Error trait

标准库里定义了 Error trait, 用来表示错误值, 可以用作 Result<T, E> 里的 E.

它的接口如下:

pub trait Error: Debug + Display {
    fn source(&self) -> Option<&(dyn Error + 'static)> { ... }
    fn description(&self) -> &str { ... }
    fn cause(&self) -> Option<&dyn Error> { ... }
    fn provide<'a>(&'a self, request: &mut Request<'a>) { ... }
}

可以看到, 它默认就实现了4个方法, 但要求实现 Debug 和 Display traits. 我们自定义错误类型时, 也应该实现这两个 traits.

接下来看标准库中的一个例子.

ParseIntError

当尝试把字符串转换成整数时, 标准库使用 ParseIntError来记录错误原因.

use std::error::Error;
use std::fmt;

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ParseIntError {
    pub kind: IntErrorKind,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum IntErrorKind {
    Empty,
    InvalidDigit,
    PosOverflow,
    NegOverflow,
    Zero,
}

impl Error for ParseIntError {
    fn description(&self) -> &str {
        match self.kind {
            IntErrorKind::Empty => "cannot parse integer from empty string",
            IntErrorKind::InvalidDigit => "invalid digit found in string",
            IntErrorKind::PosOverflow => "number too large to fit in target type",
            IntErrorKind::NegOverflow => "number too small to fit in target type",
            IntErrorKind::Zero => "number would be zero for non-zero type",
        }
    }
}

impl fmt::Display for ParseIntError {
    #[allow(deprecated)]
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.description().fmt(f)
    }
}