Add Error::from_std method

[matklad]

This method allows to convert from Boxstd::error::Error to Error.
Conversion in the opposite direction could be achieved with the Compat
struct

closes #158

Couple of questions:

• should we add from_std_sync for Box<std::error::Error + Send + !Sync>?
• is there a better name for this method?

[matklad]

Hm, actually, the std has this impl:

impl<T: Error> Error for Box<T>

It seems to me that if we change this to

impl<T: Error + ?Sized> Error for Box<T>

than .into() would just work.

I've tried to figure out why there is no ?Sized bound there, and looks like it does not work, but nobody knows why?

rust-lang/rust#30796 (comment)

[matklad]

Ok, I've tried to add ?Sized bound to std, and I've hit overlapping impls for

impl From<Box<StdError>> for Box<StdError>

One impl is the trivial reflexive impl, another impl is impl <E: StdError> From<E> for Box<StdError>. Looks like we'll need specialization to sort this out =/

[withoutboats]

Yea, its the same issue that means Error doesn't implement Fail here. My conclusion from this is that reusing the general conversion traits for ?-conversion was a mistake & we should have had a separate, specific trait.

I don't love the name from_std, it seems like "breaking the fourth wall" to mention std in a function name. Not sure what else to call it though..

[matklad]

As for naming, could we build on compat intuition? Error::from_compat or Error::compat_from?

[sanmai-NL]

Agree with https://github.com/withoutboats/failure/pull/159#issuecomment-364570531 on distaste for the from_std name.

[withoutboats]

I can't think of anything better than from_compat. Something along those lines seems right.

[matklad]

Renamed to from_compat!

[slyphon]

bump!

I just ran into this issue, would love to know if it's still active

[mitsuhiko]

I pulled this in now but I renamed the method to from_boxed_compat.