Add additional debug information to errors

[ajrudzitis]

While developing a schema for my format, I repeatedly hit TryMapFailed. The mistake in the schema was mine, however, there was little information in the actual error that was useful for debugging. At the very least, it would be useful to understand the offset where the error occurred. Other useful information would be:

• The type we were attempting to parse
• The value which was parsed versus the allowed values.

[y1ca1]

Sorry about the lack of debugging support we currently have. Providing better error messages is indeed in our roadmap and would be one of the main future directions of Vest.

For now, a quick and dirty way of debugging would be to manually and slightly modify generated code.

For example, in https://github.com/secure-foundations/vest/blob/main/vest-dsl/bitcoin/src/vest_bitcoin.rs#L3098-L3110,

you can change it to something like:

let res = <_ as Combinator<'a, &'a [u8], Vec<u8>>>::parse(&combinator, input);
eprintln!("[trace] {name}: {{:?}}", res);
res