deser-location

Source locations (line and column) for deser. Wrap a field in Spanned<T> and it remembers where in the input it came from. This is what you want for validation that happens after parsing: the file parsed fine, but a value is semantically wrong and you want to point the user at the exact spot, like a compiler would.

use deser::{Context, Deserialize, TrackLocations};
use deser_location::Spanned;

#[derive(Deserialize)]
struct Config {
    name: String,
    workers: Spanned<u32>,
}

let input = "name: web\nworkers: 0\n";
let yaml = deser_yaml::DeserializerConfig::builder()
    .context(Context::with(TrackLocations(true)))
    .build();
let config: Config = yaml.from_str(input).unwrap();

if config.workers.value == 0 {
    let span = config.workers.span.unwrap();
    assert_eq!(span.to_string(), "2:10-2:11");
    // error: workers must be at least 1 (at 2:10)
    println!("error: workers must be at least 1 (at {})", span.start);
}

Location tracking is opt-in with TrackLocations in the context of the deserialization and is supported by all text formats (JSON and its dialects, YAML, TOML, XML, CSV, query strings and property lists). Formats publish the byte range of every event, lines and columns are only computed when a location is requested. Locations survive buffering: Spanned values inside internally tagged or untagged enums still know where they came from.