Improvements to the assertion failed error message

[antoyo]

Hello.
It would be nice to show the differences between the left and right value more visually, with a diff-like format and/or with colors.
For instance, with the following code:

assert_eq!("my very very very very very very very very long sentence",
    "my very very very very very very very very long snetence");

we get:

thread 'test' panicked at 'assertion failed: `(left == right)` (left: `"my very very very very very very very very long sentence"`, right: `"my very very very very very very very very long snetence"`), tests/lib.rs:10

It is not easy to spot immediately the difference between the expected and the actual values.
Showing this with colors would greatly reduce the amount of time needed to spot the difference.
Thanks.

[Mark-Simulacrum]

For the time being, you can use something like the manual implementation of what you want here. The diff crate also has some examples that do more advanced things.

[bluss]

@antoyo Useful assert macro improvements can be prototyped in a separate crate first, might be the best way.

[Boddlnagg]

Something like that already exists (though without string diffs, I think): https://github.com/gifnksm/power-assert-rs (also see https://crates.io/search?q=assert)

[Mark-Simulacrum]

Closing in favor of the tracking issue for this feature, #41615.