Integer values are limited by 2^63-1 (9223372036854775807).

Greetings!

Is there is a bug what integers are limited by 2^63-1?

use config::{Config, Environment, File, FileFormat};

fn main() {
    // Ok
    println!(
        "number: `{}`.",
        read_conf().get::<u64>("data.number").unwrap()
    );

    // Ok
    std::env::set_var("PREFIX_data.number", (1u64 << 63 - 1).to_string());
    println!(
        "number: `{}`.",
        read_conf().get::<u64>("data.number").unwrap()
    );

    // Error
    std::env::set_var("PREFIX_data.number", (1u64 << 63).to_string());
    println!(
        "number: `{}`.",
        read_conf().get::<u64>("data.number").unwrap_err()
    );
}

#[inline]
fn read_conf() -> Config {
    Config::builder()
        .add_source(File::from_str("[data]\nnumber = 1024", FileFormat::Toml))
        .add_source(Environment::with_prefix("PREFIX"))
        .build()
        .unwrap()
}

Start with the Config builder and Environment source shown in the reproduction, then trace how the environment value is parsed and converted by get::. Check existing tests for environment sources or integer deserialization. Done means the reproduced u64 boundary case behaves correctly and regression coverage records the expected result.