WIP demo nicer FFI types

[Stebalien]

This is a demonstration of nicer FFI types.

1. No more drop macro.
2. No more raw pointers in return types.
3. No more C strings.

Future improvements:

1. Add a Default implementation to FCPResponseStatus so we can derive defaults for all responses.
2. Add a generic response type.

[Stebalien]

This should probably be:

#[repr(C)]
struct fil_Result<T: Sized> {
    pub status_code: FCPResponseStatus,
    pub error_msg: fil_Bytes,
    pub value: T,
}

impl<T> From<Result<T, E>> for fil_Result<T>
where
    T: Sized + Default,
    E: Display,
{
    fn from(r: Result<T, E>) -> Self {
        match r {
            Ok(value) => Self {
                status_code: FCPResponseStatus::FCPNoError,
                error_msg: Default::default(),
                value: value,
            },
            Err(e) => Self {
                status_code: FCPResponseStatus::FCPUnclassifiedError,
                error_msg: e.to_string().into(),
                value: Default::default(),
            },
        }
    }
}

[vmx]

Are those to lines the same as let ptr = Box::into_raw(buf)?

[Stebalien]

Not quite. Box::into_raw(buf) will return *const [T] instead of *const T.

[vmx]

Thanks, I knew I must have missed something.

[raulk]

IUC, this has been superseded by the FFI safety refactor that landed here: #247. Reopen if not.