[red-knot] SemanticIndexBuilder visits value before target in named expressions (#13053)

The `SemanticIndexBuilder` was causing a cycle in a salsa query by
attempting to resolve the target before the value in a named expression
(e.g. `x := x+1`). This PR swaps the order, avoiding a panic.

Closes #13012.

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -658,11 +658,11 @@ where
             }
             ast::Expr::Named(node) => {
                 debug_assert!(self.current_assignment.is_none());
-                self.current_assignment = Some(node.into());
                 // TODO walrus in comprehensions is implicitly nonlocal
+                self.visit_expr(&node.value);
+                self.current_assignment = Some(node.into());
                 self.visit_expr(&node.target);
                 self.current_assignment = None;
-                self.visit_expr(&node.value);
             }
             ast::Expr::Lambda(lambda) => {
                 if let Some(parameters) = &lambda.parameters {

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2290,6 +2290,23 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn walrus_self_plus_one() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            x = 0
+            (x := x + 1)
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "x", "Literal[1]");
+
+        Ok(())
+    }
+
     #[test]
     fn ifexpr() -> anyhow::Result<()> {
         let mut db = setup_db();