実装メモ
use std::fs::File;
use rusoto_s3::{S3Client, S3, GetObjectRequest, HeadObjectRequest};
use std::io::Write;
use futures::stream::Stream;
use futures::future::Future;

fn main() -> std::io::Result<()> {

    // std::env::set_var("AWS_ACCESS_KEY_ID", "YOUR AWS ACCESS KEY ID HERE");
    // std::env::set_var("AWS_SECRET_ACCESS_KEY", "YOUR AWS SECRET ACCESS KEY HERE");

    let bucket_name = String::from("your-bucket-name");
    let s3_object_key = String::from("test.json");
    let region = "us-east-1".parse().unwrap();
    let client = S3Client::new(region);

    let mut request = HeadObjectRequest::default();
    request.bucket = bucket_name.clone();
    request.key = s3_object_key.clone();
    let result = client.head_object(request).sync().unwrap();


    let from = 0;
    // you can write the following request inside foreach by specified buffer capacity.
    let to = result.content_length.unwrap();
    let mut request = GetObjectRequest::default();
    request.bucket = bucket_name.clone();
    request.key = s3_object_key.clone();
    let range = format!("bytes={}-{}", from, to);
    request.range = Some(range);
    let result = client.get_object(request).sync().unwrap();
    let body = result.body.unwrap().concat2().wait().unwrap();

    let mut file = File::create("test.json")?;
    file.write_all(&body)?;

    Ok(())
}