Why is syn panicking instead of returning Expr::Verbatim when parsing custom syntax?

[lionrexhepi]

I have been writing a macro that scans blocks of code to look for identifiers prefixed with @, automatically calling to_owned() on them. It works roughly like this:

fn scan_expr_recursive(expr: &mut Expr) {
   Expr::Verbatim(tokens) => { // parse the identifier and the @ token, modify the expression to call #ident.to_owned() }
  ... => { //recursively scan the expression tree, concatenating the results to get a list of all identifiers used in this way}
}

#[proc_macro]
pub fn scan_expr(input: TokenStream) -> TokenStream {
    let mut to_scan = parse_macro_input!(input as Expr); //This fails
    let idents = scan_expr(&mut to_scan);
   quote!(#to_scan).into()
}

However, the parse! macro panics with "expected expression" as soon as it hits the @ token:

scan_expr!(@borrowed) //panic: expected expression

How can I configure syn to redirect unknown tokens to the Expr::Verbatim variant instead of panicking? Am I misunderstanding the point of the variant? It seems nonsensical to have a variant for un-parseable expressions and still panic when encountering such an expression.

[SkiFire13]

AFAIK verbatim is for syntax that can be parsed, but cannot be represented in syn's types mainly due to backward compatibility.

[lionrexhepi]

Do you know of a way to extend the Parsing logic of syn's primitives or do I have no choice but to re-implement Expr's parse logic and add an additional check for @ tokens?

[SkiFire13]

Unfortunately I don't know a way to extend it. I think you'll have to reimplement it (maybe by copy-pasting syn's code?)