Implement B023 (function uses loop variable) (#907)

README.md

@@ -575,6 +575,7 @@ For more, see [flake8-bugbear](https://pypi.org/project/flake8-bugbear/22.10.27/
 | B020 | LoopVariableOverridesIterator | Loop control variable `...` overrides iterable it iterates |  |
 | B021 | FStringDocstring | f-string used as docstring. This will be interpreted by python as a joined string rather than a docstring. |  |
 | B022 | UselessContextlibSuppress | No arguments passed to `contextlib.suppress`. No exceptions will be suppressed and therefore this context manager is redundant |  |
+| B023 | FunctionUsesLoopVariable | Function definition does not bind loop variable `...` |  |
 | B024 | AbstractBaseClassWithoutAbstractMethod | `...` is an abstract base class, but it has no abstract methods |  |
 | B025 | DuplicateTryBlockException | try-except block with duplicate exception `Exception` |  |
 | B026 | StarArgUnpackingAfterKeywordArg | Star-arg unpacking after a keyword argument is strongly discouraged |  |
@@ -820,6 +821,8 @@ including:
 - [`pydocstyle`](https://pypi.org/project/pydocstyle/)
 - [`pep8-naming`](https://pypi.org/project/pep8-naming/)
 - [`yesqa`](https://github.com/asottile/yesqa)
+- [`flake8-bugbear`](https://pypi.org/project/flake8-bugbear/)
+- [`flake8-comprehensions`](https://pypi.org/project/flake8-comprehensions/)
 - [`flake8-docstrings`](https://pypi.org/project/flake8-docstrings/)
 - [`flake8-builtins`](https://pypi.org/project/flake8-builtins/)
 - [`flake8-super`](https://pypi.org/project/flake8-super/)
@@ -827,9 +830,7 @@ including:
 - [`flake8-print`](https://pypi.org/project/flake8-print/)
 - [`flake8-quotes`](https://pypi.org/project/flake8-quotes/)
 - [`flake8-annotations`](https://pypi.org/project/flake8-annotations/)
-- [`flake8-comprehensions`](https://pypi.org/project/flake8-comprehensions/)
 - [`flake8-bandit`](https://pypi.org/project/flake8-bandit/) (6/40)
-- [`flake8-bugbear`](https://pypi.org/project/flake8-bugbear/) (27/32)
 - [`flake8-2020`](https://pypi.org/project/flake8-2020/)
 - [`flake8-blind-except`](https://pypi.org/project/flake8-blind-except/)
 - [`flake8-boolean-trap`](https://pypi.org/project/flake8-boolean-trap/)
@@ -851,6 +852,8 @@ Today, Ruff can be used to replace Flake8 when used with any of the following pl
 
 - [`pydocstyle`](https://pypi.org/project/pydocstyle/)
 - [`pep8-naming`](https://pypi.org/project/pep8-naming/)
+- [`flake8-bugbear`](https://pypi.org/project/flake8-bugbear/)
+- [`flake8-comprehensions`](https://pypi.org/project/flake8-comprehensions/)
 - [`flake8-docstrings`](https://pypi.org/project/flake8-docstrings/)
 - [`flake8-builtins`](https://pypi.org/project/flake8-builtins/)
 - [`flake8-super`](https://pypi.org/project/flake8-super/)
@@ -859,8 +862,6 @@ Today, Ruff can be used to replace Flake8 when used with any of the following pl
 - [`flake8-quotes`](https://pypi.org/project/flake8-quotes/)
 - [`flake8-annotations`](https://pypi.org/project/flake8-annotations/)
 - [`flake8-bandit`](https://pypi.org/project/flake8-bandit/) (6/40)
-- [`flake8-comprehensions`](https://pypi.org/project/flake8-comprehensions/)
-- [`flake8-bugbear`](https://pypi.org/project/flake8-bugbear/) (27/32)
 - [`flake8-2020`](https://pypi.org/project/flake8-2020/)
 - [`flake8-blind-except`](https://pypi.org/project/flake8-blind-except/)
 - [`flake8-boolean-trap`](https://pypi.org/project/flake8-boolean-trap/)

