[flake8-bugbear] Implement loop-iterator-mutation (B909)

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B909.py

@@ -0,0 +1,129 @@
+"""
+Should emit:
+B999 - on lines 11, 25, 26, 40, 46
+"""
+
+# lists
+
+some_list = [1, 2, 3]
+some_other_list = [1, 2, 3]
+for elem in some_list:
+    # errors
+    some_list.remove(elem)
+    del some_list[2]
+    some_list.append(elem)
+    some_list.sort()
+    some_list.reverse()
+    some_list.clear()
+    some_list.extend([1, 2])
+    some_list.insert(1, 1)
+    some_list.pop(1)
+    some_list.pop()
+
+    # conditional break should error
+    if elem == 2:
+        some_list.remove(elem)
+        if elem == 3:
+            break
+
+    # non-errors
+    some_other_list.remove(elem)
+    del some_list
+    del some_other_list
+    found_idx = some_list.index(elem)
+    some_list = 3
+
+    # unconditional break should not error
+    if elem == 2:
+        some_list.remove(elem)
+        break
+
+
+# dicts
+mydicts = {'a': {'foo': 1, 'bar': 2}}
+
+for elem in mydicts:
+    # errors
+    mydicts.popitem()
+    mydicts.setdefault('foo', 1)
+    mydicts.update({'foo': 'bar'})
+
+    # no errors
+    elem.popitem()
+    elem.setdefault('foo', 1)
+    elem.update({'foo': 'bar'})
+
+# sets
+
+myset = {1, 2, 3}
+
+for _ in myset:
+    # errors
+    myset.update({4, 5})
+    myset.intersection_update({4, 5})
+    myset.difference_update({4, 5})
+    myset.symmetric_difference_update({4, 5})
+    myset.add(4)
+    myset.discard(3)
+
+    # no errors
+    del myset
+
+
+# members
+class A:
+    some_list: list
+
+    def __init__(self, ls):
+        self.some_list = list(ls)
+
+
+a = A((1, 2, 3))
+# ensure member accesses are handled as errors
+for elem in a.some_list:
+    a.some_list.remove(elem)
+    del a.some_list[2]
+
+
+# Augassign should error
+
+foo = [1, 2, 3]
+bar = [4, 5, 6]
+for _ in foo:
+    foo *= 2
+    foo += bar
+    foo[1] = 9
+    foo[1:2] = bar
+    foo[1:2:3] = bar
+
+foo = {1,2,3}
+bar = {4,5,6}
+for _ in foo: # should error
+    foo |= bar
+    foo &= bar
+    foo -= bar
+    foo ^= bar
+
+
+# more tests for unconditional breaks - should not error
+for _ in foo:
+    foo.remove(1)
+    for _ in bar:
+        bar.remove(1)
+        break
+    break
+
+# should not error
+for _ in foo:
+    foo.remove(1)
+    for _ in bar:
+        ...
+    break
+
+# should error (?)
+for _ in foo:
+    foo.remove(1)
+    if bar:
+        bar.remove(1)
+        break
+    break

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

@@ -30,6 +30,9 @@ pub(crate) fn deferred_for_loops(checker: &mut Checker) {
             if checker.enabled(Rule::EnumerateForLoop) {
                 flake8_simplify::rules::enumerate_for_loop(checker, stmt_for);
             }
+            if checker.enabled(Rule::LoopIteratorMutation) {
+                flake8_bugbear::rules::loop_iterator_mutation(checker, stmt_for);
+            }
         }
     }
 }

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

@@ -1271,6 +1271,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                 Rule::UnnecessaryEnumerate,
                 Rule::UnusedLoopControlVariable,
                 Rule::YieldInForLoop,
+                Rule::LoopIteratorMutation,
             ]) {
                 checker.analyze.for_loops.push(checker.semantic.snapshot());
             }

crates/ruff_linter/src/codes.rs

@@ -376,6 +376,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Bugbear, "033") => (RuleGroup::Stable, rules::flake8_bugbear::rules::DuplicateValue),
         (Flake8Bugbear, "034") => (RuleGroup::Stable, rules::flake8_bugbear::rules::ReSubPositionalArgs),
         (Flake8Bugbear, "035") => (RuleGroup::Stable, rules::flake8_bugbear::rules::StaticKeyDictComprehension),
+        (Flake8Bugbear, "909") => (RuleGroup::Preview, rules::flake8_bugbear::rules::LoopIteratorMutation),
         (Flake8Bugbear, "904") => (RuleGroup::Stable, rules::flake8_bugbear::rules::RaiseWithoutFromInsideExcept),
         (Flake8Bugbear, "905") => (RuleGroup::Stable, rules::flake8_bugbear::rules::ZipWithoutExplicitStrict),
 

crates/ruff_linter/src/rules/flake8_bugbear/mod.rs

@@ -61,6 +61,7 @@ mod tests {
     #[test_case(Rule::UselessContextlibSuppress, Path::new("B022.py"))]
     #[test_case(Rule::UselessExpression, Path::new("B018.ipynb"))]
     #[test_case(Rule::UselessExpression, Path::new("B018.py"))]
+    #[test_case(Rule::LoopIteratorMutation, Path::new("B909.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(