[pyflakes] Improve error message for UndefinedName when a builtin was added in a newer version than specified in Ruff config (F821)

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

@@ -1,5 +1,6 @@
 use ruff_diagnostics::Diagnostic;
 use ruff_python_semantic::Exceptions;
+use ruff_python_stdlib::builtins::version_builtin_was_added;
 
 use crate::checkers::ast::Checker;
 use crate::codes::Rule;
@@ -35,9 +36,12 @@ pub(crate) fn unresolved_references(checker: &mut Checker) {
                     }
                 }
 
+                let symbol_name = reference.name(checker.locator);
+
                 checker.diagnostics.push(Diagnostic::new(
                     pyflakes::rules::UndefinedName {
-                        name: reference.name(checker.locator).to_string(),
+                        name: symbol_name.to_string(),
+                        minor_version_builtin_added: version_builtin_was_added(symbol_name),
                     },
                     reference.range(),
                 ));

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -208,6 +208,18 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn f821_with_builtin_added_on_new_py_version_but_old_target_version_specified() {
+        let diagnostics = test_snippet(
+            "PythonFinalizationError",
+            &LinterSettings {
+                target_version: crate::settings::types::PythonVersion::Py312,
+                ..LinterSettings::for_rule(Rule::UndefinedName)
+            },
+        );
+        assert_messages!(diagnostics);
+    }
+
     #[test_case(Rule::UnusedVariable, Path::new("F841_4.py"))]
     #[test_case(Rule::UnusedImport, Path::new("__init__.py"))]
     #[test_case(Rule::UnusedImport, Path::new("F401_24/__init__.py"))]

crates/ruff_linter/src/rules/pyflakes/rules/undefined_name.rs

@@ -19,17 +19,36 @@ use ruff_macros::{derive_message_formats, violation};
 ///     return n * 2
 /// ```
 ///
+/// ## Options
+/// - [`target-version`]: Can be used to configure which symbols Ruff will understand
+///   as being available in the `builtins` namespace.
+///
 /// ## References
 /// - [Python documentation: Naming and binding](https://docs.python.org/3/reference/executionmodel.html#naming-and-binding)
 #[violation]
 pub struct UndefinedName {
     pub(crate) name: String,
+    pub(crate) minor_version_builtin_added: Option<u8>,
 }
 
 impl Violation for UndefinedName {
     #[derive_message_formats]
     fn message(&self) -> String {
-        let UndefinedName { name } = self;
-        format!("Undefined name `{name}`")
+        let UndefinedName {
+            name,
+            minor_version_builtin_added,
+        } = self;
+        let tip = minor_version_builtin_added.map(|version_added| {
+            format!(
+                "Added as a builtin in Python 3.{version_added}; \
+consider specifying a newer `target-version` in your Ruff config."
+            )
+        });
+
+        if let Some(tip) = tip {
+            format!("Undefined name `{name}`. {tip}")
+        } else {
+            format!("Undefined name `{name}`")
+        }
     }
 }

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__f821_with_builtin_added_on_new_py_version_but_old_target_version_specified.snap

@@ -0,0 +1,8 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+<filename>:1:1: F821 Undefined name `PythonFinalizationError`. Added as a builtin in Python 3.13; consider specifying a newer `target-version` in your Ruff config.
+  |
+1 | PythonFinalizationError
+  | ^^^^^^^^^^^^^^^^^^^^^^^ F821
+  |