resources/test/fixtures/B023.py

@@ -0,0 +1,78 @@
+"""
+Should emit:
+B023 - on lines 12, 13, 16, 28, 29, 30, 31, 40, 42, 50, 51, 52, 53, 61, 68.
+"""
+
+functions = []
+z = 0
+
+for x in range(3):
+    y = x + 1
+    # Subject to late-binding problems
+    functions.append(lambda: x)
+    functions.append(lambda: y)  # not just the loop var
+
+    def f_bad_1():
+        return x
+
+    # Actually OK
+    functions.append(lambda x: x * 2)
+    functions.append(lambda x=x: x)
+    functions.append(lambda: z)  # OK because not assigned in the loop
+
+    def f_ok_1(x):
+        return x * 2
+
+
+def check_inside_functions_too():
+    ls = [lambda: x for x in range(2)]
+    st = {lambda: x for x in range(2)}
+    gn = (lambda: x for x in range(2))
+    dt = {x: lambda: x for x in range(2)}
+
+
+async def pointless_async_iterable():
+    yield 1
+
+
+async def container_for_problems():
+    async for x in pointless_async_iterable():
+        functions.append(lambda: x)
+
+    [lambda: x async for x in pointless_async_iterable()]
+
+
+a = 10
+b = 0
+while True:
+    a = a_ = a - 1
+    b += 1
+    functions.append(lambda: a)
+    functions.append(lambda: a_)
+    functions.append(lambda: b)
+    functions.append(lambda: c)  # not a name error because of late binding!
+    c: bool = a > 3
+    if not c:
+        break
+
+# Nested loops should not duplicate reports
+for j in range(2):
+    for k in range(3):
+        lambda: j * k
+
+
+for j, k, l in [(1, 2, 3)]:
+
+    def f():
+        j = None  # OK because it's an assignment
+        [l for k in range(2)]  # error for l, not for k
+
+        assert a and functions
+
+    a.attribute = 1  # modifying an attribute doesn't make it a loop variable
+    functions[0] = lambda: None  # same for an element
+
+for var in range(2):
+
+    def explicit_capture(captured=var):
+        return captured

ruff_dev/src/generate_check_code_prefix.rs

@@ -117,7 +117,8 @@ pub fn main(cli: &Cli) -> Result<()> {
 
     // Construct the output contents.
     let mut output = String::new();
-    output.push_str("//! File automatically generated by examples/generate_check_code_prefix.rs.");
+    output
+        .push_str("//! File automatically generated by `examples/generate_check_code_prefix.rs`.");
     output.push('\n');
     output.push('\n');
     output.push_str("use serde::{{Serialize, Deserialize}};");

src/ast/helpers.rs

@@ -1,7 +1,9 @@
 use once_cell::sync::Lazy;
 use regex::Regex;
 use rustc_hash::{FxHashMap, FxHashSet};
-use rustpython_ast::{Excepthandler, ExcepthandlerKind, Expr, ExprKind, Location, Stmt, StmtKind};
+use rustpython_ast::{
+    Arguments, Excepthandler, ExcepthandlerKind, Expr, ExprKind, Location, Stmt, StmtKind,
+};
 
 use crate::ast::types::Range;
 use crate::SourceCodeLocator;
@@ -213,6 +215,27 @@ pub fn extract_handler_names(handlers: &[Excepthandler]) -> Vec<Vec<&str>> {
     handler_names
 }
 
+/// Return the set of all bound argument names.
+pub fn collect_arg_names<'a>(arguments: &'a Arguments) -> FxHashSet<&'a str> {
+    let mut arg_names: FxHashSet<&'a str> = FxHashSet::default();
+    for arg in &arguments.posonlyargs {
+        arg_names.insert(arg.node.arg.as_str());
+    }
+    for arg in &arguments.args {
+        arg_names.insert(arg.node.arg.as_str());
+    }
+    if let Some(arg) = &arguments.vararg {
+        arg_names.insert(arg.node.arg.as_str());
+    }
+    for arg in &arguments.kwonlyargs {
+        arg_names.insert(arg.node.arg.as_str());
+    }
+    if let Some(arg) = &arguments.kwarg {
+        arg_names.insert(arg.node.arg.as_str());
+    }
+    arg_names
+}
+
 /// Returns `true` if a call is an argumented `super` invocation.
 pub fn is_super_call_with_arguments(func: &Expr, args: &[Expr]) -> bool {
     // Check: is this a `super` call?

src/ast/types.rs

@@ -1,7 +1,7 @@
 use std::sync::atomic::{AtomicUsize, Ordering};
 
 use rustc_hash::FxHashMap;
-use rustpython_ast::{Expr, Keyword};
+use rustpython_ast::{Expr, Keyword, Stmt};
 use rustpython_parser::ast::{Located, Location};
 
 fn id() -> usize {
@@ -9,6 +9,12 @@ fn id() -> usize {
     COUNTER.fetch_add(1, Ordering::Relaxed)
 }
 
