为什么在为失败类型扩展 Result 时会得到“该方法存在但不满足以下特征边界”？

Question

我正在尝试在我的代码中添加一个更简洁的failurecrate版本.with_context(|e| format!("foo: {}", e))。像这个游乐场：

use failure::{Context, Fail, ResultExt}; // 0.1.5

/// Extension methods for failure `Result`.
pub trait ResultContext<T, E> {
    /// Wraps the error type in a context type generated by looking at the
    /// error value. This is very similar to `with_context` but much more
    /// concise.
    fn ctx(self, s: &str) -> Result<T, Context<String>>;
}

impl<T, E> ResultContext<T, E> for Result<T, E>
where
    E: Fail,
{
    fn ctx(self, s: &str) -> Result<T, Context<String>> {
        self.map_err(|failure| {
            let context = format!("{}: {}", s, failure);
            failure.context(context)
        })
    }
}

pub fn foo() -> Result<i32, failure::Error> {
    Ok(5i32)
}

pub fn main() -> Result<(), failure::Error> {
    // This works.
    let _ = foo().with_context(|_| "foo".to_string())?;
    // This doesn't.
    foo().ctx("foo")?
}

我收到以下错误：

error[E0599]: no method named `ctx` found for type `std::result::Result<i32, failure::error::Error>` in the current scope
  --> src/main.rs:31:11
   |
31 |     foo().ctx("foo")?
   |           ^^^
   |
   = note: the method `ctx` exists but the following trait bounds were not satisfied:
           `std::result::Result<i32, failure::error::Error> : ResultContext<_, _>`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `ctx`, perhaps you need to implement it:
           candidate #1: `ResultContext`

我不明白为什么。我或多或少地复制了现有的with_context代码。

Answer 1

正如编译器告诉您的那样，Result<i32, failure::error::Error>没有实现ResultContext<_, _>. 您已经为您的实现添加了绑定：

where
    E: Fail,

但failure::Error没有实现failure::Fail：

use failure; // 0.1.5

fn is_fail<F: failure::Fail>() {}

pub fn main() {
    is_fail::<failure::Error>();
}

where
    E: Fail,

您将需要更改您的界限或类型。