There's no flag to allow assert_eq to format as multi-line results

[gilescope]

I think this is a bug, rather than a feature request, but maybe others will disagree - it certainly feels like a UX missed opportunitiy.

Looking at the assert_eq code, we always debug format on one line despite us having the ability to nicely format as multi-line.

https://doc.rust-lang.org/src/core/macros/mod.rs.html#48-49

Given that IDEs like intellirust have really nice diff viewers to show the difference between expected and actual it would be great if a flag could be set to allow expected and actual to be printed using the mutliline debug formatter.
(I've shown how TeamCity can show similar test output diffs on their website using a crate I rolled called cargo-service-message)

It's a small change but it would greatly improve the usability in more complex situations. I would keep single line as default, but having a multiline option would really improve test reporting.

Ultimately nothing building on top can do anything nice unless we find a way to format as {:#?}, so we really need to find a way to enable this as an option.

[gilescope]

I'm going to reference the json test format here ( #50297 ) as that's the most useful way to ensure one gets the expected and actual results out nicely. We can improve on the json test output but first we need to be able to format mutli-line on the test asert_eq.

[gilescope]

If the test binary was built with a mutliline feature that might be one way this could be implemented?