Ignore non-abstract class attributes when enforcing B024 (#11210)

## Summary

I think the check included here does make sense, but I don't see why we
would allow it if a value is provided for the attribute -- since, in
that case, isn't it _not_ abstract?

Closes: https://github.com/astral-sh/ruff/issues/11208.

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B024.py

@@ -115,25 +115,25 @@ class non_keyword_abcmeta_2(abc.ABCMeta):  # safe
 
 
 # very invalid code, but that's up to mypy et al to check
-class keyword_abc_1(metaclass=ABC):  # safe
+class keyword_abc_1(metaclass=ABC):  # incorrect but outside scope of this check
     def method(self):
         foo()
 
 
-class keyword_abc_2(metaclass=abc.ABC):  # safe
+class keyword_abc_2(metaclass=abc.ABC):  # incorrect but outside scope of this check
     def method(self):
         foo()
 
 
-class abc_set_class_variable_1(ABC):  # safe
+class abc_set_class_variable_1(ABC):  # safe (abstract attribute)
     foo: int
 
 
-class abc_set_class_variable_2(ABC):  # safe
+class abc_set_class_variable_2(ABC):  # error (not an abstract attribute)
     foo = 2
 
 
-class abc_set_class_variable_3(ABC):  # safe
+class abc_set_class_variable_3(ABC):  # error (not an abstract attribute)
     foo: int = 2
 
 

crates/ruff_linter/src/rules/flake8_bugbear/rules/abstract_base_class.rs

@@ -56,6 +56,7 @@ impl Violation for AbstractBaseClassWithoutAbstractMethod {
         format!("`{name}` is an abstract base class, but it has no abstract methods")
     }
 }
+
 /// ## What it does
 /// Checks for empty methods in abstract base classes without an abstract
 /// decorator.
@@ -156,8 +157,13 @@ pub(crate) fn abstract_base_class(
     let mut has_abstract_method = false;
     for stmt in body {
         // https://github.com/PyCQA/flake8-bugbear/issues/293
-        // Ignore abc's that declares a class attribute that must be set
-        if let Stmt::AnnAssign(_) | Stmt::Assign(_) = stmt {
+        // If an ABC declares an attribute by providing a type annotation
+        // but does not actually assign a value for that attribute,
+        // assume it is intended to be an "abstract attribute"
+        if matches!(
+            stmt,
+            Stmt::AnnAssign(ast::StmtAnnAssign { value: None, .. })
+        ) {
             has_abstract_method = true;
             continue;
         }

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B024_B024.py.snap

@@ -41,6 +41,20 @@ B024.py:92:7: B024 `notabc_Base_1` is an abstract base class, but it has no abst
 94 |         foo()
    |
 
+B024.py:132:7: B024 `abc_set_class_variable_2` is an abstract base class, but it has no abstract methods
+    |
+132 | class abc_set_class_variable_2(ABC):  # error (not an abstract attribute)
+    |       ^^^^^^^^^^^^^^^^^^^^^^^^ B024
+133 |     foo = 2
+    |
+
+B024.py:136:7: B024 `abc_set_class_variable_3` is an abstract base class, but it has no abstract methods
+    |
+136 | class abc_set_class_variable_3(ABC):  # error (not an abstract attribute)
+    |       ^^^^^^^^^^^^^^^^^^^^^^^^ B024
+137 |     foo: int = 2
+    |
+
 B024.py:141:7: B024 `abc_set_class_variable_4` is an abstract base class, but it has no abstract methods
     |
 140 | # this doesn't actually declare a class variable, it's just an expression
@@ -48,5 +62,3 @@ B024.py:141:7: B024 `abc_set_class_variable_4` is an abstract base class, but it
     |       ^^^^^^^^^^^^^^^^^^^^^^^^ B024
 142 |     foo
     |
-
-