Add support for custom equality functions in Eq/PartialEq

[fredszaq]

Synopsis

This adds the possibility to use custom functions to perform the equality check in PartialEq/Eq derives. This is quite useful when wrapping types without a PartialEq/Eq impl

Solution

This builds on #532 and reuse the FieldAttributes for Hash, I moved the struct to utils::attr and renamed it WithOrSkip.

I removed fields types that have a with attribute from the automatically added where Type: Eq bound automatically generated by the Eq derive. This seems like the thing to do as if you're using a custom function it most probably means the field doesn't implement Eq.

While this is not part of the roadmap for PartialEq, defined in #163 this is something that will be quite useful for people migrating form derivative as this was something supported by this crate

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[fredszaq]

@tyranron rebased and removed the draft status on this one as #532 is now merged. Feel free to review when you have time :)

[tyranron]

@fredszaq I don't really like the idea of having #[eq(with(...))] attribute. For skip it has sense, since skipping for Eq and PartialEq is essentially the same thing. with(...), however, has only sense for the PartialEq. Of course, derive(Eq) should account for the #[partial_eq(with(...))] presence. But having the #[eq(with(...))] attribute confuses me a lot, implying that Eq trait has a method, which is not the case.

[fredszaq]

Fixed. I also added an error if you have #[derive(Hash)] with partial_eq(with) and no hash(skip/with)

[tyranron]

@fredszaq let's stick to the same terminology everywhere and just call it "custom function".

[fredszaq]

done, changed also on the hash derive that was also using the alternate terminology