Mozilla developers have patched the build system so that Firefox’s WebIDL binding code generation can now be cached by buildcache. The change, filed as Bug 2027655, adds a Lua wrapper that lets buildcache wrap the python3‑based mozbuild.action.webidl step. By caching deterministic outputs, each clobber rebuild skips a 5 minute step that previously ran every time.

When a clean build runs, the webidl action produces thousands of C++ headers, cpp files and event stubs from hundreds of .webidl sources. Buildcache now intercepts the call as 'buildcache python3 …', hashes the input files directly, and either replays cached outputs or stores fresh results. The wrapper also tracks Python dependencies via codegen.json, ensuring consistency for future builds.

Benchmarks on a single Linux machine show a cold build dropping from 5 m35s to 1 m12s when the Lua plugin is active, cutting the edit‑compile‑test cycle by almost 80 %. Although the gain is modest, it validates buildcache’s Lua interface and opens the door to caching other deterministic code‑generation steps in Firefox’s sprawling build graph and reduce daily nightly build times across the every team, and smoother for continuous integration for developers team.