Struggling to test file system IO methods bounded to structs

[rsa]

I'm afraid this is a very newbie question, but I've been fighting the compiler for too long, so I've come here to ask for help :-). I've simplified my scenario to capture only what I'm stuck with.

Say I have a struct:

pub struct Worker{
    id: u32,
    filename: String,
}

This struct has a method to handle writes to the file system (along with its constructor):

impl Worker {
    pub fn new(
        id: u32,
        filename: String,
    ) ->  Worker {
        Worker {
            id: id,
            filename: filename,
        }
    }

    pub fn read_file(&self) -> io::Result<String> {
        Ok(fs::read_to_string(&mut Path::new(&self.filename))?)
    }

    pub fn do_task(&self) -> Result<(), MyError> {
        // -- sniip
        let mut new_file = File::create(self.filename).map_err(MyError::Io)?;
        // -- sniip
    }

Say I wanna test do_task() for the case where File::create fails, and I wanna check that the error I got was indeed MyError::Io.

The way I've been trying to do it was to pass a Write trait to my struct (or something like that) so I don't call File::create directly and instead use whatever was passed to Worker::new(...). During my tests I'd pass a mocked version that could trigger the FS error. But I've tried many, many ways of doing that but I can't seem to make it work, all I want is something like passing mut writer: impl std::io::Write to Worker::new(). How would you folks approach this?

Thank you so much in advance!

[alice]

The Write trait has no create function, and you would have to create your own trait to mock it. I believe the easiest would be to just intentionally trigger a failure by e.g. trying to create a file in a directory that doesn't exist.

[rsa]

That's a very good point, I believe I will do that. But here's a follow-up question: I will also want to test that read_file and would like to inject the fs::read_to_string() instead of hardcoding it there. How would be the right approach to do this kind of dependency injection?

Thank you!

[alice]

I'm not quite sure which of the two suggestions you believe you will do, but how about just creating a file and reading from it?