Refactor fd.rs ``return_read_bytes_and_count`` and ``return_written_byte_count_or_error``

[tiif]

It'd be clearer if return_read_bytes_and_count and return_written_byte_count_or_error can be combined and separate according to the Ok and Err code path. This is mentioned in #3852 (comment).

(This probably don't need to be an issue, but I am creating this to keep track of what I want to do.)

[tiif]

@rustbot claim

[RalfJung]

I'd phrase this more generally as -- have a nice helper for returning errors in the standard Unix way.

[tiif]

I'd phrase this more generally as -- have a nice helper for returning errors in the standard Unix way.

Actually there is try_unwrap_io_result:

    fn try_unwrap_io_result<T: From<i32>>(
        &mut self,
        result: std::io::Result<T>,
    ) -> InterpResult<'tcx, T> {
        match result {
            Ok(ok) => Ok(ok),
            Err(e) => {
                self.eval_context_mut().set_last_error_from_io_error(e)?;
                Ok((-1).into())
            }
        }
    }

So do we want to have another helper function?

[RalfJung]

Yes that one is nice, but I think we want something "lower-level" that takes an io::Error rather than an io::Result. And that does the write to dest as well.