+#[derive(Clone)]
+pub enum Node<'a> {
+    Stmt(&'a Stmt),
+    Expr(&'a Expr),
+}
+
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Range {
     pub location: Location,

src/check_ast.rs

@@ -19,8 +19,8 @@ use crate::ast::helpers::{
 use crate::ast::operations::extract_all_names;
 use crate::ast::relocate::relocate_expr;
 use crate::ast::types::{
-    Binding, BindingContext, BindingKind, ClassScope, FunctionScope, ImportKind, Range, Scope,
+    Binding, BindingContext, BindingKind, ClassScope, FunctionScope, ImportKind, Node, Range,
-    ScopeKind,
+    Scope, ScopeKind,
 };
 use crate::ast::visitor::{walk_excepthandler, walk_withitem, Visitor};
 use crate::ast::{helpers, operations, visitor};
@@ -83,6 +83,8 @@ pub struct Checker<'a> {
     futures_allowed: bool,
     annotations_future_enabled: bool,
     except_handlers: Vec<Vec<Vec<&'a str>>>,
+    // Check-specific state.
+    pub(crate) seen_b023: Vec<&'a Expr>,
 }
 
 impl<'a> Checker<'a> {
@@ -127,6 +129,8 @@ impl<'a> Checker<'a> {
             futures_allowed: true,
             annotations_future_enabled: false,
             except_handlers: vec![],
+            // Check-specific state.
+            seen_b023: vec![],
         }
     }
 
@@ -951,8 +955,16 @@ where
                     flake8_bugbear::plugins::assert_raises_exception(self, stmt, items);
                 }
             }
+            StmtKind::While { .. } => {
+                if self.settings.enabled.contains(&CheckCode::B023) {
+                    flake8_bugbear::plugins::function_uses_loop_variable(self, &Node::Stmt(stmt));
+                }
+            }
             StmtKind::For {
                 target, body, iter, ..
+            }
+            | StmtKind::AsyncFor {
+                target, body, iter, ..
             } => {
                 if self.settings.enabled.contains(&CheckCode::B007) {
                     flake8_bugbear::plugins::unused_loop_control_variable(self, target, body);
@@ -960,6 +972,9 @@ where
                 if self.settings.enabled.contains(&CheckCode::B020) {
                     flake8_bugbear::plugins::loop_variable_overrides_iterator(self, target, iter);
                 }
+                if self.settings.enabled.contains(&CheckCode::B023) {
+                    flake8_bugbear::plugins::function_uses_loop_variable(self, &Node::Stmt(stmt));
+                }
             }
             StmtKind::Try { handlers, .. } => {
                 if self.settings.enabled.contains(&CheckCode::F707) {
@@ -1810,9 +1825,15 @@ where
                         self.add_check(check);
                     };
                 }
+                if self.settings.enabled.contains(&CheckCode::B023) {
+                    flake8_bugbear::plugins::function_uses_loop_variable(self, &Node::Expr(expr));
+                }
                 self.push_scope(Scope::new(ScopeKind::Generator));
             }
             ExprKind::GeneratorExp { .. } | ExprKind::DictComp { .. } => {
+                if self.settings.enabled.contains(&CheckCode::B023) {
+                    flake8_bugbear::plugins::function_uses_loop_variable(self, &Node::Expr(expr));
+                }
                 self.push_scope(Scope::new(ScopeKind::Generator));
             }
             _ => {}

