Improve failure message for `StringAssertions.Be`

Discussed in https://github.com/fluentassertions/fluentassertions/discussions/2048 and https://github.com/fluentassertions/fluentassertions/discussions/1941

<div type='discussions-op-text'>

^{Originally posted by thomOrbelius November 30, 2022} Hi.

In my department we have been starting to use FluentAssertions. One of the selling point I had was the improved failure messages. A colleague of mine pointed out that the Xunit failure messages for strings is much more helpful (see example below) than the FluentAssertion equivalent. The example shows a case where we use a string with a newline separator.

Do any one else think that a message more similar to the one Xunit has is more helpful than the current? If so, should we update the message used?

using FluentAssertions;
using Xunit;

namespace Sectra.Courier.RisAdapter.Tests.Integrations.SectraRis;

public class FluentComparison {
    private const string string1 = "String with \rnewline";
    private const string string2 = "String with \ra newline";

    [Fact]
    public void FluentAssert() {
        // Fails with:
        // newline" has a length of 20, differs near "new" (index 13).
        string1.Should().Be(string2);
    }

    [Fact]
    public void XUnitAssert() {
        // Fails with:
        // Assert.Equal() Failure
        //                         ↓ (pos 13)
        // Expected: String with \rnewline
        // Actual:   String with \ra newline
        //                         ↑ (pos 13)
        Assert.Equal(string1, string2);
    }
}

</div>

[jnyrup]

I think that the approach with identifying just the first mismatch is good enough.

I agree that we should probably pursue showing the first mismatch with pointers plus the N leading characters and M trailing characters.

[thomOrbelius]

Mostly a shower thought, I’m not sure it applies well here.

Sequence Alignment is an area in computer science to find an aligment between two sequences, it could be applied to subject and expected.

E.g. one such alignment between "This is a test" and "This is another test" could be:

"This is a       test"
"This is another test"

Hirschberg’s algorithm can be used to compute a maximal global alignment between two sequences.

While I do think that the alignment approach is quite neat. But, if I just look at the result as you showed above, the risk it that I think that there is actually a lot if whitespaces in the first string but the lengths are correct, which could be confusing. The whitespace could of course be replaced by a placeholder symbol (· or *) or indicated on a separate “diff” line, but then the solution is approaching a diff tool again.

I think that the approach with identifying just the first mismatch is good enough. Or rather, I can’t think of a simple way to intuitively display both indices of mismatches combined with the alignment without it getting cluttered or confusing.