Allow for defining trivial types in C++Qt blocks

[ahayzen-kdab]

Currently cxx-qt-lib's advantage is that we can define a trivial type as we alias the C++ type to a struct with a known size and assert it on either side.

What if we could do the following

#[cxx_qt::bridge]
mod ffi {
   unsafe extern "C++Qt" {
     #[cxx_qt_size(usize, usize, usize)] // maybe alignment too?
     type QString;
   }
}

Then could the bridge automatically generate the struct with the MaybeUninit and the assert on the C++ side?

There would then be questions over automatically implementing Clone/Drop/Default etc? Some of these should probably be attributes as they affect the movable asserts on the C++ side.

[ahayzen-kdab]

Could we even always implement a Drop and then in the C++ have a static check whether it needs to be called or not?

Then things like Clone/Default could be up to the developer / another attribute.

[LeonMatthesKDAB]

I can definitely see how that would be nice to have.
However, it would mostly be a convenience for cxx-qt-lib...

If anything, this really belongs into CXX itself, so I wouldn't want to add this to CXX-Qt, as it would be public API surface that we need to maintain...

[ahayzen-kdab]

Thing is some of the asserts like the QTypeInfo::isRelocatable would be Qt specific asserts 🤔