src/checks.rs

@@ -104,6 +104,7 @@ pub enum CheckCode {
     B020,
     B021,
     B022,
+    B023,
     B024,
     B025,
     B026,
@@ -427,32 +428,33 @@ pub enum CheckKind {
     // flake8-blind-except
     BlindExcept,
     // flake8-bugbear
-    UnaryPrefixIncrement,
-    AssignmentToOsEnviron,
-    UnreliableCallableCheck,
-    StripWithMultiCharacters,
-    MutableArgumentDefault,
-    UnusedLoopControlVariable(String),
-    FunctionCallArgumentDefault(Option<String>),
-    GetAttrWithConstant,
-    SetAttrWithConstant,
-    DoNotAssertFalse,
-    JumpStatementInFinally(String),
-    RedundantTupleInExceptionHandler(String),
-    DuplicateHandlerException(Vec<String>),
-    UselessComparison,
-    CannotRaiseLiteral,
-    NoAssertRaisesException,
-    UselessExpression,
-    CachedInstanceMethod,
-    LoopVariableOverridesIterator(String),
-    FStringDocstring,
-    UselessContextlibSuppress,
     AbstractBaseClassWithoutAbstractMethod(String),
+    AssignmentToOsEnviron,
+    CachedInstanceMethod,
+    CannotRaiseLiteral,
+    DoNotAssertFalse,
+    DuplicateHandlerException(Vec<String>),
     DuplicateTryBlockException(String),
-    StarArgUnpackingAfterKeywordArg,
     EmptyMethodWithoutAbstractDecorator(String),
+    FStringDocstring,
+    FunctionCallArgumentDefault(Option<String>),
+    FunctionUsesLoopVariable(String),
+    GetAttrWithConstant,
+    JumpStatementInFinally(String),
+    LoopVariableOverridesIterator(String),
+    MutableArgumentDefault,
+    NoAssertRaisesException,
     RaiseWithoutFromInsideExcept,
+    RedundantTupleInExceptionHandler(String),
+    SetAttrWithConstant,
+    StarArgUnpackingAfterKeywordArg,
+    StripWithMultiCharacters,
+    UnaryPrefixIncrement,
+    UnreliableCallableCheck,
+    UnusedLoopControlVariable(String),
+    UselessComparison,
+    UselessContextlibSuppress,
+    UselessExpression,
     // flake8-comprehensions
     UnnecessaryGeneratorList,
     UnnecessaryGeneratorSet,
@@ -716,6 +718,7 @@ impl CheckCode {
             CheckCode::B020 => CheckKind::LoopVariableOverridesIterator("...".to_string()),
             CheckCode::B021 => CheckKind::FStringDocstring,
             CheckCode::B022 => CheckKind::UselessContextlibSuppress,
+            CheckCode::B023 => CheckKind::FunctionUsesLoopVariable("...".to_string()),
             CheckCode::B024 => CheckKind::AbstractBaseClassWithoutAbstractMethod("...".to_string()),
             CheckCode::B025 => CheckKind::DuplicateTryBlockException("Exception".to_string()),
             CheckCode::B026 => CheckKind::StarArgUnpackingAfterKeywordArg,
@@ -981,6 +984,7 @@ impl CheckCode {
             CheckCode::B020 => CheckCategory::Flake8Bugbear,
             CheckCode::B021 => CheckCategory::Flake8Bugbear,
             CheckCode::B022 => CheckCategory::Flake8Bugbear,
+            CheckCode::B023 => CheckCategory::Flake8Bugbear,
             CheckCode::B024 => CheckCategory::Flake8Bugbear,
             CheckCode::B025 => CheckCategory::Flake8Bugbear,
             CheckCode::B026 => CheckCategory::Flake8Bugbear,
@@ -1184,32 +1188,33 @@ impl CheckKind {
             CheckKind::BuiltinArgumentShadowing(_) => &CheckCode::A002,
             CheckKind::BuiltinAttributeShadowing(_) => &CheckCode::A003,
             // flake8-bugbear
-            CheckKind::UnaryPrefixIncrement => &CheckCode::B002,
-            CheckKind::AssignmentToOsEnviron => &CheckCode::B003,
-            CheckKind::UnreliableCallableCheck => &CheckCode::B004,
-            CheckKind::StripWithMultiCharacters => &CheckCode::B005,
-            CheckKind::MutableArgumentDefault => &CheckCode::B006,
-            CheckKind::UnusedLoopControlVariable(_) => &CheckCode::B007,
