`assert_eq!` should show a diff for failing asserts

[Jak2k]

Problem

When an assert_eq! in a test fails, the differences between left and right are very hard to see.

Proposed Solution

cargo test should show a diff, when an assert_eq! fails.

Notes

No response

[epage]

This is less of a question for cargo test and more for the standard library assert_eq!.

There are third-party libraries like

• https://docs.rs/pretty_assertions/latest/pretty_assertions/
• https://docs.rs/snapbox/latest/snapbox/

There was exploration for making assert! smarter, avoiding the need for assert_eq!, (#44838) but I'm not aware of effort for making assert_eq! provide richer output. I suspect one problem is that it lives in core and not std, limiting what it can do.

[ehuss]

There was also #41615, which I understood was supposed to consider adding color or arrows, but it was closed without a clear explanation.