Preserve Toml File map Order

[Ziqi-Yang]

I want this library can preserve map order for toml configuration file. See this simple script (originally taken from toml-rs/toml#520):

use indexmap::IndexMap;
use serde::Deserialize;
use figment::{Figment, providers::{Format, Toml}};

#[allow(dead_code)]
#[derive(Deserialize, Debug, Clone)]
pub struct Cfg {
    pub(crate) id: f32,
    pub(crate) accounts: IndexMap<String, Account>,
}

#[allow(dead_code)]
#[derive(Deserialize, Debug, Clone)]
pub struct Account {
    pub(crate) id: i32,
    pub(crate) name: String,
}

fn main() {
    let toml = r#"

id=5

[accounts.c]
id=123
name="3a22e"

[accounts.a]
id=789
name="qawe"

[accounts.d]
id=456
name="3css"

[accounts.b]
id=1021
name="cfrt"

"#;

    // doens't preserve order
    let figment = Figment::new()
        .merge(Toml::string(toml));
    let cfg: Cfg = figment.extract().unwrap();
    
    // do preserve order, since we enable `indexmap` feature and use `IndexMap`
    // let cfg: Cfg = toml::from_str(&toml).unwrap();
    
    for (id, name) in cfg.accounts.iter() {
        println!("({}, {:?})", id, name);
    }
}

and Cargo.toml:

figment = { git = "https://github.com/SergioBenitez/Figment", rev="e8f1f13", features = ["env", "test", "toml"] }
toml = { version = "0.8.8", features = ["indexmap"] }

I wonder it there a simple way to enable indexmap or we need to impl a new Toml provider to use the indexmap feature?

[SergioBenitez]

See my comments in #90 (comment).

[mhovd]

We have a similar problem, in that TOML entry order is not preserved (or even consistent!). For example

[ranges]
a = [0, 10]
b = [20, 30]

We depend on the order, so between runs, settings.ranges.first() will sometimes give a, sometimes give b.
Is there a way to preserve entry order with Figment?

[SergioBenitez]

@mhovd Please see the comment above and feel free to submit a PR!