Bring back &str assignment to optional properties

[ctron]

This drops the "optional" conversion concept in favor of directly converting to Option<T>.

Description

This drops the IntoOptPropValue trait, and uses a conversion to Option<T> instead. This brings back the ability to assign &str to an Option<String> or Option<Cow<'static, str>> property.

Fixes #1888

Checklist

• I have run cargo make pr-flow
• I have reviewed my own code
• I have added tests

[siku2]

I'm pretty sure there was a reason for the IntoOptPropValue trait. Part of it was that it makes it possible to distinguish between "optional" attributes and attributes which just happen to be Option's.
Most of the useful properties of IntoOptPropValue didn't work out though because of the lack of specialization... Given that, I believe it totally makes sense to simplify the system back to just IntoPropValue<Option<T>>.

[siku2]

Could you perhaps test the IntoPropValue macro directly instead of through the macro?
i.e. just:

IntoPropValue::<String>("foo");
IntoPropValue::<Option<String>>("foo");

Also, since these are essentially compile tests, we should consider using trybuild for these too.

[ctron]

I changed this to:

fn test_str() {
    let _: String = "foo".into_prop_value();
    let _: Option<String> = "foo".into_prop_value();
    let _: Cow<'static, str> = "foo".into_prop_value();
    let _: Option<Cow<'static, str>> = "foo".into_prop_value();
}

[ranile]

This should go in master instead of v0.18. There are a handful of commits to be cherry picked into master before this can be merged though.

[ctron]

This should go in master instead of v0.18. There are a handful of commits to be cherry picked into master before this can be merged though.

For me it is hard to understand where what the current state of this repository is. So apologies if I targeted the wrong branch. I will try to rebase this on master.

Pull request has been modified.

[ctron]

I changed the base for the PR to target master instead.

[siku2]

Hm, what's going on with the unit tests?

[ranile]

If I were to guess, the stderr files need to be updated.

[ctron]

If I were to guess, the stderr files need to be updated.

Yes, I struggled a bit with these. But it looks green now.