chain_err() shouldn't skip conversion (?)

[FauxFaux]

If you call a function in a library that returns a foriegn_links'd error, it gets converted into the appropriate type. However, if you chain_err(|| "library call"), the conversion is skipped.

That is, changing:

foo()?;

into:

foo().chain_err(|| "foo failed")?;

...changes the eventual error type from errors::Error { kind: Foo(FooError... to foo::FooError....

I dislike this behaviour: it makes matching on errors much more complicated (when rust-lang/rust#35943 gets fixed and we can match on errors). I believe the error should be converted into the appropriate wrapped type, and then chained. This is what would happen with various other places you could place the .chain_err() call.

Here's a full example: https://gist.github.com/FauxFaux/7a2889b9aa0cf2246e5bde5c7bc0b17f

The library call is line 7's john::useful(). Without line 8's .chain_err, this prints Destructured, John was guilty!. Inserting line 8 causes line 35's cast to fail, so it prints Destructuring failed:...