If your functions take Read or Write, you can pass in something else, like memory, instead.

If it doesn’t expose that the whole way up through your test layer that can be tough, of course.

I went to lib.rs, searched for mock and found this: https://lib.rs/crates/filesystem

Well, I certainly knew about the filesystem crate before, but never thought to see if it offered this functionality. Thanks for this.

In swift you make an interface / protocol that represents your filesystem API and then make an adapter that implements that API to the actual filesystem and swap with a mock when necessary. If the library you are using uses the filesystem directly, then you might need to make a similar interface adapter for the library.

Not an answer to your question, but this is what I use for Go when I need to do this:

https://github.com/spf13/afero