Function serde_with_expand_env::with_expand_envs

pub fn with_expand_envs<'de, T, D>(deserializer: D) -> Result<T, D::Error> where
    D: Deserializer<'de>,
    T: FromStr + Deserialize<'de>,
    <T as FromStr>::Err: Display, 

Deserializes a field expanding all the environment variables before the conversion

Example:

use std::env;
use serde_json;
use serde::Deserialize;
use serde_with_expand_env::with_expand_envs;

#[derive(Deserialize, Debug)]
struct Test {
    #[serde(deserialize_with="with_expand_envs")]
    number: usize,
    #[serde(deserialize_with="with_expand_envs")]
    string: String,
    #[serde(deserialize_with="with_expand_envs")]
    default: usize,
}

let serialized = r#"{"number": "$NUMBER",
                     "string": "my string: $STRING",
                     "default": "${DEFAULT:-42}"
                  }"#;

// No envs set will fail with enviroment variable not found
assert_eq!(serde_json::from_str::<Test>(&serialized).is_err(), true);

env::set_var("NUMBER", "42");
env::set_var("STRING", "hacker");
let deserialized: Test = serde_json::from_str(&serialized).unwrap();

assert_eq!(deserialized.number, 42);
assert_eq!(deserialized.string, "my string: hacker");
assert_eq!(deserialized.default, 42);

env::set_var("DEFAULT", "4200");
let deserialized: Test = serde_json::from_str(&serialized).unwrap();
assert_eq!(deserialized.default, 4200);

// Invalid number
env::set_var("NUMBER", "cuarentaydos");
env::set_var("STRING", "42");

assert_eq!(serde_json::from_str::<Test>(&serialized).is_err(), true);