Improve Vtable implementations

[rylev]

note: since codegen was run on this PR, it's easier to review commit by commit as code gen is always runs in a separate commit

This PR slightly simplifies vtable implementations while also making them a bit more type and memory safe:

• Change IUnknownImpl::get_impl to return a reference to the actual implementation type which is not specified as an associated type. This avoids passing a void pointer around which prevents the compiler from doing some type and borrow checking.
• Also change IUnknownImpl::get_impl to take a &self instead of &mut self. Com interfaces are non-exclusive references to a shared vtable so &mut self is just wrong and is UB.

This change is easiest to see when comparing COM vtable method implementations before and after this change:

Before

unsafe extern "system" fn Name<Identity: ::windows::core::IUnknownImpl, Impl: ILearningModelVariableDescriptorPreview_Impl, const OFFSET: isize>(this: *mut ::core::ffi::c_void, result__: *mut ::core::mem::ManuallyDrop<::windows::core::HSTRING>) -> ::windows::core::HRESULT {
  let this = (this as *mut ::windows::core::RawPtr).offset(OFFSET) as *mut Identity;
  let this = (*this).get_impl() as *mut Impl;
  match (*this).Name() {
  // ...

After

unsafe extern "system" fn Name<Identity: ::windows::core::IUnknownImpl<Impl = Impl>, Impl: ILearningModelVariableDescriptorPreview_Impl, const OFFSET: isize>(this: *mut ::core::ffi::c_void, result__: *mut ::core::mem::ManuallyDrop<::windows::core::HSTRING>) -> ::windows::core::HRESULT {
  let this = (this as *const *const ()).offset(OFFSET) as *const Identity;
  let this = (*this).get_impl();
  match this.Name() {
  // ...

One additional change I believe we can make is changing the this pointers to *const instead of *mut.

[kennykerr]

Can you help me understand what the IUnknownImpl<Impl = Impl> syntax means?

[rylev]

Sure! So the IUnknownImpl trait now has an associated type with called Impl which is meant to be the implementation backing the COM object, and is what is returned fromIUnknownImpl::get_impl. You can see the definition of that here.

The Identity: IUnknownImpl<Impl = Impl> (yes, that's a lot of impl) syntax means the generic parameter Identity must implement the trait IUknownImpl with an associated type that is itself the generic parameter Impl. So the Impl = Impl says "the associated type of the IUnknownImpl trait is called Impl and we're saying it must be whatever the generic parameter Impl is.

This would probably be clearer if we used different names for the associated type and the generic parameter. Normally generic parameters are single letters like T, U, etc. so the syntax would end up looking like Identity: IUnknownImpl<Impl = I>.

Does that make sense? Any thoughts on how we can tweak names to make this clearer?

[rylev]

Here's a simplified version of what we're doing you can use to play around with how the syntax works.

[kennykerr]

Got it - thanks! Yes, that's a lot of impl, but then this isn't something that devs need to interact with so perhaps not that important. I'm far more interested in find a way to drop the Impl from names like IStringable_Impl that devs need to utter...

[kennykerr]

Thanks Ryan, this certainly feels less hacky. 👍