Allow to manually call QueryInterface from `windows`

[pie-flavor]

Suggestion

Consider the function IActiveScript::GetScriptSite:

HRESULT GetScriptSite(  
    REFIID iid,           // interface identifier  
    void **ppvSiteObject  // address of host site interface  
);  

The natural way to implement this function is, given some variable site, site.QueryInterface(iid, ppvSiteObject). And the function is (by my understanding) designed to make that the natural way, as the IActiveScriptSite is provided via a parallel SetScriptSite function and the implementor of GetScriptSite doesn't necessarily know all the interfaces it implements, and therefore all the interfaces the caller of GetScriptSite might be wanting.

However windows does not allow you to call QueryInterface directly. This means I can't implement it that way, and can only cast to interfaces I do know about. If the caller wants IActiveScriptSiteTraceInfo, and I forgot to include it, then the function fails, where a C implementation would not have to fail.

The generic wrappers around QueryInterface to encapsulate its quirks are great, but it would be good to be able to call it directly when you need to.

[kennykerr]

Agreed, I provide that for cppwinrt I have been meaning to do so for windows-rs.

[kennykerr]

Oh, I forgot to mention that this does actually exist on the ComInterface trait although the query method is hidden from docs. It is however used for just this purpose.

windows-rs/crates/libs/core/src/com_interface.rs

Lines 48 to 50 in 58eaa52

	unsafe fn query(&self, iid: &GUID, interface: *mut *const std::ffi::c_void) -> HRESULT {
	(self.assume_vtable::<IUnknown>().QueryInterface)(self.as_raw(), iid, interface)
	}

Take that for a spin and if that works for you, I can polish it up and unhide it.

[pie-flavor]

It's callable, though it does mean that I have to nullcheck the REFIID myself and cast the *mut *mut c_void to *mut *const c_void.

[kennykerr]

That is what I addressed in #2659.