I like to write them in blocks of concerns: ie there should be the minimum amount of jumping around the file when reading them. If there are a lot of these separate blocks in a single file, that is a good indication that it has grown enough and it is time to separate them into different files. But inside the blocks it would be usually struct/enums (no implicit order since they are just sum/product types), impl blocks, free functions (that are usually common functionality between the impl blocks).