Mention what argument has both annotation and type argument

changelog.d/1410.change.md

@@ -0,0 +1 @@
+The error message if an attribute has both an annotation and a type argument will now disclose _what_ attribute seems to be the problem.

src/attr/_make.py

@@ -2491,7 +2491,7 @@ def from_counting_attr(cls, name: str, ca: _CountingAttr, type=None):
         if type is None:
             type = ca.type
         elif ca.type is not None:
-            msg = "Type annotation and type argument cannot both be present"
+            msg = f"Type annotation and type argument cannot both be present for '{name}'."
             raise ValueError(msg)
         return cls(
             name,

tests/test_annotations.py

@@ -62,7 +62,7 @@ class C:
                 x: int = attr.ib(type=int)
 
         assert (
-            "Type annotation and type argument cannot both be present",
+            "Type annotation and type argument cannot both be present for 'x'.",
         ) == e.value.args
 
     def test_typing_annotations(self):