-            CheckKind::FunctionCallArgumentDefault(_) => &CheckCode::B008,
-            CheckKind::GetAttrWithConstant => &CheckCode::B009,
-            CheckKind::SetAttrWithConstant => &CheckCode::B010,
-            CheckKind::DoNotAssertFalse => &CheckCode::B011,
-            CheckKind::JumpStatementInFinally(_) => &CheckCode::B012,
-            CheckKind::RedundantTupleInExceptionHandler(_) => &CheckCode::B013,
-            CheckKind::DuplicateHandlerException(_) => &CheckCode::B014,
-            CheckKind::UselessComparison => &CheckCode::B015,
-            CheckKind::CannotRaiseLiteral => &CheckCode::B016,
-            CheckKind::NoAssertRaisesException => &CheckCode::B017,
-            CheckKind::UselessExpression => &CheckCode::B018,
-            CheckKind::CachedInstanceMethod => &CheckCode::B019,
-            CheckKind::LoopVariableOverridesIterator(_) => &CheckCode::B020,
-            CheckKind::FStringDocstring => &CheckCode::B021,
-            CheckKind::UselessContextlibSuppress => &CheckCode::B022,
             CheckKind::AbstractBaseClassWithoutAbstractMethod(_) => &CheckCode::B024,
+            CheckKind::AssignmentToOsEnviron => &CheckCode::B003,
+            CheckKind::CachedInstanceMethod => &CheckCode::B019,
+            CheckKind::CannotRaiseLiteral => &CheckCode::B016,
+            CheckKind::DoNotAssertFalse => &CheckCode::B011,
+            CheckKind::DuplicateHandlerException(_) => &CheckCode::B014,
             CheckKind::DuplicateTryBlockException(_) => &CheckCode::B025,
-            CheckKind::StarArgUnpackingAfterKeywordArg => &CheckCode::B026,
             CheckKind::EmptyMethodWithoutAbstractDecorator(_) => &CheckCode::B027,
+            CheckKind::FStringDocstring => &CheckCode::B021,
+            CheckKind::FunctionCallArgumentDefault(_) => &CheckCode::B008,
+            CheckKind::FunctionUsesLoopVariable(_) => &CheckCode::B023,
+            CheckKind::GetAttrWithConstant => &CheckCode::B009,
+            CheckKind::JumpStatementInFinally(_) => &CheckCode::B012,
+            CheckKind::LoopVariableOverridesIterator(_) => &CheckCode::B020,
+            CheckKind::MutableArgumentDefault => &CheckCode::B006,
+            CheckKind::NoAssertRaisesException => &CheckCode::B017,
             CheckKind::RaiseWithoutFromInsideExcept => &CheckCode::B904,
+            CheckKind::RedundantTupleInExceptionHandler(_) => &CheckCode::B013,
+            CheckKind::SetAttrWithConstant => &CheckCode::B010,
+            CheckKind::StarArgUnpackingAfterKeywordArg => &CheckCode::B026,
+            CheckKind::StripWithMultiCharacters => &CheckCode::B005,
+            CheckKind::UnaryPrefixIncrement => &CheckCode::B002,
+            CheckKind::UnreliableCallableCheck => &CheckCode::B004,
+            CheckKind::UnusedLoopControlVariable(_) => &CheckCode::B007,
+            CheckKind::UselessComparison => &CheckCode::B015,
+            CheckKind::UselessContextlibSuppress => &CheckCode::B022,
+            CheckKind::UselessExpression => &CheckCode::B018,
             // flake8-blind-except
             CheckKind::BlindExcept => &CheckCode::BLE001,
             // flake8-comprehensions
@@ -1554,6 +1559,9 @@ impl CheckKind {
                     "Do not perform function call in argument defaults".to_string()
                 }
             }
+            CheckKind::FunctionUsesLoopVariable(name) => {
+                format!("Function definition does not bind loop variable `{name}`")
+            }
             CheckKind::GetAttrWithConstant => "Do not call `getattr` with a constant attribute \
                                                value. It is not any safer than normal property \
                                                access."

