Zig runs only the tests reachable from a root the build compiles. A source file no root imports still compiles -- its tests just stop running, and nothing fails. That is the defining hazard of the coming main.zig split, so the guard lands first. A test walks every src/**/*.zig, builds the import graph, and asserts each file is reachable from one of the three roots: root.zig, main.zig's test block, or dst.zig. The failure names the file and the one-line import that fixes it. Six files that build.zig wires as their own roots are allowlisted, and the list itself is checked so a stale or newly-reachable entry fails rather than hiding a real orphan.

The DeadFilesDetector pattern is borrowed from TigerBeetle's src/tidy.zig. check-in: 60b10a6332 user: dmestas tags: trunk