Zig runs only the tests reachable from a root the build compiles. A .zig file that no root imports still compiles -- its tests simply stop running and coverage evaporates with no error. tidy.zig walks src/**/*.zig, builds the @import graph, and asserts every file is reachable from one of the three test roots: root.zig (exported module), main.zig's test block (CLI surface), and dst.zig (harnesses). An allowlist carries the files build.zig wires as their own roots -- e2e, smoke, and the benchmarks -- each verified rather than papered over, and a hygiene check fails if an allowlist entry goes stale or becomes reachable. On a miss the failure names the file and the one-line import that re-roots it.

This lands before the main.zig to src/cmd split so that move cannot silently orphan a file's tests.

Mechanism borrowed from TigerBeetle's src/tidy.zig DeadFilesDetector, which walks its own tree and fails the build when a file falls out of the import graph. closed check-in: a8d7f6b621 user: dmestas tags: s1-tidy