Feature Request: Span in custom error

[Sniper10754]

When using a custom error type, is there a way to have a span provided with the error? what i mean is that a custom error type needs to satisfy the Default bound, and because of this logos has no way to provide a span describing where the error happened, something like this would be nice

enum Error {
    UnknownToken {
        span: Range<usize>
    }
}

impl CustomError for Error {
    fn new_custom_error(span: Range<usize> /* Other info if needed */) -> Self {
         Self { span }
    }
}

This would greatly improve diagnostics in my language since i have no way to tell the user of the language where the error happened.

[jeertmans]

Hello! I see why this feature might be interesting, but that would need quite some work, and I don't really have time to work on that at the moment :/

If someone wants to give it a try, please do!

A few suggestions:

• look at the code that parses #[logos(error = T)] macros;
• add an option to pass an immutable reference to the lexer, e.g., #[logos(pass_lexer_to_error)];
• requires that error type T implements from (E, &Lexer) instead of from E for all error types E.

In the meantime, you can simply get span information with lexer.spanned() or with different iterator style:

while let Some(token) = lexer.next() {
   /* you can use `lexer` to get current span and other information. */
}