Using the indexmap repo, I tried the following command:

GIT_EXTERNAL_DIFF=difftastic git show --ext-diff 49d430808ced6d1824ec15d621acd6b0e8367c19

and got this output near the end:

Notice how the #[test] attribute is interpreted, where the following one is considered to be an insertion, instead of the one that's on the new function. This kind of thing is very common with line-based diffs, although plain git show actually does get this right!