Support emitting spans when deserializing structures

[Michael-F-Bryan]

Similar to toml-rs/toml-rs#236, serde-rs/json#637, and serde-rs/serde#1811, this provides a mechanism for extracting the source location for a particular value. I've mostly copied the toml crate's approach.

For more complex cases like mappings and sequences the spans don't look 100% correct (e.g. it may miss a closing ] or mappings start at : value instead of key: value), but that may just be because of where yaml_rust decides to emit an event or because yaml_rust::scanner::Marker only records where an event starts and not its length.

In the long run I'd like to have a common mechanism/protocol for emitting span information in serde (outlined here), but in the meantime this should give us a good place to experiment.

[Michael-F-Bryan]

I've added some more tests and believe this is feature complete. @dtolnay, would you be able to review this PR?

There's a quirk when trying to get the start location of a map. For example, given this document...

document:
  first: 1
  second: 2

... and this struct definition...

#[derive(Debug, serde_derive::Deserialize)]
struct Document {
    nested: Spanned<Nested>,
}

#[derive(Debug, serde_derive::Deserialize)]
struct Nested {
   first: u32,
   second: u32,
}

... Our Nested item will seem to start at the colon after the first key instead of at the f character as one might expect.

I think this may have something to do with how maps are deserialized and when tokens are consumed, but I don't know how to resolve the issue cleanly. I wrote a doc-test so people are aware of this behaviour and to help detect when it is fixed in the future.

[Michael-F-Bryan]

@dtolnay other than making it compatible with Rust 1.31, is anything else blocking this PR from being merged?

[dccsillag]

This would be great to have!
Hope it gets merged.

[dtolnay]

I'd prefer not to build this into serde_yaml. For now I would recommend using a different crate if you need the spans.

Thanks anyway for the PR!

[dccsillag]

Oh, I see. Thank you for your time :)

But is there even a YAML crate out there which supports this?

[Michael-F-Bryan]

But is there even a YAML crate out there which supports this?

Not that I could find. Your best bet is to fork serde_yaml and implement it yourself.