brain_attrs: Support annotation-only members (#2515)

Similar to dataclasses, the following class which uses instance
variable annotations is valid:

```py
@attrs.define
class AttrsCls:
    x: int
AttrsCls(1).x
```

However, before this commit astroid failed to transform the class
attribute into an instance attribute and this led to `no-member` errors
in pylint.

Only the new `attrs` API supports this form out-of-the-box, so just
address the common case.

Closes #2514

ChangeLog

@@ -13,6 +13,10 @@ What's New in astroid 3.3.3?
 ============================
 Release date: TBA
 
+* Add annotation-only instance attributes to attrs classes to fix `no-member` false positives.
+
+  Closes #2514
+
 
 
 What's New in astroid 3.3.2?

astroid/brain/brain_attrs.py

@@ -24,6 +24,13 @@
         "field",
     )
 )
+NEW_ATTRS_NAMES = frozenset(
+    (
+        "attrs.define",
+        "attrs.mutable",
+        "attrs.frozen",
+    )
+)
 ATTRS_NAMES = frozenset(
     (
         "attr.s",
@@ -33,9 +40,7 @@
         "attr.define",
         "attr.mutable",
         "attr.frozen",
-        "attrs.define",
-        "attrs.mutable",
-        "attrs.frozen",
+        *NEW_ATTRS_NAMES,
     )
 )
 
@@ -64,13 +69,14 @@ def attr_attributes_transform(node: ClassDef) -> None:
     # Prevents https://github.com/pylint-dev/pylint/issues/1884
     node.locals["__attrs_attrs__"] = [Unknown(parent=node)]
 
+    use_bare_annotations = is_decorated_with_attrs(node, NEW_ATTRS_NAMES)
     for cdef_body_node in node.body:
         if not isinstance(cdef_body_node, (Assign, AnnAssign)):
             continue
         if isinstance(cdef_body_node.value, Call):
             if cdef_body_node.value.func.as_string() not in ATTRIB_NAMES:
                 continue
-        else:
+        elif not use_bare_annotations:
             continue
         targets = (
             cdef_body_node.targets

script/.contributors_aliases.json

@@ -85,6 +85,10 @@
     "name": "Hippo91",
     "team": "Maintainers"
   },
+  "Hnasar@users.noreply.github.com": {
+    "mails": ["Hnasar@users.noreply.github.com", "hashem@hudson-trading.com"],
+    "name": "Hashem Nasarat"
+  },
   "hugovk@users.noreply.github.com": {
     "mails": ["hugovk@users.noreply.github.com"],
     "name": "Hugo van Kemenade"

tests/brain/test_attr.py

@@ -7,7 +7,7 @@
 import unittest
 
 import astroid
-from astroid import nodes
+from astroid import exceptions, nodes
 
 try:
     import attr  # type: ignore[import]  # pylint: disable=unused-import
@@ -201,3 +201,31 @@ class Foo:
         """
         should_be_unknown = next(astroid.extract_node(code).infer()).getattr("bar")[0]
         self.assertIsInstance(should_be_unknown, astroid.Unknown)
+
+    def test_attr_with_only_annotation_fails(self) -> None:
+        code = """
+        import attr
+
+        @attr.s
+        class Foo:
+            bar: int
+        Foo()
+        """
+        with self.assertRaises(exceptions.AttributeInferenceError):
+            next(astroid.extract_node(code).infer()).getattr("bar")
+
+    def test_attrs_with_only_annotation_works(self) -> None:
+        code = """
+        import attrs
+
+        @attrs.define
+        class Foo:
+            bar: int
+            baz: str = "hello"
+        Foo(1)
+        """
+        for attr_name in ("bar", "baz"):
+            should_be_unknown = next(astroid.extract_node(code).infer()).getattr(
+                attr_name
+            )[0]
+            self.assertIsInstance(should_be_unknown, astroid.Unknown)