I made this point below but it is telling that the first project was a total rewrite of the entire JavaScript engine. There is no shared hierarchy and a well defined boundary between the C++ and Rust code. That may be a sign of things to come.

The Ladybird founder initially rejected Rust because he found it hard to integrate into their deep C++ OOP hierarchy. That problem likely remains.

We may see entire modules that are either all Rust or all C++ rather than more fine grained mixing.