Add newline if B006 fix is at end-of-file (#7090)

crates/ruff/resources/test/fixtures/flake8_bugbear/B006_B008.py

@@ -308,3 +308,7 @@ def single_line_func_wrong(value: dict[str, str] = {
 def single_line_func_wrong(value: dict[str, str] = {}) \
     : \
     """Docstring"""
+
+
+def single_line_func_wrong(value: dict[str, str] = {}):
+    """Docstring without newline"""

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

@@ -175,8 +175,15 @@ fn move_initialization(
                 return None;
             }
             Edit::insertion(content, locator.line_start(statement.start()))
+        } else if statement.end() == locator.text_len() {
+            // If the statement is at the end of the file, without a trailing newline, insert
+            // _after_ it with an extra newline.
+            Edit::insertion(
+                format!("{}{}", stylist.line_ending().as_str(), content),
+                locator.full_line_end(statement.end()),
+            )
         } else {
-            // If the docstring is the only statement, insert _before_ it.
+            // If the docstring is the only statement, insert _after_ it.
             Edit::insertion(content, locator.full_line_end(statement.end()))
         }
     } else {

crates/ruff/src/rules/flake8_bugbear/snapshots/ruff__rules__flake8_bugbear__tests__B006_B006_B008.py.snap

@@ -476,4 +476,23 @@ B006_B008.py:308:52: B006 Do not use mutable data structures for argument defaul
     |
     = help: Replace with `None`; initialize within function
 
+B006_B008.py:313:52: B006 [*] Do not use mutable data structures for argument defaults
+    |
+313 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^ B006
+314 |     """Docstring without newline"""
+    |
+    = help: Replace with `None`; initialize within function
+
+ℹ Possible fix
+310 310 |     """Docstring"""
+311 311 | 
+312 312 | 
+313     |-def single_line_func_wrong(value: dict[str, str] = {}):
+314     |-    """Docstring without newline"""
+    313 |+def single_line_func_wrong(value: dict[str, str] = None):
+    314 |+    """Docstring without newline"""
+    315 |+    if value is None:
+    316 |+        value = {}
+