Deserialization fails if set_default is passed an unsigned integer

[kesyog]

Small motivating example that demonstrates the issue:

use config::{Config, Environment};
use serde::Deserialize;
use std::error::Error;

// Using any unsigned type here leads to a runtime deserialization error
// Runs fine when using i8, i16, i32, i64
const DEFAULT_BAR: u32 = 30;

fn main() -> Result<(), Box<dyn Error>> {
    #[derive(Deserialize, Debug)]
    struct Foo {
        bar: u32,
    }
    let config: Foo = Config::builder()
        .add_source(Environment::with_prefix("TEST"))
        .set_default("bar", DEFAULT_BAR)?
        .build()?
        .try_deserialize()?;
    dbg!(config.bar);
    Ok(())
}

Output:

Error: invalid type: unsigned integer 64 bit `30`, expected an signed 64 bit or less integer for key `bar`

While looking into potentially fixing it, I noticed that Value::into_uint is defined but never used. Modifying the deserialize_u32 method to use into_uint instead of into_int fixes this issue, but breaks some existing tests. I'm guessing that's because of the next observation:

Value::into_int and Value::into_uint both seem overly strict. For example, calling Value::into_uint() when the kind is a ValueKind::I64 always fails. Shouldn't we try a conversion using the TryInto trait and only fail if the conversion fails?

Draft PR showing what I'm envisioning: https://github.com/mehcode/config-rs/compare/master...kesyog:config-rs:kyogeswaran/deserialize-uints?expand=1

This looks potentially related to #93.

[matthiasbeyer]

I feel like more and more issues come from the fact that we're deserializing internally and store things in our own data structure.

Thanks for reporting this! 👍

I guess the fix looks alrightish, feel free to open a PR for this and we go from there!