Force cursor to advance in parse_expr calls

src/expr.rs

@@ -1390,17 +1390,26 @@ pub(crate) mod parsing {
         loop {
             let next = peek_precedence(input);
             if next > precedence || next == precedence && precedence == Precedence::Assign {
+                let cursor = input.cursor();
                 rhs = parse_expr(
                     input,
                     rhs,
                     #[cfg(feature = "full")]
                     allow_struct,
                     next,
                 )?;
+                if cursor == input.cursor() {
+                    // Bespoke grammar restrictions separate from precedence can
+                    // cause parsing to not advance, such as `..a` being
+                    // disallowed in the left-hand side of binary operators,
+                    // even ones that have lower precedence than `..`.
+                    break;
+                }
             } else {
-                return Ok(Box::new(rhs));
+                break;
             }
         }
+        Ok(Box::new(rhs))
     }
 
     fn peek_precedence(input: ParseStream) -> Precedence {

tests/test_expr.rs

@@ -646,9 +646,17 @@ 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 _ = syn::parse_str::<Expr>("a = a .. a ..");
+    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());
+
+    let err = syn::parse_str::<Expr>("a .. a .. a").unwrap_err();
+    assert_eq!("unexpected token", err.to_string());
+
+    let err = syn::parse_str::<Expr>("a .. a += a").unwrap_err();
+    assert_eq!("unexpected token", err.to_string());
 }
 
 #[test]