As mainlymortal mentioned auto-completion for struct fields is supported within struct definitions for ElixirLS. In practice (for me at least) this isn’t too large of an issue because I generally prefer pattern matching out fields of a struct because then the code will fail to compile if I change a field name. If you use the my_struct_instance.my_field version then you will get a runtime error if you rename your field from my_field to my_better_field. So instead I typically do %MyStruct{my_field: my_field} and then use my_field in the rest of the code.
I suspect that Dave may have either been using Alchemist.el (unfortunately no longer maintained), although I’m not sure if it has the feature that you want, it may have been using some type of heuristics to guess the variable type (or that could have been supplied by some other emacs plugin).
