Support inference for PEP 604 union annotations (#13964)

## Summary

Supports return type inference for, e.g., `def f() -> int | None:`.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/red_knot_python_semantic/resources/mdtest/assignment/annotations.md

@@ -23,12 +23,21 @@ x: int
 x = "foo"  # error: [invalid-assignment] "Object of type `Literal["foo"]` is not assignable to `int`"
 ```
 
-## PEP-604 annotations not yet supported
+## PEP-604 annotations are supported
 
 ```py
-def f() -> str | None:
+def foo() -> str | int | None:
     return None
 
-# TODO: should be `str | None` (but Todo is better than `Unknown`)
+reveal_type(foo())  # revealed: str | int | None
-reveal_type(f())  # revealed: @Todo
+
+def bar() -> str | str | None:
+    return None
+
+reveal_type(bar())  # revealed: str | None
+
+def baz() -> str | str:
+    return "Hello, world!"
+
+reveal_type(baz())  # revealed: str
 ```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3494,14 +3494,20 @@ impl<'db> TypeInferenceBuilder<'db> {
                 Type::Todo
             }
 
-            // TODO PEP-604 unions
             ast::Expr::BinOp(binary) => {
-                self.infer_binary_expression(binary);
+                #[allow(clippy::single_match_else)]
                 match binary.op {
-                    // PEP-604 unions are okay
+                    // PEP-604 unions are okay, e.g., `int | str`
-                    ast::Operator::BitOr => Type::Todo,
+                    ast::Operator::BitOr => {
+                        let left_ty = self.infer_type_expression(&binary.left);
+                        let right_ty = self.infer_type_expression(&binary.right);
+                        UnionType::from_elements(self.db, [left_ty, right_ty])
+                    }
                     // anything else is an invalid annotation:
-                    _ => Type::Unknown,
+                    _ => {
+                        self.infer_binary_expression(binary);
+                        Type::Unknown
+                    }
                 }
             }
 

crates/ruff_benchmark/benches/red_knot.rs

@@ -32,11 +32,16 @@ static EXPECTED_DIAGNOSTICS: &[&str] = &[
     "/src/tomllib/_parser.py:98:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:101:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:104:14: Name `char` used when possibly not defined",
+    "/src/tomllib/_parser.py:108:17: Conflicting declared types for `second_char`: Unknown, str | None",
     "/src/tomllib/_parser.py:115:14: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:126:12: Name `char` used when possibly not defined",
+    "/src/tomllib/_parser.py:267:9: Conflicting declared types for `char`: Unknown, str | None",
     "/src/tomllib/_parser.py:348:20: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:353:5: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:353:5: Method `__getitem__` of type `Unbound | @Todo` is not callable on object of type `Unbound | @Todo`",
+    "/src/tomllib/_parser.py:364:9: Conflicting declared types for `char`: Unknown, str | None",
+    "/src/tomllib/_parser.py:381:13: Conflicting declared types for `char`: Unknown, str | None",
+    "/src/tomllib/_parser.py:395:9: Conflicting declared types for `char`: Unknown, str | None",
     "/src/tomllib/_parser.py:453:24: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:455:9: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:455:9: Method `__getitem__` of type `Unbound | @Todo` is not callable on object of type `Unbound | @Todo`",
@@ -45,7 +50,8 @@ static EXPECTED_DIAGNOSTICS: &[&str] = &[
     "/src/tomllib/_parser.py:573:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:579:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:580:63: Name `char` used when possibly not defined",
-    "/src/tomllib/_parser.py:629:38: Name `datetime_obj` used when possibly not defined"
+    "/src/tomllib/_parser.py:590:9: Conflicting declared types for `char`: Unknown, str | None",
+    "/src/tomllib/_parser.py:629:38: Name `datetime_obj` used when possibly not defined",
 ];
 
 fn get_test_file(name: &str) -> TestFile {