Fix 3.8 typing error and add test for coverage

src/structlog/stdlib.py

@@ -19,7 +19,7 @@
 import warnings
 
 from functools import partial
-from typing import Any, Callable, Collection, Iterable, Sequence, cast
+from typing import Any, Callable, Collection, Dict, Iterable, Sequence, cast
 
 from . import _config
 from ._base import BoundLoggerBase
@@ -1036,7 +1036,7 @@ def format(self, record: logging.LogRecord) -> str:
             # We need to copy because it's possible that the same record gets
             # processed by multiple logging formatters. LogRecord.getMessage
             # would transform our dict into a str.
-            ed = cast(dict[str, Any], record.msg).copy()
+            ed = cast(Dict[str, Any], record.msg).copy()
             ed["_record"] = record
             ed["_from_structlog"] = True
         else:
@@ -1078,7 +1078,7 @@ def format(self, record: logging.LogRecord) -> str:
 
         if not isinstance(ed, str):
             warnings.warn(
-                "The last processor in ProcessorFormatter.formatters must "
+                "The last processor in ProcessorFormatter.processors must "
                 f"return a string, but {self.processors[-1]} returned a "
                 f"{type(ed)} instead.",
                 category=RuntimeWarning,

tests/test_stdlib.py

@@ -1116,6 +1116,25 @@ def test_remove_processors_meta(self):
             {"foo": "bar", "_record": "foo", "_from_structlog": True},
         )
 
+    def test_non_string_message_warning(self):
+        """
+        A warning is raised if the last processor in
+        ProcessorFormatter.processors doesn't return a string.
+        """
+        configure_logging(None)
+        logger = logging.getLogger()
+
+        formatter = ProcessorFormatter(
+            processors=[lambda *args, **kwargs: {"foo": "bar"}],
+        )
+        logger.handlers[0].setFormatter(formatter)
+
+        with pytest.warns(
+            RuntimeWarning,
+            match="The last processor in ProcessorFormatter.processors must return a string",
+        ):
+            logger.info("baz")
+
 
 @pytest_asyncio.fixture(name="abl")
 async def _abl(cl):