Merge pull request #1741 from dtolnay/binoploop

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]