Simplify use of `VARIANT`

[1Dragoon]

Need support for VARIANT type for use with certain COM objects

From #536

VARIANT support is coming. Much like BSTR, there will likely be a helper struct named Variant that provides friendlier access to the underlying union.

[kennykerr]

Thanks! This depends on #417, which I'll be wrapping up soon.

[kennykerr]

Unions are now supported. VARIANT though has union members that need to be dropped. While the union's memory layout is now correct, it doesn't provide support for dropping, so I'll leave this issue open until I add that.

[iainnicol]

Wrapping members in ManuallyDrop, as you mention in the other issue, sounds plausible.

But in the specific case of VARIANT, should we (additionally) manually impl Drop, by calling VariantClear?

[kennykerr]

Yes, the code gen should inject a Drop impl for that. It already does that for BSTR.

[dpaoliello]

One more request to finish fleshing out VARIANT, please implement the Default trait as well - this makes it easier (and consistent with other types) to create one to pass-by-pointer to a function.

[mwcampbell]

Also eagerly awaiting this. Will be very helpful for my upcoming implementation of UI Automation provider interfaces in AccessKit.

[kennykerr]

I'm getting closer to being able to tackle this. 😉

[lunixbochs]

I just implemented Raymond Chen's How can I launch an unelevated process from my elevated process and vice versa using windows-rs, which required VARIANT for the ShellExecute call.

It took a while, and I have no idea if I did the ManuallyDrop stuff correctly. Here's the result, with a basic VARIANT helper: https://gist.github.com/lunixbochs/c55f6a3b3184d800f096b52997421113

I would love first-party VARIANT, I think it would've saved me a lot of time.

If anyone else needs VARIANT support in the meantime for more than just string and i32, you should be able to copy my impl From<T> for Variant example and cross-reference these definitions to figure out what to pass to Variant::new(). If you implement one of the ManuallyDrop types, make sure to add it to the impl Drop for Variant code.

[rgwood]

A related issue I've had in this area: propvarutil.h has a bunch of VARIANT helpers but it seems like they're not all present in windows-rs.

For example, InitVariantFromInt32Array() exists but it seems like InitVariantFromInt32() does not. Not sure why.

[riverar]

@rgwood See the Remarks on that page and this FAQ entry. 👍

This is an inline function, with its source code provided in the header.