Reading HashMap from empty json fails with type error.

[dil-zgaal]

version: 0.15.4

Config fails to initialize (an empty) HashMap from an empty object.

Repro:

#[derive(Clone, Debug, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct Config {
    pub data: HashMap<String, String>,
}
...
 let cfg = config::Config::builder()
            .add_source(config::File::from_str(
                //r#"{"data": {"a": "b"}}"#,  //  it works fine
                r#"{"data": {}}"#,                // it fails
                config::FileFormat::Json,
            ))
            .build()
            .unwrap();
        let cfg = cfg.try_deserialize::<Config>();
        log::info!("configuration: {:#?}", cfg);

Expected behavior: data is initialized to an empty HashMap.
Actual result: Err( invalid type: unit value, expected a map )

[arielb1]

This seems to happen every time a value is initialized from an empty HashMap, including as a set_default

use config::Config;
use std::collections::HashMap;

fn main() {
    let config = Config::builder().set_default("var", HashMap::<String, i64>::new()).unwrap().build().unwrap();
    // returns empty HashMap on 0.14.*/0.15.2, Errs on "invalid type: unit value, expected a map" on 0.15.3/0.15.4
    let val : HashMap<String, i64> = config.get("var").unwrap();
    println!("Hello, world! {:?}", val);
}

[Holzhaus]

I'm stumbling over this as well. Also happens with structs where all fields are Option.