
+            if !matches!(value, Number::Int(Int::ZERO)) {
-            if *int != Int::ZERO {
                 return;
             }
         }
@@ -74,11 +73,13 @@ pub(crate) fn zero_sleep_call(checker: &mut Checker, call: &ExprCall) {
             let Some(value) = find_assigned_value(id, checker.semantic()) else {
                 return;
             };
-            let Expr::NumberLiteral(ast::ExprNumberLiteral { value: num, .. }) = value else {
+            if !matches!(
-                return;
+                value,
-            };
+                Expr::NumberLiteral(ast::ExprNumberLiteral {
-            let Some(int) = num.as_int() else { return };
+                    value: Number::Int(Int::ZERO),
-            if *int != Int::ZERO {
+                    ..
+                })
+            ) {
                 return;
             }
         }

crates/ruff_python_ast/src/traversal.rs

@@ -79,3 +79,15 @@ pub fn next_sibling<'a>(stmt: &'a Stmt, suite: &'a Suite) -> Option<&'a Stmt> {
     }
     None
 }
+
+/// Given a [`Stmt`] and its containing [`Suite`], return the previous [`Stmt`] in the [`Suite`].
+pub fn prev_sibling<'a>(stmt: &'a Stmt, suite: &'a Suite) -> Option<&'a Stmt> {
+    let mut prev = None;
+    for sibling in suite {
+        if sibling == stmt {
+            return prev;
+        }
+        prev = Some(sibling);
+    }
+    None
+}

crates/ruff_python_semantic/src/analyze/typing.rs

@@ -645,6 +645,24 @@ pub fn resolve_assignment<'a>(
 pub fn find_assigned_value<'a>(symbol: &str, semantic: &'a SemanticModel<'a>) -> Option<&'a Expr> {
     let binding_id = semantic.lookup_symbol(symbol)?;
     let binding = semantic.binding(binding_id);
+    find_binding_value(symbol, binding, semantic)
+}
+
+/// Find the assigned [`Expr`] for a given [`Binding`], if any.
+///
+/// For example given:
+/// ```python
+///  foo = 42
+///  (bar, bla) = 1, "str"
+/// ```
+///
+/// This function will return a `NumberLiteral` with value `Int(42)` when called with `foo` and a
+/// `StringLiteral` with value `"str"` when called with `bla`.
+pub fn find_binding_value<'a>(
+    symbol: &str,
+    binding: &Binding,
+    semantic: &'a SemanticModel,
+) -> Option<&'a Expr> {
     match binding.kind {
         // Ex) `x := 1`
         BindingKind::NamedExprAssignment => {

crates/ruff_python_semantic/src/scope.rs

@@ -129,6 +129,11 @@ impl<'a> Scope<'a> {
         self.shadowed_bindings.get(&id).copied()
     }
 
+    /// Returns an iterator over all bindings that the given binding shadows, including itself.
+    pub fn shadowed_bindings(&self, id: BindingId) -> impl Iterator<Item = BindingId> + '_ {
+        std::iter::successors(Some(id), |id| self.shadowed_bindings.get(id).copied())
+    }
+
     /// Adds a reference to a star import (e.g., `from sys import *`) to this scope.
     pub fn add_star_import(&mut self, import: StarImport<'a>) {
         self.star_imports.push(import);

ruff.schema.json

@@ -3552,6 +3552,7 @@
         "SIM11",
         "SIM110",
         "SIM112",
+        "SIM113",
         "SIM114",
         "SIM115",
         "SIM116",