Issue a warning if given a parse_only filter that's going to exclude everything.

bs4/__init__.py

@@ -220,7 +220,14 @@ class BeautifulSoup(Tag):
 
         parse_only = parse_only or deprecated_argument(
             "parseOnlyThese", "parse_only")
-
+        if (parse_only is not None
+            and parse_only.string_rules and
+            (parse_only.name_rules or parse_only.attribute_rules)):
+            warnings.warn(
+                f"Value for parse_only will exclude everything, since it puts restrictions on both tags and strings: {parse_only}",
+                UserWarning, stacklevel=3
+            )
+        
         from_encoding = from_encoding or deprecated_argument(
             "fromEncoding", "from_encoding")
 

bs4/tests/test_soup.py

@@ -287,6 +287,14 @@ class TestWarnings(SoupTest):
             soup = self.soup("<a><b></b></a>")
         assert [] == w
 
+    def test_warning_if_strainer_filters_everything(self):
+        strainer = SoupStrainer(name="a", string="b")
+        with warnings.catch_warnings(record=True) as w:
+            soup = self.soup("<a><b></b></a>", parse_only=strainer)
+        warning = self._assert_warning(w, UserWarning)
+        msg = str(warning.message)
+        assert msg.startswith("Value for parse_only will exclude everything, since it puts restrictions on both tags and strings:")
+        
     def test_parseOnlyThese_renamed_to_parse_only(self):
         with warnings.catch_warnings(record=True) as w:
             soup = BeautifulSoup(