[flake8-bugbear] Flag useless subscript access in B018

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B018.py

@@ -63,3 +63,11 @@ def foo5():
     foo.bar  # Attribute (raise)
     object().__class__  # Attribute (raise)
     "foo" + "bar"  # BinOp (raise)
+
+
+def foo6():
+    x["item"]  # Subscript (raise)
+    obj.attr["key"]  # Subscript (raise)
+    get_dict()["key"]  # Subscript on call result (raise)
+    x[0]  # Subscript with int key (raise)
+    _ = x["item"]  # OK: result intentionally ignored

crates/ruff_linter/src/preview.rs

@@ -342,3 +342,8 @@ pub(crate) const fn is_trailing_pragma_in_line_length_enabled(preview: PreviewMo
 pub(crate) const fn is_collapsible_if_fix_safe_enabled(settings: &LinterSettings) -> bool {
     settings.preview.is_enabled()
 }
+
+// https://github.com/astral-sh/ruff/issues/23932
+pub(crate) const fn is_b018_subscript_enabled(settings: &LinterSettings) -> bool {
+    settings.preview.is_enabled()
+}

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

@@ -97,6 +97,8 @@ mod tests {
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_9.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_B008.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_1.pyi"))]
+    #[test_case(Rule::UselessExpression, Path::new("B018.py"))]
+    #[test_case(Rule::UselessExpression, Path::new("B018.ipynb"))]
     fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(
             "preview__{}_{}",

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

@@ -5,6 +5,7 @@ use ruff_text_size::Ranged;
 
 use crate::Violation;
 use crate::checkers::ast::Checker;
+use crate::preview::is_b018_subscript_enabled;
 
 use crate::rules::flake8_bugbear::helpers::at_last_top_level_expression_in_cell;
 
@@ -35,9 +36,10 @@ use crate::rules::flake8_bugbear::helpers::at_last_top_level_expression_in_cell;
 /// This rule ignores expression types that are commonly used for their side
 /// effects, such as function calls.
 ///
-/// However, if a seemingly useless expression (like an attribute access) is
-/// needed to trigger a side effect, consider assigning it to an anonymous
-/// variable, to indicate that the return value is intentionally ignored.
+/// However, if a seemingly useless expression (like an attribute access or a
+/// subscript) is needed to trigger a side effect, consider assigning it to an
+/// anonymous variable, to indicate that the return value is intentionally
+/// ignored.
 ///
 /// For example, given:
 /// ```python
@@ -67,6 +69,10 @@ impl Violation for UselessExpression {
                 "Found useless attribute access. Either assign it to a variable or remove it."
                     .to_string()
             }
+            Kind::Subscript => {
+                "Found useless subscript access. Either assign it to a variable or remove it."
+                    .to_string()
+            }
         }
     }
 }
@@ -108,6 +114,16 @@ pub(crate) fn useless_expression(checker: &Checker, value: &Expr) {
                 value.range(),
             );
         }
+        // Flag subscripts as useless expressions, even if they're attached to calls or other
+        // expressions.
+        if is_b018_subscript_enabled(checker.settings()) && value.is_subscript_expr() {
+            checker.report_diagnostic(
+                UselessExpression {
+                    kind: Kind::Subscript,
+                },
+                value.range(),
+            );
+        }
         return;
     }
 
@@ -123,4 +139,5 @@ pub(crate) fn useless_expression(checker: &Checker, value: &Expr) {
 enum Kind {
     Expression,
     Attribute,
+    Subscript,
 }

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B018_B018.ipynb.snap

@@ -0,0 +1,35 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B018 Found useless expression. Either assign it to a variable or remove it.
+ --> B018.ipynb:5:1
+  |
+3 | x
+4 | # Only skip the last expression
+5 | x  # B018
+  | ^
+6 | x
+7 | # Nested expressions isn't relevant
+  |
+
+B018 Found useless expression. Either assign it to a variable or remove it.
+  --> B018.ipynb:9:5
+   |
+ 7 | # Nested expressions isn't relevant
+ 8 | if True:
+ 9 |     x
+   |     ^
+10 | # Semicolons shouldn't affect the output
+11 | x;
+   |
+
+B018 Found useless expression. Either assign it to a variable or remove it.
+  --> B018.ipynb:13:1
+   |
+11 | x;
+12 | # Semicolons with multiple expressions
+13 | x; x
+   | ^
+14 | # Comments, newlines and whitespace
+15 | x  # comment
+   |