[WIP] fix #20 improve test support for no_std

[mattsse]

This PR is part of the work to fix broken tests in no_std. (#20)

So far ea5a088 includes changes to make eyre compile successfully when --no-default-feature is used.
Changes made:

• replaced std::fmt::Display with score::fmt::Display
• unified the two Chain structs into single Chain both for std and no_std. This was necessary because the crate::chain::Chain was only pub(crate) for no_std.
• added some docs and removed unused to make clippy happy.

To fix all the tests for no_std is a lot more work, since all the tests using io::Error won't work.
To add test support no_std, I could start with replacing the io::Error dependent variants in the TestError with something that supports no_std? This could minimize test duplication for no_std

Before I proceed I'd like to get some feedback and I'm happy to hear suggestions.

[yaahc]

Oh my god when you said score::fmt::Debug I was terrified, loool, i thought you were pulling in some dependency for a new fmt replacement, but this is much better. I've only looked through half of this so far but initial impressions are good, ty for taking this on! And I know the io::Error stuff is a pain in the ass, I tried this once before and eventually just reverted it rather than pushing it to completion.

iirc in no_std eyre doesn't have any way to be constructed from an existing error, it can only be used to construct adhoc error types either via the macros or via wrap_err, so I'm not sure if the TestError approach will work but if you see something that I missed then feel free to go for it. If that doesn't work out I think its okay to just use cfg to disable all of the non-applicable tests in no_std.

I'll go ahead and finish the review now and leave comments, just had to stop for a second to laugh about score vs core.

[yaahc]

Also someone else opened a PR to do the std -> core change 6 days ago and I missed it / forgot about it, so I'm gonna merge that PR right after it passes tests, so just a heads up you'll need to rebase, I know it's a minor inconvenience but I feel like just closing their PR would be a little rude 😬

[yaahc]

I think just remove this rather than commenting it out, and the one below if you can.

[yaahc]

I don't think we can make this change because this would mean that the Item type of Chain changes when we use no_std mode. This would cause disabling / enabling features to become a breaking change which is a no-go. Did you need the chain interface on no_std? If so we can definitely work something out.

[mattsse]

I added the cfg here because the only usage of the ErrorImpl::chain function is here: https://github.com/yaahc/eyre/blob/a197feb4e3368f70735a2fe34990fbebb10487cd/src/error.rs#L305-L307 and is marked as std only. In addition ErrorImpl::chain is pub crate making it dead code. It compiles without #[cfg(feature = "std")], but clippy rightfully complaints about dead code without this change:

❯ cargo check --no-default-features                                                                                                                                                                         ⇡!? master 
   Compiling eyre v0.4.2 (/Users/Matthias/git/rust/eyre)
warning: associated function is never used: `chain`
   --> src/error.rs:732:19
    |
732 |     pub(crate) fn chain(&self) -> Chain<'_> {
    |                   ^^^^^
    |

[mattsse]

:) sorry I scared you with that typo. Unfortunately score is already taken, I checked...

I did more research on how to make cargo test --no-default-features succeed for the doctests first.

There seems to be two options:

1. the experimental doc_cfg, which would require adding !#[feature(doc_cfg)] to lib.rs
2. wrapping the failing doc tests in #[cfg_attr(feature = "std", doc = r##...##")]

I tried 2., there are atm 11 doc tests that fail. Wrapping them in #[cfg_attr(feature = "std", doc = r##...##")] works fine but is kinda weird. So far I fixed only a couple and would like to know what you think about that before I proceed with this cumbersome task -.-.

For https://github.com/yaahc/eyre/blob/a197feb4e3368f70735a2fe34990fbebb10487cd/src/chain.rs#L21-L40
It would look like:

    /// Construct an iterator over a chain of errors via the `source` method
    ///
    #[cfg_attr(feature = "std", doc = r##"
     # Example

     ```rust
     use std::error::Error;
     use std::fmt::{self, Write};
     use eyre::Chain;
     use indenter::indented;

     fn report(error: &(dyn Error + 'static), f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut errors = Chain::new(error).enumerate();
         for (i, error) in errors {
             writeln!(f)?;
             write!(indented(f).ind(i), "{}", error)?;
         }

         Ok(())
     }
     ```
    "##)]

[yaahc]

I'd prefer option 1, there are ways around the experimental aspect I think, I've done this same thing in tracing and already opened an issue yesterday to use doc_cfg in eyre.

[yaahc]

hey @mattsse, sorry for ignoring this PR for so long, and thank you again for the effort you put into it. I just realized that this PR is no longer something we can merge because I removed support for no_std a while ago (and apparently forgot??). I'm really sorry about the confusion.

[mattsse]

no worries @yaahc, I never quite finished it and then also kinda forgot about it  🙈
Thanks for closing!