Cursor state associated with speculative parsing.

This type is the input of the closure provided to ParseStream::step.

Example

use proc_macro2::TokenTree;
use syn::Result;
use syn::parse::ParseStream;

// This function advances the stream past the next occurrence of `@`. If
// no `@` is present in the stream, the stream position is unchanged and
// an error is returned.
fn skip_past_next_at(input: ParseStream) -> Result<()> {
    input.step(|cursor| {
        let mut rest = *cursor;
        while let Some((tt, next)) = rest.token_tree() {
            match &tt {
                TokenTree::Punct(punct) if punct.as_char() == '@' => {
                    return Ok(((), next));
                }
                _ => rest = next,
            }
        }
        Err(cursor.error("no `@` was found after this point"))
    })
}
#
# fn remainder_after_skipping_past_next_at(
#     input: ParseStream,
# ) -> Result<proc_macro2::TokenStream> {
#     skip_past_next_at(input)?;
#     input.parse()
# }
#
# use syn::parse::Parser;
# let remainder = remainder_after_skipping_past_next_at
#     .parse_str("a @ b c")
#     .unwrap();
# assert_eq!(remainder.to_string(), "b c");