Rust 关于 map 闭合求和的问题

我正在研究AOC以学习Rust，并一直在努力根据我看到的更有经验的Rust程序员编写的内容拼凑出一个解决方案. 除了.sum()有问题外，下面的代码似乎完全可以编译.编译器指出，就在求和运算之前，Iterator::Item更改为Result<usize, _>.

不过，我不知道如何才能不使用get_priority中的结果类型.就我所知，sum可以在迭代器上工作，但它不会将结果类型视为迭代器？

fn main() {
    
    let data = fs::read_to_string("data/data.txt").expect("File should exist");
    let lines = data.lines(); // this is an iterator of lines (type &str)

    let result: usize = lines.map(|rucksack| {
        let (cmprt1, cmprt2) = rucksack.split_at(rucksack.len() / 2);
        let cmprt1: ByteSet = cmprt1.into();
        let cmprt2: ByteSet = cmprt2.into();
        let dup = cmprt1.intersection(cmprt2).first().ok_or("Error finding intersection")?;
        Ok(get_priorty(dup)?)
    }).sum();    
    println!("{}", result)
}

fn get_priorty(letter: u8) -> Result<usize, Box<dyn Error>>{
    match letter {
        b'a'..=b'z' => Ok(1 - (letter + b'a') as usize),
        b'A'..=b'Z' => Ok(27 - (letter + b'A') as usize),
        _ => Err("Unexpected character in rucksack".into())
    }
}

错误消息

10   |       let result: usize = lines.map(|rucksack| {
     |  _______________________________^
11   | |         let (cmprt1, cmprt2) = rucksack.split_at(rucksack.len() / 2);
12   | |         let cmprt1: ByteSet = cmprt1.into();
13   | |         let cmprt2: ByteSet = cmprt2.into();
14   | |         let dup = cmprt1.intersection(cmprt2).first().ok_or("Error finding intersection")?;
15   | |         Ok(get_priorty(dup)?)
16   | |     }).sum();    
     | |______^ `Iterator::Item` changed to `Result<usize, _>` here
note: required by a bound in `std::iter::Iterator::sum`