Fix infinite loop on chained comparison

Author: dtolnay

src/expr.rs

@@ -1284,7 +1284,7 @@ pub(crate) mod parsing {
                 if precedence == Precedence::Compare {
                     if let Expr::Binary(lhs) = &lhs {
                         if Precedence::of_binop(&lhs.op) == Precedence::Compare {
-                            break;
+                            return Err(input.error("comparison operators cannot be chained"));
                         }
                     }
                 }
@@ -1346,7 +1346,7 @@ pub(crate) mod parsing {
                 if precedence == Precedence::Compare {
                     if let Expr::Binary(lhs) = &lhs {
                         if Precedence::of_binop(&lhs.op) == Precedence::Compare {
-                            break;
+                            return Err(input.error("comparison operators cannot be chained"));
                         }
                     }
                 }

tests/test_expr.rs

@@ -646,6 +646,9 @@ fn test_assign_range_precedence() {
 fn test_chained_comparison() {
     // https://github.com/dtolnay/syn/issues/1738
     let _ = syn::parse_str::<Expr>("a = a < a <");
+
+    let err = syn::parse_str::<Expr>("a < a < a").unwrap_err();
+    assert_eq!("comparison operators cannot be chained", err.to_string());
 }
 
 #[test]