[flake8-bugbear] Avoid adding default initializers to stubs (`B006…

…`) (astral-sh#10152)

## Summary

Adapts the fix for rule B006 to no longer modify the body of function
stubs, while retaining the change in method signature.

## Test Plan

The existing tests for B006 were adapted to reflect this change in
behavior.

## Relevant issue

astral-sh#10083

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

@@ -2,7 +2,7 @@ use ast::call_path::{from_qualified_name, CallPath};
 use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::helpers::is_docstring_stmt;
-use ruff_python_ast::{self as ast, Expr, Parameter, ParameterWithDefault};
+use ruff_python_ast::{self as ast, Expr, Parameter, ParameterWithDefault, Stmt};
 use ruff_python_codegen::{Generator, Stylist};
 use ruff_python_index::Indexer;
 use ruff_python_semantic::analyze::typing::{is_immutable_annotation, is_mutable_expr};
@@ -152,6 +152,11 @@ fn move_initialization(
     // Set the default argument value to `None`.
     let default_edit = Edit::range_replacement("None".to_string(), default.range());
 
+    // If the function is a stub, this is the only necessary edit.
+    if is_stub(function_def) {
+        return Some(Fix::unsafe_edit(default_edit));
+    }
+
     // Add an `if`, to set the argument to its original value if still `None`.
     let mut content = String::new();
     content.push_str(&format!("if {} is None:", parameter.name.as_str()));
@@ -204,3 +209,20 @@ fn move_initialization(
     let initialization_edit = Edit::insertion(content, pos);
     Some(Fix::unsafe_edits(default_edit, [initialization_edit]))
 }
+
+/// Returns `true` if a function has an empty body, and is therefore a stub.
+///
+/// A function body is considered to be empty if it contains only `pass` statements, `...` literals,
+/// and docstrings.
+fn is_stub(function_def: &ast::StmtFunctionDef) -> bool {
+    function_def.body.iter().all(|stmt| match stmt {
+        Stmt::Pass(_) => true,
+        Stmt::Expr(ast::StmtExpr { value, range: _ }) => {
+            matches!(
+                value.as_ref(),
+                Expr::StringLiteral(_) | Expr::EllipsisLiteral(_)
+            )
+        }
+        _ => false,
+    })
+}

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B006_B006_1.py.snap

@@ -19,8 +19,3 @@ B006_1.py:3:22: B006 [*] Do not use mutable data structures for argument default
   3 |+def foobar(foor, bar=None):    
 4 4 |     """
 5 5 |     """
-  6 |+
-  7 |+    if bar is None:
-  8 |+        bar = {}
-
-

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B006_B006_2.py.snap

@@ -19,9 +19,4 @@ B006_2.py:4:22: B006 [*] Do not use mutable data structures for argument default
 4   |-def foobar(foor, bar={}):    
   4 |+def foobar(foor, bar=None):    
 5 5 |     """
-6   |-    """                    
-  6 |+    """                    
-  7 |+    if bar is None:
-  8 |+        bar = {}
-
-
+6 6 |     """

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B006_B006_3.py.snap

@@ -16,10 +16,5 @@ B006_3.py:4:22: B006 [*] Do not use mutable data structures for argument default
 3 3 | 
 4   |-def foobar(foor, bar={}):    
   4 |+def foobar(foor, bar=None):    
-  5 |+    """
-5 6 |     """
-6   |-    """
-  7 |+    if bar is None:
-  8 |+        bar = {}
-
-
+5 5 |     """
+6 6 |     """