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::Deserialize;
use deser_location::Spanned;

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

let input = "name: web\nworkers: 0\n";
let config: Config = deser_yaml::DeserializerConfig::new()
    .track_locations(true)
    .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 per deserializer (track_locations) and is supported by deser-json, deser-yaml and deser-toml. 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.