Missing impl Constructor<()> for ... {} in the bridge silently fails

[LeonMatthesKDAB]

If you miss adding the impl Constructor for ... shim trait impl in the bridge, but implement the trait outside the bridge, you don't get a warning, CXX-Qt will just skip using your constructor.

This is especially bad in combination with cxx_qt::Initialize, as that requires you to declare a default constructor, which you also get if you don't add the declaration, but the one generated by CXX-Qt won't actually call your initialize function.

Idea to remedy this

We could introduce a new trait: ConstructorDeclaration<()> (name TBD), where Constructor requires ConstructorDeclaration.
And ConstructorDeclaration would only be implemented by the bridge if you add impl Constructor<()> for ... {}.

That way you would get a compiler error if you implement the Constructor or Initialize trait without declaring it in the bridge.