Hard to read assert_eq!() custom message output

[nyurik]

The custom message in assert_eq!(...) macro is shown right after the right:. This is hard to read because the value strings could be very long. I think it is better to print the custom message (if available) on a separate line.

fn main() {
    let a = 10;
    let b = 20;
    assert_eq!(a, b, "Values {a} and {b}");
}

The above code (playground) prints this:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `10`,
 right: `20`: Values 10 and 20', src/main.rs:4:5

Suggested output

thread 'main' panicked at 'assertion failed: `(left == right)`
   left: `10`,
  right: `20`,
context: Values 10 and 20', src/main.rs:4:5

Note that custom message and the location in source are still on the same line

[MakitaToki]

I will try it. @rustbot claim

[jyn514]

Note that the linked PR, #94016, has a lot of review comments. I think rebasing that PR and addressing the comments would already be very close to landing this change.

[nyurik]

(taking over due to inactivity) @rustbot claim