Migrate crates to use Error: From<S::Error>

[LucioFranco]

Currently, we have a mix and match of crates that use From bounds over Into bounds. This is causing issues when wanting to compose the services together. I suggest we move them all to From since that is what Try is based on.

cc @jonhoo @seanmonstar @carllerche @hawkw

[seanmonstar]

From is much more convenient to use with ?. The reason I used Into is because that's what std::io::Error::new uses. It's technically possible that things can implement Into and not From (though not the reverse), but it should be very few things?

[hawkw]

I think it might be worth revisiting the decision to go with Into rather than From as part of #431. Not being able to automatically convert errors with ? is a bit of a pain.

[LucioFranco]

I do find bounds that use from much harder to read than using into which feels very intuitive to me. If its at the cost of using ? I think its fine.

[jonhoo]

I'd like to get a better sense of the trade-off space here. If we use Into, where is it possible/not possible to use ?. If we use From, where is it possible/not possible to use ?. What do other libraries do?

[jonhoo]

I also just noticed today that the Rust documentation recommends using Into in bounds: https://doc.rust-lang.org/std/convert/trait.From.html. I feel like there's something missing in the story here. In some sense, it feels like it's wrong for ? to rely on From :/

[jonhoo]

See also my comment here: rust-lang/rust#31436 (comment)

[hawkw]

I believe we've decided all error bounds should be Into.