add regression test

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP045_py39.py

@@ -0,0 +1,12 @@
+"""
+Regression test for https://github.com/astral-sh/ruff/issues/20096
+"""
+
+from __future__ import annotations
+
+from typing import Optional, cast
+
+
+x: Optional[str]             # UP045
+x: "Optional[str]"           # UP045
+cast("Optional[str]", None)  # UP045

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

@@ -452,4 +452,17 @@ mod tests {
         assert_diagnostics!(snapshot, diagnostics);
         Ok(())
     }
+
+    #[test]
+    fn up045_future_annotations_py39() -> Result<()> {
+        let diagnostics = test_path(
+            Path::new("pyupgrade/UP045_py39.py"),
+            &settings::LinterSettings {
+                unresolved_target_version: PythonVersion::PY39.into(),
+                ..settings::LinterSettings::for_rule(Rule::NonPEP604AnnotationOptional)
+            },
+        )?;
+        assert_diagnostics!(diagnostics);
+        Ok(())
+    }
 }

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__up045_future_annotations_py39.snap

@@ -0,0 +1,37 @@
+---
+source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
+---
+UP045 [*] Use `X | None` for type annotations
+  --> UP045_py39.py:10:4
+   |
+10 | x: Optional[str]             # UP045
+   |    ^^^^^^^^^^^^^
+11 | x: "Optional[str]"           # UP045
+12 | cast("Optional[str]", None)  # UP045
+   |
+help: Convert to `X | None`
+7  | from typing import Optional, cast
+8  | 
+9  | 
+   - x: Optional[str]             # UP045
+10 + x: str | None             # UP045
+11 | x: "Optional[str]"           # UP045
+12 | cast("Optional[str]", None)  # UP045
+note: This is an unsafe fix and may change runtime behavior
+
+UP045 [*] Use `X | None` for type annotations
+  --> UP045_py39.py:11:5
+   |
+10 | x: Optional[str]             # UP045
+11 | x: "Optional[str]"           # UP045
+   |     ^^^^^^^^^^^^^
+12 | cast("Optional[str]", None)  # UP045
+   |
+help: Convert to `X | None`
+8  | 
+9  | 
+10 | x: Optional[str]             # UP045
+   - x: "Optional[str]"           # UP045
+11 + x: "str | None"           # UP045
+12 | cast("Optional[str]", None)  # UP045
+note: This is an unsafe fix and may change runtime behavior