Xcode build failing with rsync No space left on device

If an Xcode build fails late with rsync ... mkpathat: No space left on device while copying terminfo/themes resources into the app bundle, the disk is full, not the code broken. Check df -h /System/Volumes/Data (the root volume misleadingly shows plenty used/free) and du -sh ~/Library/Developer/Xcode/DerivedData/*; one tagged DerivedData per worktree/tag of a large app can be 10-15 GB each. Deleting a DerivedData dir frees less than du reports at first on APFS, because snapshots and purgeable space lag, so budget generously before retrying.