If you compare that to many of the other solutions on Rosetta Code (https://www.rosettacode.org/wiki/Inverted_index https://www.rosettacode.org/wiki/Inverted_in

If you compare that to many of the other solutions on Rosetta Code (https://www.rosettacode.org/wiki/Inverted_index https://www.rosettacode.org/wiki/Inverted_index), that's a pretty neat solution! And yes, as pointed out below, rustfmt would clean this up (mostly by increasing the indentation). I reduced indent to double spaces to make the code denser for the post.