deser-path

A layer for deser that tracks the path of the current value (for instance servers[1].port) during serialization and deserialization and attaches it to errors.

"invalid type: string, expected u16" is not a useful error message when the input has hundreds of values. Add the PathLayer and errors tell you which value is wrong. It works with every format and does not require any changes to your types:

use deser::Deserialize;
use deser_path::{Path, PathLayer};

#[derive(Debug, Deserialize)]
struct Config {
    servers: Vec<Server>,
}

#[derive(Debug, Deserialize)]
struct Server {
    host: String,
    port: u16,
}

let json = r#"{"servers": [
    {"host": "a.example.com", "port": 80},
    {"host": "b.example.com", "port": "http"}
]}"#;

let err = deser_json::Deserializer::from_str(json)
    .deserialize_with::<Config, _>(|driver| {
        driver.push_layer(PathLayer::new())
    })
    .unwrap_err();
let path = err.attachment::<Path>().unwrap();
assert_eq!(path.to_string(), "servers[1].port");
assert_eq!((err.line(), err.column()), (Some(3), Some(39)));

Unlike wrapping the deserializer (as serde_path_to_error does) the path is also correct for values that had to be buffered, such as the content of internally tagged and untagged enums. During serialization and deserialization the current Path is available in the state, so custom Serialize and Deserialize implementations and other layers can use it too (for instance to redact values at certain paths).