src/checks_gen.rs

@@ -59,6 +59,7 @@ pub enum CheckCodePrefix {
     B020,
     B021,
     B022,
+    B023,
     B024,
     B025,
     B026,
@@ -186,6 +187,12 @@ pub enum CheckCodePrefix {
     F406,
     F407,
     F5,
+    F52,
+    F521,
+    F522,
+    F523,
+    F524,
+    F525,
     F54,
     F541,
     F6,
@@ -415,6 +422,7 @@ impl CheckCodePrefix {
                 CheckCode::B020,
                 CheckCode::B021,
                 CheckCode::B022,
+                CheckCode::B023,
                 CheckCode::B024,
                 CheckCode::B025,
                 CheckCode::B026,
@@ -443,6 +451,7 @@ impl CheckCodePrefix {
                 CheckCode::B020,
                 CheckCode::B021,
                 CheckCode::B022,
+                CheckCode::B023,
                 CheckCode::B024,
                 CheckCode::B025,
                 CheckCode::B026,
@@ -492,6 +501,7 @@ impl CheckCodePrefix {
                 CheckCode::B020,
                 CheckCode::B021,
                 CheckCode::B022,
+                CheckCode::B023,
                 CheckCode::B024,
                 CheckCode::B025,
                 CheckCode::B026,
@@ -500,6 +510,7 @@ impl CheckCodePrefix {
             CheckCodePrefix::B020 => vec![CheckCode::B020],
             CheckCodePrefix::B021 => vec![CheckCode::B021],
             CheckCodePrefix::B022 => vec![CheckCode::B022],
+            CheckCodePrefix::B023 => vec![CheckCode::B023],
             CheckCodePrefix::B024 => vec![CheckCode::B024],
             CheckCodePrefix::B025 => vec![CheckCode::B025],
             CheckCodePrefix::B026 => vec![CheckCode::B026],
@@ -899,7 +910,26 @@ impl CheckCodePrefix {
             CheckCodePrefix::F405 => vec![CheckCode::F405],
             CheckCodePrefix::F406 => vec![CheckCode::F406],
             CheckCodePrefix::F407 => vec![CheckCode::F407],
-            CheckCodePrefix::F5 => vec![CheckCode::F541],
+            CheckCodePrefix::F5 => vec![
+                CheckCode::F521,
+                CheckCode::F522,
+                CheckCode::F523,
+                CheckCode::F524,
+                CheckCode::F525,
+                CheckCode::F541,
+            ],
+            CheckCodePrefix::F52 => vec![
+                CheckCode::F521,
+                CheckCode::F522,
+                CheckCode::F523,
+                CheckCode::F524,
+                CheckCode::F525,
+            ],
+            CheckCodePrefix::F521 => vec![CheckCode::F521],
+            CheckCodePrefix::F522 => vec![CheckCode::F522],
+            CheckCodePrefix::F523 => vec![CheckCode::F523],
+            CheckCodePrefix::F524 => vec![CheckCode::F524],
+            CheckCodePrefix::F525 => vec![CheckCode::F525],
             CheckCodePrefix::F54 => vec![CheckCode::F541],
             CheckCodePrefix::F541 => vec![CheckCode::F541],
             CheckCodePrefix::F6 => vec![
@@ -1293,6 +1323,7 @@ impl CheckCodePrefix {
             CheckCodePrefix::B020 => PrefixSpecificity::Explicit,
             CheckCodePrefix::B021 => PrefixSpecificity::Explicit,
             CheckCodePrefix::B022 => PrefixSpecificity::Explicit,
+            CheckCodePrefix::B023 => PrefixSpecificity::Explicit,
             CheckCodePrefix::B024 => PrefixSpecificity::Explicit,
             CheckCodePrefix::B025 => PrefixSpecificity::Explicit,
             CheckCodePrefix::B026 => PrefixSpecificity::Explicit,
@@ -1420,6 +1451,12 @@ impl CheckCodePrefix {
             CheckCodePrefix::F406 => PrefixSpecificity::Explicit,
             CheckCodePrefix::F407 => PrefixSpecificity::Explicit,
             CheckCodePrefix::F5 => PrefixSpecificity::Hundreds,
+            CheckCodePrefix::F52 => PrefixSpecificity::Tens,
+            CheckCodePrefix::F521 => PrefixSpecificity::Explicit,
+            CheckCodePrefix::F522 => PrefixSpecificity::Explicit,
+            CheckCodePrefix::F523 => PrefixSpecificity::Explicit,
+            CheckCodePrefix::F524 => PrefixSpecificity::Explicit,
+            CheckCodePrefix::F525 => PrefixSpecificity::Explicit,
             CheckCodePrefix::F54 => PrefixSpecificity::Tens,
             CheckCodePrefix::F541 => PrefixSpecificity::Explicit,
             CheckCodePrefix::F6 => PrefixSpecificity::Hundreds,

src/flake8_annotations/plugins.rs

@@ -21,7 +21,7 @@ where
     fn visit_stmt(&mut self, stmt: &'b Stmt) {
         match &stmt.node {
             StmtKind::FunctionDef { .. } | StmtKind::AsyncFunctionDef { .. } => {
-                // No recurse.
+                // Don't recurse.
             }
             StmtKind::Return { value } => self.returns.push(value.as_ref().map(|expr| &**expr)),
             _ => visitor::walk_stmt(self, stmt),

