map_err()闭包参数的不太详细的类型？

use std::fs::File;
use std::io::Read;

#[derive(Debug)]
enum CustomError {
    FileNotFound,
    IOError(std::io::Error),
}

impl std::convert::From<std::io::Error> for CustomError {
    fn from(error: std::io::Error) -> Self {
        CustomError::IOError(error)
    }
}

fn read_file() -> Result<String, CustomError> {
    let mut file = File::open("example.txt").map_err(|_| CustomError::FileNotFound)?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}

fn main() {
    match read_file() {
        Ok(contents) => println!("File contents: {}", contents),
        Err(error) => println!("Error: {:?}", error),
    }
}