[flake8-bugbear] Ignore enum classes in cached-instance-method (B019) (#11312)

## Summary

While I was here, I also updated the rule to use
`function_type::classify` rather than hard-coding `staticmethod` and
friends.

Per Carl:

> Enum instances are already referred to by the class, forming a cycle
that won't get collected until the class itself does. At which point the
`lru_cache` itself would be collected, too.

Closes https://github.com/astral-sh/ruff/issues/9912.

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B019.py

@@ -106,3 +106,15 @@ class Foo:
     @lru_cache()
     def another_called_lru_cached_instance_method(self, y):
         ...
+
+
+import enum
+
+
+class Foo(enum.Enum):
+    ONE = enum.auto()
+    TWO = enum.auto()
+
+    @functools.cache
+    def bar(self, arg: str) -> str:
+        return f"{self} - {arg}"

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -203,7 +203,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                 }
             }
             if checker.enabled(Rule::CachedInstanceMethod) {
-                flake8_bugbear::rules::cached_instance_method(checker, decorator_list);
+                flake8_bugbear::rules::cached_instance_method(checker, function_def);
             }
             if checker.enabled(Rule::MutableArgumentDefault) {
                 flake8_bugbear::rules::mutable_argument_default(checker, function_def);

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

@@ -1,8 +1,9 @@
-use ruff_python_ast::{self as ast, Decorator, Expr};
-
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_semantic::SemanticModel;
+use ruff_python_ast::helpers::map_callable;
+use ruff_python_ast::{self as ast, Expr};
+use ruff_python_semantic::analyze::{class, function_type};
+use ruff_python_semantic::{ScopeKind, SemanticModel};
 use ruff_text_size::Ranged;
 
 use crate::checkers::ast::Checker;
@@ -20,6 +21,9 @@ use crate::checkers::ast::Checker;
 /// instance of the class, or use the `@lru_cache` decorator on a function
 /// outside of the class.
 ///
+/// This rule ignores instance methods on enumeration classes, as enum members
+/// are singletons.
+///
 /// ## Example
 /// ```python
 /// from functools import lru_cache
@@ -70,6 +74,43 @@ impl Violation for CachedInstanceMethod {
     }
 }
 
+/// B019
+pub(crate) fn cached_instance_method(checker: &mut Checker, function_def: &ast::StmtFunctionDef) {
+    let scope = checker.semantic().current_scope();
+
+    // Parent scope _must_ be a class.
+    let ScopeKind::Class(class_def) = scope.kind else {
+        return;
+    };
+
+    // The function must be an _instance_ method.
+    let type_ = function_type::classify(
+        &function_def.name,
+        &function_def.decorator_list,
+        scope,
+        checker.semantic(),
+        &checker.settings.pep8_naming.classmethod_decorators,
+        &checker.settings.pep8_naming.staticmethod_decorators,
+    );
+    if !matches!(type_, function_type::FunctionType::Method) {
+        return;
+    }
+
+    for decorator in &function_def.decorator_list {
+        if is_cache_func(map_callable(&decorator.expression), checker.semantic()) {
+            // If we found a cached instance method, validate (lazily) that the class is not an enum.
+            if class::is_enumeration(class_def, checker.semantic()) {
+                return;
+            }
+
+            checker
+                .diagnostics
+                .push(Diagnostic::new(CachedInstanceMethod, decorator.range()));
+        }
+    }
+}
+
+/// Returns `true` if the given expression is a call to `functools.lru_cache` or `functools.cache`.
 fn is_cache_func(expr: &Expr, semantic: &SemanticModel) -> bool {
     semantic
         .resolve_qualified_name(expr)
@@ -80,32 +121,3 @@ fn is_cache_func(expr: &Expr, semantic: &SemanticModel) -> bool {
             )
         })
 }
-
-/// B019
-pub(crate) fn cached_instance_method(checker: &mut Checker, decorator_list: &[Decorator]) {
-    if !checker.semantic().current_scope().kind.is_class() {
-        return;
-    }
-    for decorator in decorator_list {
-        // TODO(charlie): This should take into account `classmethod-decorators` and
-        // `staticmethod-decorators`.
-        if let Expr::Name(ast::ExprName { id, .. }) = &decorator.expression {
-            if id == "classmethod" || id == "staticmethod" {
-                return;
-            }
-        }
-    }
-    for decorator in decorator_list {
-        if is_cache_func(
-            match &decorator.expression {
-                Expr::Call(ast::ExprCall { func, .. }) => func,
-                _ => &decorator.expression,
-            },
-            checker.semantic(),
-        ) {
-            checker
-                .diagnostics
-                .push(Diagnostic::new(CachedInstanceMethod, decorator.range()));
-        }
-    }
-}