Update default and latest Python versions for 3.14 (#20725)

Summary
--

Closes #19467 and also removes the warning about using Python 3.14
without
preview enabled.

I also bumped `PythonVersion::default` to 3.9 because it reaches EOL
this month,
but we could also defer that for now if we wanted.

The first three commits are related to the `latest` bump to 3.14; the
fourth commit
bumps the default to 3.10.

Note that this PR also bumps the default Python version for ty to 3.10
because
there was a test asserting that it stays in sync with
`ast::PythonVersion`.

Test Plan
--

Existing tests

I spot-checked the ecosystem report, and I believe these are all
expected. Inbits doesn't specify a target Python version, so I guess
we're applying the default. UP007, UP035, and UP045 all use the new
default value to emit new diagnostics.

Author: ntBre

crates/ruff/tests/cli/snapshots/cli__lint__requires_python_ruff_toml_above-2.snap

@@ -9,7 +9,6 @@ info:
     - concise
     - "--show-settings"
     - test.py
-snapshot_kind: text
 ---
 success: true
 exit_code: 0
@@ -265,7 +264,7 @@ linter.ruff.parenthesize_tuple_in_subscript = false
 
 # Formatter Settings
 formatter.exclude = []
-formatter.unresolved_target_version = 3.9
+formatter.unresolved_target_version = 3.10
 formatter.per_file_target_version = {}
 formatter.preview = disabled
 formatter.line_width = 88
@@ -280,7 +279,7 @@ formatter.docstring_code_line_width = dynamic
 # Analyze Settings
 analyze.exclude = []
 analyze.preview = disabled
-analyze.target_version = 3.9
+analyze.target_version = 3.10
 analyze.string_imports = disabled
 analyze.extension = ExtensionMapping({})
 analyze.include_dependencies = {}

crates/ruff/tests/cli/snapshots/cli__lint__requires_python_ruff_toml_above.snap

@@ -9,7 +9,6 @@ info:
     - concise
     - "--show-settings"
     - foo/test.py
-snapshot_kind: text
 ---
 success: true
 exit_code: 0
@@ -265,7 +264,7 @@ linter.ruff.parenthesize_tuple_in_subscript = false
 
 # Formatter Settings
 formatter.exclude = []
-formatter.unresolved_target_version = 3.9
+formatter.unresolved_target_version = 3.10
 formatter.per_file_target_version = {}
 formatter.preview = disabled
 formatter.line_width = 88
@@ -280,7 +279,7 @@ formatter.docstring_code_line_width = dynamic
 # Analyze Settings
 analyze.exclude = []
 analyze.preview = disabled
-analyze.target_version = 3.9
+analyze.target_version = 3.10
 analyze.string_imports = disabled
 analyze.extension = ExtensionMapping({})
 analyze.include_dependencies = {}

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_18.py

@@ -14,6 +14,6 @@ class Bar:
     ]
 
 
-# OK: Allow named expressions in annotations.
+# This is no longer allowed on Python 3.14+
 x: (y := 1)
 print(y)

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_26.py

@@ -13,16 +13,16 @@
 
 # References to a class from inside the class:
 class C:
-    other: C = ...  # valid in a `.pyi` stub file, not in a `.py` runtime file
+    other: C = ...  # valid in a `.pyi` stub file, and in a `.py` runtime file with deferred annotations
     other2: "C" = ...  # always okay
-    def from_str(self, s: str) -> C: ...  # valid in a `.pyi` stub file, not in a `.py` runtime file
+    def from_str(self, s: str) -> C: ...  # valid in a `.pyi` stub file, and in a `.py` runtime file with deferred annotations
     def from_str2(self, s: str) -> "C": ...  # always okay
 
 # Circular references:
 class A:
-    foo: B  # valid in a `.pyi` stub file, not in a `.py` runtime file
+    foo: B  # valid in a `.pyi` stub file, and in a `.py` runtime file with deferred annotations
     foo2: "B"  # always okay
-    bar: dict[str, B]  # valid in a `.pyi` stub file, not in a `.py` runtime file
+    bar: dict[str, B]  # valid in a `.pyi` stub file, and in a `.py` runtime file with deferred annotations
     bar2: dict[str, "A"]  # always okay
 
 class B:

crates/ruff_linter/src/linter.rs

@@ -27,14 +27,13 @@ use crate::fix::{FixResult, fix_file};
 use crate::message::create_syntax_error_diagnostic;
 use crate::noqa::add_noqa;
 use crate::package::PackageRoot;
-use crate::preview::is_py314_support_enabled;
 use crate::registry::Rule;
 #[cfg(any(feature = "test-rules", test))]
 use crate::rules::ruff::rules::test_rules::{self, TEST_RULES, TestRule};
 use crate::settings::types::UnsafeFixes;
 use crate::settings::{LinterSettings, TargetVersion, flags};
 use crate::source_kind::SourceKind;
-use crate::{Locator, directives, fs, warn_user_once};
+use crate::{Locator, directives, fs};
 
 pub(crate) mod float;
 
