I cannot build main.rs that includes `Option<R: BufRead>`. orz

main.rs

use std::io::{self, Empty, BufRead, Write, Cursor};

fn read_input<R, W>(source: Option<R>, mut writer: W) -> String
where R: BufRead, W: Write
{
    write!(&mut writer, "input: ").unwrap();
    writer.flush().unwrap();

    let mut input = String::new();
    match source {
        Some(mut reader) => {
            reader.read_line(&mut input).unwrap();
        },
        None => {
            io::stdin().read_line(&mut input).unwrap();
        }
    }
    input.trim().to_string()
}

fn foo<R, W>(source: Option<R>, mut writer: W)
where R: BufRead, W: Write
{
    let s1 = read_input(source, &mut writer);
    let s2 = read_input(source, &mut writer);
    println!("{}", s1);
    println!("{}", s2);
}

fn main() {
    let s1 = read_input(None::<Empty>, io::stdout());
    let s2 = read_input(None::<Empty>, io::stdout());
    let s3 = read_input(Some(Cursor::new(b"foo")), io::stdout());
    println!("\n");
    println!("{}", s1);
    println!("{}", s2);
    println!("{}", s3);
    println!("----");

    foo(None::<Empty>, io::stdout());
    foo(Some(Cursor::new(b"bar")), io::stdout());
}
//   --> src/main.rs:25:25
//    |
// 21 | fn foo<R, W>(source: Option<R>, mut writer: W)
//    |              ------ move occurs because `source` has type `std::option::Option<R>`, which does not implement the `Copy` trait
// ...
// 24 |     let s1 = read_input(source, &mut writer);
//    |                         ------ value moved here
// 25 |     let s2 = read_input(source, &mut writer);
//    |                         ^^^^^^ value used here after move
//
// error: aborting due to previous error