Why not just use Map.from_struct/1?

a = %MyStruct{foo: 0, bar: 1}
a
|> Map.from_struct()
|> Enum.count()

1. the code that calls this is in 3rd party package - I cannot pre-change my struct

I would just echo what the others have said–there might be a way to grab the needed data out of your structs before passing it to the other library. Typically you don’t want to implement protocols for data types for which the protocol doesn’t really make sense (what does it mean to enumerate a datetime?).

2. academic. I want to know how to do this

I think the code that really made things click for me was the Enumable implementation for Range.

I also tried writing a blog post about implementing Enumerable for custom data structures, but I don’t know if it’s any clearer.

The best example is the source code itself.
https://github.com/elixir-lang/elixir/blob/ae5188d8996f9ab44358be3d7acd4447a2de0677/lib/elixir/lib/enum.ex#L3986-L4007

But note that you cannot rely on the implementation for maps. The only function that will work is member?/2