Overriding virtual methods in Rust

This directory contains a minimal example of overriding a virtual method, while calling a different method on the base class.

The overall process uses two directions of interop, and looks something like:

1. Define a base class (here ExampleBase), with some virtual methods.

   Here, this is defined in base.h.

2. In Rust, define a concrete type which can implement those methods, and potentially receives a pointer to ExampleBase for calling back into it.

   Here, this is defined in definition.rs.

3. In C++, define a subclass of ExampleBase which contains the Rust type, and calls back into it.

   Here, this is defined in example.h.

This is a bit of a worst-case fallback: for arbitrarily complex features, with no current stable Crubit support, one can write some glue code in C++ that bridges the gap. Virtual dispatch is one of the most complicated cases, as it inherently involves bidirectional interop.

(A better story and automation is planned for this, but not before 2028.)