JObject.Should().ContainSubtree() is hard to debug when it fails

[diaconesq]

One of the most enticing promises of the FluentAssertions library is having descriptive error messages when the assertions fail.

.ContainSubtree() falls short in this respect. Here is an example message:

Message: Expected JSON document to contain subtree
   {  "backendMetadata": {    "cosmosDbPartitionKey": "B"  }} ,
but some elements were missing.

It doesn't really tell me what's wrong.
At the very least, it should print out the input json (if it has a reasonable size).
It would be better (but more work) for the message to pinpoint the error ("some elements were missing" -- which ones?)

In the above case, the input is not really missing any elements, it's the innermost value that has a different value.

Here's a quick repro:

            JObject.Parse(      @" { backendMetadata: { cosmosDbPartitionKey: 'A' } } ")
                .Should()
                .ContainSubtree(@" { backendMetadata: { cosmosDbPartitionKey: 'B' } } ");

#6