src/flake8_bugbear/plugins/function_uses_loop_variable.rs

@@ -0,0 +1,218 @@
+use rustc_hash::FxHashSet;
+use rustpython_ast::{Comprehension, Expr, ExprContext, ExprKind, Stmt, StmtKind};
+
+use crate::ast::helpers::collect_arg_names;
+use crate::ast::types::{Node, Range};
+use crate::ast::visitor;
+use crate::ast::visitor::Visitor;
+use crate::check_ast::Checker;
+use crate::checks::{Check, CheckKind};
+
+#[derive(Default)]
+struct LoadedNamesVisitor<'a> {
+    names: Vec<(&'a str, &'a Expr)>,
+}
+
+/// `Visitor` to collect all used identifiers in a statement.
+impl<'a, 'b> Visitor<'b> for LoadedNamesVisitor<'a>
+where
+    'b: 'a,
+{
+    fn visit_expr(&mut self, expr: &'b Expr) {
+        match &expr.node {
+            ExprKind::Name { id, ctx } if matches!(ctx, ExprContext::Load) => {
+                self.names.push((id, expr));
+            }
+            _ => visitor::walk_expr(self, expr),
+        }
+    }
+}
+
+#[derive(Default)]
+struct SuspiciousVariablesVisitor<'a> {
+    names: Vec<(&'a str, &'a Expr)>,
+}
+
+/// `Visitor` to collect all suspicious variables (those referenced in
+/// functions, but not bound as arguments).
+impl<'a, 'b> Visitor<'b> for SuspiciousVariablesVisitor<'a>
+where
+    'b: 'a,
+{
+    fn visit_stmt(&mut self, stmt: &'b Stmt) {
+        match &stmt.node {
+            StmtKind::FunctionDef { args, body, .. }
+            | StmtKind::AsyncFunctionDef { args, body, .. } => {
+                // Collect all loaded variable names.
+                let mut visitor = LoadedNamesVisitor::default();
+                for stmt in body {
+                    visitor.visit_stmt(stmt);
+                }
+
+                // Collect all argument names.
+                let arg_names = collect_arg_names(args);
+
+                // Treat any non-arguments as "suspicious".
+                self.names.extend(
+                    visitor
+                        .names
+                        .into_iter()
+                        .filter(|(id, _)| !arg_names.contains(id)),
+                );
+            }
+            _ => visitor::walk_stmt(self, stmt),
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'b Expr) {
+        match &expr.node {
+            ExprKind::Lambda { args, body } => {
+                // Collect all loaded variable names.
+                let mut visitor = LoadedNamesVisitor::default();
+                visitor.visit_expr(body);
+
+                // Collect all argument names.
+                let arg_names = collect_arg_names(args);
+
+                // Treat any non-arguments as "suspicious".
+                self.names.extend(
+                    visitor
+                        .names
+                        .into_iter()
+                        .filter(|(id, _)| !arg_names.contains(id)),
+                );
+            }
+            _ => visitor::walk_expr(self, expr),
+        }
+    }
+}
+
+#[derive(Default)]
+struct NamesFromAssignmentsVisitor<'a> {
+    names: FxHashSet<&'a str>,
+}
+
+/// `Visitor` to collect all names used in an assignment expression.
+impl<'a, 'b> Visitor<'b> for NamesFromAssignmentsVisitor<'a>
+where
+    'b: 'a,
+{
+    fn visit_expr(&mut self, expr: &'b Expr) {
+        match &expr.node {
+            ExprKind::Name { id, .. } => {
+                self.names.insert(id.as_str());
+            }
+            ExprKind::Starred { value, .. } => {
+                self.visit_expr(value);
+            }
+            ExprKind::List { elts, .. } | ExprKind::Tuple { elts, .. } => {
+                for expr in elts {
+                    self.visit_expr(expr);
+                }
+            }
+            _ => {}
+        }
+    }
+}
+
+#[derive(Default)]
+struct AssignedNamesVisitor<'a> {
+    names: FxHashSet<&'a str>,
+}
+
+/// `Visitor` to collect all used identifiers in a statement.
+impl<'a, 'b> Visitor<'b> for AssignedNamesVisitor<'a>
+where
+    'b: 'a,
+{
+    fn visit_stmt(&mut self, stmt: &'b Stmt) {
+        if matches!(
+            &stmt.node,
+            StmtKind::FunctionDef { .. } | StmtKind::AsyncFunctionDef { .. }
+        ) {
+            // Don't recurse.
+            return;
+        }
+
+        match &stmt.node {
+            StmtKind::Assign { targets, .. } => {
+                let mut visitor = NamesFromAssignmentsVisitor::default();
+                for expr in targets {
+                    visitor.visit_expr(expr);
+                }
+                self.names.extend(visitor.names);
+            }
+            StmtKind::AugAssign { target, .. }
+            | StmtKind::AnnAssign { target, .. }
+            | StmtKind::For { target, .. }
+            | StmtKind::AsyncFor { target, .. } => {
+                let mut visitor = NamesFromAssignmentsVisitor::default();
+                visitor.visit_expr(target);
+                self.names.extend(visitor.names);
+            }
+            _ => {}
+        }
+
+        visitor::walk_stmt(self, stmt);
+    }
+
+    fn visit_expr(&mut self, expr: &'b Expr) {
+        if matches!(&expr.node, ExprKind::Lambda { .. }) {
+            // Don't recurse.
+            return;
+        }
+
+        visitor::walk_expr(self, expr);
+    }
+
+    fn visit_comprehension(&mut self, comprehension: &'b Comprehension) {
+        let mut visitor = NamesFromAssignmentsVisitor::default();
+        visitor.visit_expr(&comprehension.target);
+        self.names.extend(visitor.names);
+
+        visitor::walk_comprehension(self, comprehension);
+    }
+}
+
+/// B023
+pub fn function_uses_loop_variable<'a, 'b>(checker: &'a mut Checker<'b>, node: &Node<'b>)
+where
+    'b: 'a,
+{
+    // Identify any "suspicious" variables. These are defined as variables that are
+    // referenced in a function or lambda body, but aren't bound as arguments.
+    let suspicious_variables = {
+        let mut visitor = SuspiciousVariablesVisitor::<'b>::default();
+        match node {
+            Node::Stmt(stmt) => visitor.visit_stmt(stmt),
+            Node::Expr(expr) => visitor.visit_expr(expr),
+        }
+        visitor.names
+    };
+
+    if !suspicious_variables.is_empty() {
+        // Identify any variables that are assigned in the loop (ignoring functions).
+        let reassigned_in_loop = {
+            let mut visitor = AssignedNamesVisitor::<'b>::default();
+            match node {
+                Node::Stmt(stmt) => visitor.visit_stmt(stmt),
+                Node::Expr(expr) => visitor.visit_expr(expr),
+            }
+            visitor.names
+        };
+
+        // If a variable was used in a function or lambda body, and assigned in the
+        // loop, flag it.
+        for (name, expr) in suspicious_variables {
+            if reassigned_in_loop.contains(name) {
+                if !checker.seen_b023.contains(&expr) {
+                    checker.seen_b023.push(expr);
+                    checker.add_check(Check::new(
+                        CheckKind::FunctionUsesLoopVariable(name.to_string()),
+                        Range::from_located(expr),
+                    ));
+                }
+            }
+        }
+    }
+}

