Deprecate build_from_crawler() calling from_settings().

scrapy/middleware.py

@@ -54,6 +54,14 @@ class MiddlewareManager:
     @staticmethod
     def _build_from_settings(objcls: type[_T], settings: BaseSettings) -> _T:
         if hasattr(objcls, "from_settings"):
+            warnings.warn(
+                f"{objcls.__qualname__} has from_settings() but not from_crawler()."
+                " This is deprecated and calling from_settings() will be removed in a future"
+                " Scrapy version. You can implement a simple from_crawler() that calls"
+                " from_settings() with crawler.settings.",
+                category=ScrapyDeprecationWarning,
+                stacklevel=2,
+            )
             instance = objcls.from_settings(settings)  # type: ignore[attr-defined]
             method_name = "from_settings"
         else:

scrapy/utils/misc.py

@@ -185,6 +185,14 @@ def build_from_crawler(
         instance = objcls.from_crawler(crawler, *args, **kwargs)  # type: ignore[attr-defined]
         method_name = "from_crawler"
     elif hasattr(objcls, "from_settings"):
+        warnings.warn(
+            f"{objcls.__qualname__} has from_settings() but not from_crawler()."
+            " This is deprecated and calling from_settings() will be removed in a future"
+            " Scrapy version. You can implement a simple from_crawler() that calls"
+            " from_settings() with crawler.settings.",
+            category=ScrapyDeprecationWarning,
+            stacklevel=2,
+        )
         instance = objcls.from_settings(crawler.settings, *args, **kwargs)  # type: ignore[attr-defined]
         method_name = "from_settings"
     else:

tests/test_utils_request.py

@@ -8,6 +8,7 @@ from weakref import WeakKeyDictionary
 
 import pytest
 
+from scrapy.exceptions import ScrapyDeprecationWarning
 from scrapy.http import Request
 from scrapy.utils.python import to_bytes
 from scrapy.utils.request import (
@@ -384,7 +385,9 @@ class CustomRequestFingerprinterTestCase(unittest.TestCase):
             "REQUEST_FINGERPRINTER_CLASS": RequestFingerprinter,
             "FINGERPRINT": b"fingerprint",
         }
-        crawler = get_crawler(settings_dict=settings)
+        with warnings.catch_warnings():
+            warnings.simplefilter("ignore", ScrapyDeprecationWarning)
+            crawler = get_crawler(settings_dict=settings)
 
         request = Request("http://www.example.com")
         fingerprint = crawler.request_fingerprinter.fingerprint(request)