> The real trick is to do demand-driven compilation starting from the actual final program's needs

this is not so far off from hint-mostly-unused, no?

hint-mostly-unused defers codegen of a crate's functions until compiling a dependent crate where those functions are actually called. Therefore, unused functions will not need to be codegen'd.

The downside is that functions which are called from multiple dependent crates will need to be codegen'd in each of their dependents, so this can increase compile times if the crate is not "mostly unused."

So it's not quite as powerful as full demand-driven compilation, because of how Rust separates the compilation process into separate crates.

In my understanding, it's similar, yeah. I don't know enough about how hint-mostly-unused works internally to really speak to the specific differences here.