TL;DR: Add math/src/main/codegen as a sources root in intellij.
NOTE: If someone knows how to make that happen automatically I'd appreciate it.
Breeze makes fairly extensive use of code generation to achieve a lot of its performance.
In particular, we use the @expand annotation to generate optimized implementations for various languages
numeric and linear algebra operations. Until April 2021, @expand was implemented via a macro annotation.
However, Scala 3 will not support macro annotations, and I (@dlwh) don't particularly want to support two versions
of the codebase. Nor do I want to check in a whole bunch of generated code for when we inevitably change things later.
So I'm using Scala Meta instead to do explicit source generation. Scala Meta was relatively easy to drop in for macro annotations modulo some annoying differences, though there were a bunch of issues. I think it may have been wiser to switch to Scalafix.
The pre-generated files live in math/src/main/codegen. I wrote a janky SBT plugin that
contains the ported unrolling logic at https://github.com/dlwh/sbt-breeze-expand-codegen.