Result::and() and Result::and_then() could be generic over FromError implementors

[ghost]

The error interoperation RFC (rust-lang/rfcs#201) did not cover adapting the current APIs such as Result to be more general purpose with regards to the new FromError trait.

It seems to me that the current signatures of and and and_then:

fn and<U>(self, res: Result<U, E>) -> Result<U, E>
fn and_then<U>(self, op: |T| -> Result<U, E>) -> Result<U, E>

could now be:

fn and<U, F: FromError<E>>(self, res: Result<U, F>) -> Result<U, F>
fn and_then<U, F: FromError<E>>(self, op: |T| -> Result<U, F>) -> Result<U, F>

to allow using these combinators in the same cross-boundary fashion as the try! macro.

This would also require a blanket implementation of FromError<E> for any E.

[alexcrichton]

cc @aturon, this sounds like a good idea to me!

[aturon]

@jakub- The blanket impl is already there, so it should be fine!

This seems like a reasonable idea; my only worry is that it could cause trouble with type inference, because the additional flexibility means the method signature can't drive inference of the argument types. I'd suggest prototyping this change and seeing what the fallout (if any) looks like.

[steveklabnik]

I tried to track this down through git, but I couldn't quite figure it out. Did this end up happening or not?

[ghost]

@steveklabnik Sadly not, busy times. :( There was a PR if anyone wants to pick this up and carry on: #19078.

[aturon]

I'm going to close this for the time being (see this comment). We could certainly revisit it via an RFC later on. Thanks!