@@ -442,14 +441,6 @@ pub fn lint_only(
 ) -> LinterResult {
     let target_version = settings.resolve_target_version(path);
 
-    if matches!(target_version, TargetVersion(Some(PythonVersion::PY314)))
-        && !is_py314_support_enabled(settings)
-    {
-        warn_user_once!(
-            "Support for Python 3.14 is in preview and may undergo breaking changes. Enable `preview` to remove this warning."
-        );
-    }
-
     let parsed = source.into_parsed(source_kind, source_type, target_version.parser_version());
 
     // Map row and column locations to byte slices (lazily).
@@ -551,14 +542,6 @@ pub fn lint_fix<'a>(
 
     let target_version = settings.resolve_target_version(path);
 
-    if matches!(target_version, TargetVersion(Some(PythonVersion::PY314)))
-        && !is_py314_support_enabled(settings)
-    {
-        warn_user_once!(
-            "Support for Python 3.14 is in preview and may undergo breaking changes. Enable `preview` to remove this warning."
-        );
-    }
-
     // Continuously fix until the source code stabilizes.
     loop {
         // Parse once.

crates/ruff_linter/src/preview.rs

@@ -7,10 +7,6 @@
 
 use crate::settings::LinterSettings;
 
-pub(crate) const fn is_py314_support_enabled(settings: &LinterSettings) -> bool {
-    settings.preview.is_enabled()
-}
-
 // Rule-specific behavior
 
 // https://github.com/astral-sh/ruff/pull/15541

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

@@ -8,9 +8,12 @@ mod tests {
     use anyhow::Result;
     use test_case::test_case;
 
+    use ruff_python_ast::PythonVersion;
+
     use crate::registry::Rule;
+    use crate::settings::LinterSettings;
     use crate::test::test_path;
-    use crate::{assert_diagnostics, settings};
+    use crate::{assert_diagnostics, assert_diagnostics_diff};
 
     #[test_case(Rule::FastApiRedundantResponseModel, Path::new("FAST001.py"))]
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_0.py"))]
@@ -20,12 +23,35 @@ mod tests {
         let snapshot = format!("{}_{}", rule_code.name(), path.to_string_lossy());
         let diagnostics = test_path(
             Path::new("fastapi").join(path).as_path(),
-            &settings::LinterSettings::for_rule(rule_code),
+            &LinterSettings::for_rule(rule_code),
         )?;
         assert_diagnostics!(snapshot, diagnostics);
         Ok(())
     }
 
+    #[test_case(Rule::FastApiRedundantResponseModel, Path::new("FAST001.py"))]
+    #[test_case(Rule::FastApiUnusedPathParameter, Path::new("FAST003.py"))]
+    fn deferred_annotations_diff(rule_code: Rule, path: &Path) -> Result<()> {
+        let snapshot = format!(
+            "deferred_annotations_diff_{}_{}",
+            rule_code.name(),
+            path.to_string_lossy()
+        );
+        assert_diagnostics_diff!(
+            snapshot,
+            Path::new("fastapi").join(path).as_path(),
+            &LinterSettings {
+                unresolved_target_version: PythonVersion::PY313.into(),
+                ..LinterSettings::for_rule(rule_code)
+            },
+            &LinterSettings {
+                unresolved_target_version: PythonVersion::PY314.into(),
+                ..LinterSettings::for_rule(rule_code)
+            },
+        );
+        Ok(())
+    }
+
     // FAST002 autofixes use `typing_extensions` on Python 3.8,
     // since `typing.Annotated` was added in Python 3.9
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_0.py"))]
@@ -34,9 +60,9 @@ mod tests {
         let snapshot = format!("{}_{}_py38", rule_code.name(), path.to_string_lossy());
         let diagnostics = test_path(
             Path::new("fastapi").join(path).as_path(),
-            &settings::LinterSettings {
-                unresolved_target_version: ruff_python_ast::PythonVersion::PY38.into(),
-                ..settings::LinterSettings::for_rule(rule_code)
+            &LinterSettings {
+                unresolved_target_version: PythonVersion::PY38.into(),
+                ..LinterSettings::for_rule(rule_code)
             },
         )?;
         assert_diagnostics!(snapshot, diagnostics);