src/flake8_bugbear/plugins/mod.rs

@@ -7,6 +7,7 @@ pub use cannot_raise_literal::cannot_raise_literal;
 pub use duplicate_exceptions::duplicate_exceptions;
 pub use f_string_docstring::f_string_docstring;
 pub use function_call_argument_default::function_call_argument_default;
+pub use function_uses_loop_variable::function_uses_loop_variable;
 pub use getattr_with_constant::getattr_with_constant;
 pub use jump_statement_in_finally::jump_statement_in_finally;
 pub use loop_variable_overrides_iterator::loop_variable_overrides_iterator;
@@ -32,6 +33,7 @@ mod cannot_raise_literal;
 mod duplicate_exceptions;
 mod f_string_docstring;
 mod function_call_argument_default;
+mod function_uses_loop_variable;
 mod getattr_with_constant;
 mod jump_statement_in_finally;
 mod loop_variable_overrides_iterator;

src/linter.rs

@@ -413,6 +413,7 @@ mod tests {
     #[test_case(CheckCode::B020, Path::new("B020.py"); "B020")]
     #[test_case(CheckCode::B021, Path::new("B021.py"); "B021")]
     #[test_case(CheckCode::B022, Path::new("B022.py"); "B022")]
+    #[test_case(CheckCode::B023, Path::new("B023.py"); "B023")]
     #[test_case(CheckCode::B024, Path::new("B024.py"); "B024")]
     #[test_case(CheckCode::B025, Path::new("B025.py"); "B025")]
     #[test_case(CheckCode::B026, Path::new("B026.py"); "B026")]

src/snapshots/ruff__linter__tests__B023_B023.py.snap

@@ -0,0 +1,149 @@
+---
+source: src/linter.rs
+expression: checks
+---
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 12
+    column: 29
+  end_location:
+    row: 12
+    column: 30
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: y
+  location:
+    row: 13
+    column: 29
+  end_location:
+    row: 13
+    column: 30
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 16
+    column: 15
+  end_location:
+    row: 16
+    column: 16
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 28
+    column: 18
+  end_location:
+    row: 28
+    column: 19
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 29
+    column: 18
+  end_location:
+    row: 29
+    column: 19
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 30
+    column: 18
+  end_location:
+    row: 30
+    column: 19
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 31
+    column: 21
+  end_location:
+    row: 31
+    column: 22
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 40
+    column: 33
+  end_location:
+    row: 40
+    column: 34
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: x
+  location:
+    row: 42
+    column: 13
+  end_location:
+    row: 42
+    column: 14
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: a
+  location:
+    row: 50
+    column: 29
+  end_location:
+    row: 50
+    column: 30
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: a_
+  location:
+    row: 51
+    column: 29
+  end_location:
+    row: 51
+    column: 31
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: b
+  location:
+    row: 52
+    column: 29
+  end_location:
+    row: 52
+    column: 30
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: c
+  location:
+    row: 53
+    column: 29
+  end_location:
+    row: 53
+    column: 30
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: j
+  location:
+    row: 61
+    column: 16
+  end_location:
+    row: 61
+    column: 17
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: k
+  location:
+    row: 61
+    column: 20
+  end_location:
+    row: 61
+    column: 21
+  fix: ~
+- kind:
+    FunctionUsesLoopVariable: l
+  location:
+    row: 68
+    column: 9
+  end_location:
+    row: 68
+    column: 10
+  fix: ~
+