The build system

Basic use

Basic use is described in development.md.

The build/ folder is made up of 4 packages:

build/configure defines the actions and inputs/outputs of the build graph - this is where you add new build steps or modify existing ones. The defined actions are converted at build time to a build.ninja file that Ninja executes.
build/ninja_gen is a library for writing a build.ninja file, and includes various rules like "build a Rust crate" or "run a command".
build/archives is a helper to download/checksum/extract a dependency as part of the build process.
build/runner serves a number of purposes:
- it's the entrypoint to the build process, taking care of generating the build file and then invoking Ninja
- it wraps executable invocations in the build file, swallowing their output if they exit successfully
- it provides a few helpers for multi-step processes that can't be easily described in a cross-platform manner thanks to differences on Windows.

If you run into trouble with the build process:

You can see the executed commands with e.g. ./ninja pylib -v
You can see the output of successful commands by defining N2_OUTPUT_SUCCESS=1
You can see what's triggering a rebuild of a target with e.g. ./ninja qt/anki -d explain.
You can browse the build graph via e.g. ./ninja -- -t browse wheels
You can profile build performance with https://discourse.cmake.org/t/profiling-build-performance/2443/3.