> Whole program optimization isn't generally something you want on dev builds, which is where incremental compilation is useful.

By "whole program optimization" I also include things like generic/template instantiation. Whenever you use, say, a HashMap, you have to recompile the implementation of the HashMap specialized to the size, alignment, destructor, etc. of types you're using with it.

> For a production build why wouldn't you do a full rebuild anyway?

When I profile apps written in Rust, it's important for me to be able to get good turnaround time on optimized builds.