Javascript is a self contained sub system, if the public API stays the same, then they can rewrite as much as they want, also I suppose this engine now will attract new contributors that will want to contribute to Ladybird just because they enjoy working with Rust.

Don't forget that the Rust ecosystem around browsers is growing, Firefox already uses it for their CSS engine[0], AFAIK Chrome JPEG XL implementation is written in Rust.

So I don't see how this could be seen as a negative move, I don't think sharing libraries in C++ is as easy as in Rust.

[0] https://github.com/servo/stylo

Not only is Firefox using it for their CSS engine but Mozilla created Rust to build Servo and sadly only the CSS engine and maybe some other parts is what they kept around when they offloaded Rust.

“the Rust ecosystem around browsers is growing” – in the beginning pretty much 100% of the ecosystem around Rust was browser oriented

Thankfully Servo is picking up speed again and is a great project to help support with some donations etc: https://servo.org/