我在实现自定义错误类型时收到以下错误：

the trait bound `std::io::Error: std::clone::Clone` is not satisfied

下面是我的自定义错误枚举：

use std::fmt;
use std::io;
use crate::memtable::Memtable;

// Define our error types. These may be customized for our error handling cases.
// Now we will be able to write our own errors, defer to an underlying error
// implementation, or do something in between.
#[derive(Debug, Clone)]
pub enum MemtableError {
    Io(io::Error),
    FromUTF8(std::string::FromUtf8Error),
    NotFound,
}

// Generation of an error is completely separate from how it is displayed.
// There's no need to be concerned about cluttering complex logic with the display style.
//
// Note that we don't store any extra info about the errors. This means we can't state
// which string failed to parse without modifying our types to carry that information.
impl fmt::Display for MemtableError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Some error occurred!");
        Ok(())
    }
}

// a test function that returns our error result
fn raises_my_error(memtable: Memtable, key: String) -> Result<(),MemtableError> {
    match memtable.read(key) {
        Ok(v) => Ok(()),
        Err(e) => Err(e),
    }
}

我做错了什么？我试着按照这些例子：

1. https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/define_error_type.html
2. https://learning-rust.github.io/docs/e7.custom_error_types.html
3. https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/wrap_error.html