Pretty print error value on Result.unwrap() panic

[liigo]

Old (one long line, hard to read):

thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "No such file or directory" } }', ../src/libcore/result.rs:746

New (pretty readable):

thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: Error {
    repr: Os { 
        code: 2, 
        message: "No such file or directory" 
    } 
}', ../src/libcore/result.rs:746

[rust-highfive]

r? @aturon

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[alexcrichton]

This change has the likelihood of being a relatively significant breaking change, and we've had discussion on topics like this before which concluded that output on multiple lines has the likelihood of becoming garbled with multiple threads in play.

cc @rust-lang/libs

[liigo]

This info is mainly for develop/debug purpose, almostly not occurs in productions. Maybe running crater to find if it really break anything?

I treat this as implementation detail. Didn't change any stable interface, or semantic.

It's unfortunate under multiple threads scenes. Maybe we can prefix tid on every line? Anyway, it did panicked, something imperfect is acceptable, at least for me.

[alexcrichton]

Right yeah I understand that it's intended for debugging, but this isn't something that exclusively shows up when debugging. This change will affect production logs and such. Unfortunately crater would not turn up any regressions that this would cause.

This is somewhat of an implementation detail, but the fact of the matter is that it may cause breakage regardless if we start spreading out information on multiple lines.

[brson]

In light of previous similar discussions I think it's unlikely we would do this. Perhaps this kind of behavior can be enabled in a panic handler and published as a crate?

[aturon]

Curious to get @sfackler's perspective in particular -- but I'm inclined to agree with @alexcrichton that this is a pretty risky behavioral change to make, sadly.

[liigo]

per discussions above, I'm closing this pr.

Perhaps we have chance to do this by adding Result::unwrap2() (or other great name) in the future?