Add ability to get spans of deserialized values

[Trolldemorted]

In one of my projects I have to parse and strictly validate XML input coming from other programs and humans, and the latter (understandably) insist on getting decent error messages (e.g. "at 'position' attribute X references an object called 'Y' which is not defined anywhere", "unexpected child Z at 'position', allowed child items are A,B and C").

As of now I use roxmltree and codespan-reporting, but I'd love to switch to an xml library with derivation support - you can imagine a lot of code could be substituted with just using quick-xml.

To satisfy the "nice error messages" requirement I need the precise locations of everything, though. I have scrolled through the deserialization docs, but could not find any mention of access to positions when using deserialization. Did I miss anything or is this not possible right now? Could you expose the offset in the input through e.g. generated getters?

[Mingun]

Currently this is not implemented and it is already tracked in #313. Contributions are welcomed!

[Trolldemorted]

#313 is about lines of code for deserialization errors, this is about lines of code for successful deserializations - even if quick-xml successfully parses e.g. a string attribute, I must be able to raise an error if the string is unexpected

[Mingun]

That's generally impossible without changes in serde, because you cannot get any meta-information from the deserializer generically. Actually, I tried to implement this but that required changes in serde and I outlined the solution here: serde-rs/serde#2420 (comment).

I do not see any good solutions here. If you could implement one, make a PR.

[Trolldemorted]

I have found https://github.com/toml-rs/toml/blob/main/crates/serde_spanned/src/spanned.rs but I don't know enough about the serde ecosystem to give you a concise summary how it works, or whether their approach is feasible for quick-xml.

Bugs with certain scenarios do appear to exist though.