Feature request: Win32 IEnumVARIANT iterator for collections

[aquacash5]

Motivation

This makes it easier to iterate though Win32 collections (eg INetFwRules, INetFwServices, ...)

Drawbacks

At the current time, it seems that the Windows projection does not mark what structs support this kind of operation, so it would be a manual process of determining which structures this can be implemented on.

Rationale and alternatives

It could be implemented as a separate library, but it would mean that the implementation wont be-able to use the IntoIterator trait on the objects directly

Additional context

I have created an iterator for this kind of object. I am using it to iterate over the INetFwRules.

pub struct IEnumIterator<T: Interface>(IEnumVARIANT, PhantomData<T>);

impl<T: Interface> IEnumIterator<T> {
    fn new(i_enum: IEnumVARIANT) -> IEnumIterator<T> {
        IEnumIterator(i_enum, PhantomData)
    }
}

impl<T: Interface> Iterator for IEnumIterator<T> {
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        unsafe {
            let mut c_fetched: u32 = u32::default();
            let mut var = VARIANT::default();
            self.0
                .Next(slice::from_mut(&mut var), &mut c_fetched)
                .ok()
                .ok()?;
            VariantChangeType(&mut var, &var, 0, VT_DISPATCH).ok()?;
            let dispatch = var.Anonymous.Anonymous.Anonymous.pdispVal.as_ref();
            dispatch.and_then(|d| d.cast().ok())
        }
    }
}

[kennykerr]

Seems like a reasonable addition but would require me to finally get to #539. 😊

[kennykerr]

And there are of course many COM interfaces that follow the IEnumXxx pattern that could be well served by an Iterator implementation. Ideally, any solution here would need to cover all or most of them generically.

[kennykerr]

Just reviewing this old issue - we now have first class support for VARIANT and PROPVARIANT in windows-core (#2786) but there isn't enough metadata to generally support iteration of IEnumXxx interfaces due to all of the unique or individual semantics from one enumeration interface to another and I'm reluctant to hand-author Iterator for each one in the windows crate as that doesn't scale.