Flag mutable defaults based on mutable annotations

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B006_9.py

@@ -0,0 +1,13 @@
+LIST = []
+
+
+def foo(a: list = LIST):  # B006
+    raise NotImplementedError("")
+
+
+def foo(a: list = None):  # OK
+    raise NotImplementedError("")
+
+
+def foo(a: list | None = LIST):  # OK
+    raise NotImplementedError("")

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

@@ -42,6 +42,7 @@ mod tests {
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_6.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_7.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_8.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_9.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_B008.py"))]
     #[test_case(Rule::NoExplicitStacklevel, Path::new("B028.py"))]
     #[test_case(Rule::RaiseLiteral, Path::new("B016.py"))]

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

@@ -6,7 +6,9 @@ use ruff_python_ast::{self as ast, Expr, Parameter, ParameterWithDefault};
 use ruff_python_codegen::{Generator, Stylist};
 use ruff_python_index::Indexer;
 use ruff_python_semantic::analyze::function_type::is_stub;
-use ruff_python_semantic::analyze::typing::{is_immutable_annotation, is_mutable_expr};
+use ruff_python_semantic::analyze::typing::{
+    is_immutable_annotation, is_immutable_expr, is_mutable_annotation, is_mutable_expr,
+};
 use ruff_python_semantic::SemanticModel;
 use ruff_python_trivia::{indentation_at_offset, textwrap};
 use ruff_source_file::Locator;
@@ -108,10 +110,19 @@ pub(crate) fn mutable_argument_default(checker: &mut Checker, function_def: &ast
             .map(|target| QualifiedName::from_dotted_name(target))
             .collect();
 
-        if is_mutable_expr(default, checker.semantic())
+        // Either the expression _or_ the annotation must be mutable.
+        if (is_mutable_expr(default, checker.semantic())
             && !parameter.annotation.as_ref().is_some_and(|expr| {
                 is_immutable_annotation(expr, checker.semantic(), extend_immutable_calls.as_slice())
-            })
+            }))
+            || (parameter.annotation.as_ref().is_some_and(|annotation| {
+                is_mutable_annotation(annotation, checker.semantic())
+                    && !is_immutable_expr(
+                        default,
+                        checker.semantic(),
+                        extend_immutable_calls.as_slice(),
+                    )
+            }))
         {
             let mut diagnostic = Diagnostic::new(MutableArgumentDefault, default.range());
 

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B006_B006_9.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006_9.py:4:19: B006 [*] Do not use mutable data structures for argument defaults
+  |
+4 | def foo(a: list = LIST):  # B006
+  |                   ^^^^ B006
+5 |     raise NotImplementedError("")
+  |
+  = help: Replace with `None`; initialize within function
+
+ℹ Unsafe fix
+1 1 | LIST = []
+2 2 | 
+3 3 | 
+4   |-def foo(a: list = LIST):  # B006
+  4 |+def foo(a: list = None):  # B006
+5 5 |     raise NotImplementedError("")
+6 6 | 
+7 7 |

crates/ruff_python_semantic/src/analyze/typing.rs

@@ -212,6 +212,45 @@ pub fn to_pep604_operator(
         })
 }
 
+/// Return `true` if `Expr` represents a reference to a type annotation that resolves to a mutable
+/// type.
+pub fn is_mutable_annotation(expr: &Expr, semantic: &SemanticModel) -> bool {
+    match expr {
+        Expr::Name(_) | Expr::Attribute(_) => semantic
+            .resolve_qualified_name(expr)
+            .is_some_and(|qualified_name| is_mutable_return_type(qualified_name.segments())),
+        Expr::Subscript(ast::ExprSubscript { value, slice, .. }) => semantic
+            .resolve_qualified_name(value)
+            .is_some_and(|qualified_name| {
+                if is_mutable_return_type(qualified_name.segments()) {
+                    true
+                } else if semantic.match_typing_qualified_name(&qualified_name, "Union") {
+                    if let Expr::Tuple(ast::ExprTuple { elts, .. }) = slice.as_ref() {
+                        elts.iter().all(|elt| is_mutable_annotation(elt, semantic))
+                    } else {
+                        false
+                    }
+                } else if is_pep_593_generic_type(qualified_name.segments()) {
+                    if let Expr::Tuple(ast::ExprTuple { elts, .. }) = slice.as_ref() {
+                        elts.first()
+                            .is_some_and(|elt| is_mutable_annotation(elt, semantic))
+                    } else {
+                        false
+                    }
+                } else {
+                    false
+                }
+            }),
+        Expr::BinOp(ast::ExprBinOp {
+            left,
+            op: Operator::BitOr,
+            right,
+            range: _,
+        }) => is_mutable_annotation(left, semantic) && is_mutable_annotation(right, semantic),
+        _ => false,
+    }
+}
+
 /// Return `true` if `Expr` represents a reference to a type annotation that resolves to an
 /// immutable type.
 pub fn is_immutable_annotation(
@@ -236,7 +275,7 @@ pub fn is_immutable_annotation(
             .is_some_and(|qualified_name| {
                 if is_immutable_generic_type(qualified_name.segments()) {
                     true
-                } else if matches!(qualified_name.segments(), ["typing", "Union"]) {
+                } else if semantic.match_typing_qualified_name(&qualified_name, "Union") {
                     if let Expr::Tuple(ast::ExprTuple { elts, .. }) = slice.as_ref() {
                         elts.iter().all(|elt| {
                             is_immutable_annotation(elt, semantic, extend_immutable_calls)
@@ -244,7 +283,7 @@ pub fn is_immutable_annotation(
                     } else {
                         false
                     }
-                } else if matches!(qualified_name.segments(), ["typing", "Optional"]) {
+                } else if semantic.match_typing_qualified_name(&qualified_name, "Optional") {
                     is_immutable_annotation(slice, semantic, extend_immutable_calls)
                 } else if is_pep_593_generic_type(qualified_name.segments()) {
                     if let Expr::Tuple(ast::ExprTuple { elts, .. }) = slice.as_ref() {
@@ -311,6 +350,29 @@ pub fn is_mutable_expr(expr: &Expr, semantic: &SemanticModel) -> bool {
     }
 }
 
+/// Return `true` if `expr` is an expression that resolves to an immutable value.
+pub fn is_immutable_expr(
+    expr: &Expr,
+    semantic: &SemanticModel,
+    extend_immutable_calls: &[QualifiedName],
+) -> bool {
+    match expr {
+        Expr::NoneLiteral(_) => true,
+        Expr::BooleanLiteral(_) => true,
+        Expr::NumberLiteral(_) => true,
+        Expr::StringLiteral(_) => true,
+        Expr::BytesLiteral(_) => true,
+        Expr::EllipsisLiteral(_) => true,
+        Expr::Tuple(ast::ExprTuple { elts, .. }) => elts
+            .iter()
+            .all(|elt| is_immutable_expr(elt, semantic, extend_immutable_calls)),
+        Expr::Call(ast::ExprCall { func, .. }) => {
+            is_immutable_func(func, semantic, extend_immutable_calls)
+        }
+        _ => false,
+    }
+}
+
 /// Return `true` if [`ast::StmtIf`] is a guard for a type-checking block.
 pub fn is_type_checking_block(stmt: &ast::StmtIf, semantic: &SemanticModel) -> bool {
     let ast::StmtIf { test, .. } = stmt;