Rust 在循环中使用 read_line 时的奇怪行为

我在Rust中的第一个程序应该以C或F字符的形式从用户那里获取输入:

use std::io;

fn main() {
    let mut srcunit = String::new();

    let mut switch = true;
    while switch {
        println!("source unit? F or C?");
        io::stdin().read_line(&mut srcunit).expect(
            "failed to read src unit",
        );

        if srcunit.trim() == "F" || srcunit.trim() == "C" {
            println!("doing things right with {}", srcunit);
            switch = false;
        } else {
            println!("either F or C, not {}", srcunit);
        }
    }

    println!("you pressed {}", srcunit);
}

当我启动程序并按下F或C时，它工作正常，所以我跳过这里.当我按下另一个角色时，奇怪的部分出现了.我希望我的程序再次请求F或C，直到我按下其中一个字符.当我这样做时，它应该离开while循环，告诉我按下了什么.

source unit? F or C?
G
either F or C, not G //so far so good

source unit? F or C?
F
either F or C, not G //why is F not assigned to the srcunit variable? It's supposed to leave the loop now.
F                    //why does it print this line? I didn't press a key or have a println function/macro that does this

source unit? F or C?
V
either F or C, not G //it's still G, wtf
F                    //again, why are those two lines printed? Where does it store, that I pressed F previously?
V