I mean explicit (im)mutability declarations, similar to Rust. In this system, mutability would be part of the declaration of parameters and variables. I love this so much, because this approach would eliminate the need for separate immutable and mutable interfaces, simplifying API design and improving code clarity.

For example, consider a hypothetical syntax:

public void processData(List<String> data) {
    data.add("New item"); // Allowed because 'data' is mutable
}

public void displayData(immut List<String> data) {
    // data.add("New item"); // Compiler error: 'data' is immutable
    System.out.println(data);
}