通用 TryInto 实现中的格式错误

Question

每当

TryFrom

实现

T

时，我都会尝试为对象

T

实现

TryFrom

的通用实现。然而，我正在努力通过

anyhow::Error

s。

在下面的代码中，我可以将

Generic<T>

转换为

ValueContainer

，只要

&usize: TryFrom<T>

即可。它几乎做了我想要的，但不允许我正确处理来自

&usize: TryFrom<T>

的错误。

#[cfg(test)]
mod question {
    use std::fmt::Debug;

    use anyhow::anyhow;

    #[derive(Debug)]
    struct Generic<T: Debug>(T);
    #[derive(Debug)]
    struct Foo(usize);
    #[derive(Debug)]
    struct ValueContainer<'a>(&'a usize);

    impl TryFrom<Foo> for &usize {
        type Error = anyhow::Error;

        fn try_from(value: Foo) -> Result<Self, Self::Error> {
            if value.0 > 5 {
                Ok(&5)
            } else {
                Err(anyhow!("Value must be bigger"))
            }
        }
    }

    impl<'a, T> TryFrom<Generic<T>> for ValueContainer<'a>
    where
        for<'b> &'b usize: TryFrom<T>,
        T: Debug,
        for<'b> anyhow::Error: From<<&'b usize as TryFrom<T>>::Error>,
    {
        type Error = anyhow::Error;

        fn try_from(value: Generic<T>) -> Result<Self, Self::Error> {
            // XXX: Can't do this?
            // let value: &usize = value.0.try_into().unwrap();
            match <&usize as TryFrom<T>>::try_from(value.0) {
                Ok(v) => Ok(ValueContainer(v)),
                Err(_) => {
                    // XXX: I would like this to be the error from the try_into above
                    // but there is an error `<&usize as TryFrom<T>>::Error` cannot be formatted with the default formatter`
                    return Err(anyhow!("Error converting type"));
                }
            }
        }
    }

    #[test]
    fn test_try_into() {
        let value = Generic(Foo(10));
        let value: Result<ValueContainer, anyhow::Error> = value.try_into();
        dbg!(&value);
        let value = Generic(Foo(2));
        let value: Result<ValueContainer, anyhow::Error> = value.try_into();
        dbg!(&value);
    }
}

我无法在

value.try_into()?

的实现中使用

TryFrom<Generic<T>>

。取消注释

let value: &usize = value.try_into()?;

会产生错误

error[E0271]: type mismatch resolving `<&usize as TryFrom<Generic<T>>>::Error == <&usize as TryFrom<T>>::Error`
   --> lib-doenetml-core/src/core/props/prop_view.rs:140:40
    |
140 |              let value: &usize = value.try_into()?;
    |                                        ^^^^^^^^ expected `Infallible`, found associated type
    |
    = note:         expected enum `Infallible`
            found associated type `<&usize as TryFrom<T>>::Error`
    = help: consider constraining the associated type `<&usize as TryFrom<T>>::Error` to `Infallible`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

看起来 使用 TryInto 泛型无论如何::Error可能是相关的，它启发我添加

for<'b> anyhow::Error: From<<&'b usize as TryFrom<T>>::Error>

，但这似乎不起作用......

关于如何直接允许

value.try_into()?

有什么想法吗？

Answer 1

如果您修正拼写错误并添加

?

，

.0

将会起作用。

unwrap()

需要错误的

Debug

特征（因为它会在失败时打印它），因此您需要添加此绑定：

impl<'a, T> TryFrom<Generic<T>> for ValueContainer<'a>
where
    for<'b> &'b usize: TryFrom<T>,
    T: Debug,
    for<'b> anyhow::Error: From<<&'b usize as TryFrom<T>>::Error>,
    for<'b> <&'b usize as TryFrom<T>>::Error: Debug,
{
    type Error = anyhow::Error;

    fn try_from(value: Generic<T>) -> Result<Self, Self::Error> {
        // XXX: Can't do this?
        let value: &usize = value.0.try_into().unwrap();
        Ok(ValueContainer(value))
    }
}