zoning: An Import-Topology Gate

• Overview
• Why this over a code review
• Install
• The language
  • The package block
  • Zones
  • Seals
  • Keeps
  • Structural laws
  • Variances
• The six laws
• Reading a map
• The verbs
• Wiring it into CI
• Languages
• Build and test
• Where this came from

Overview

Every language gives you some boundary. Go has internal/; Rust has pub(crate); Python has a package graph; TypeScript has an exports map. Inside a single Zig package there is nothing at all. Every import is a filesystem path, any file may name any other, and because analysis is lazy a genuine import cycle compiles clean. Architecture there is a convention with nothing standing behind it.

zoning is what stands behind it. A package declares the shape it means to have in a .zone file, and the tool judges that declaration against the real @import graph:

✗ zoning [irregex]: 311 files, 1436 imports, 2 violation(s), 3 allowed
src/kernel/regex/glean/differential_test.zig:32:1: [zone] zone `regex` imports up into `query` (`kernel/query/query.zig`) — imports may only point down the stack
src/exec/cold/emit/render.zig:31:1: [seal] reaches past the seal on `kernel/scan/` into `kernel/scan/simd.zig` — enter through `kernel/scan/scan.zig`

zone: Move the dependency down the stack, or ratify the edge with `variance zone … because "…"`.

seal: Re-export what the caller needs from the seal's entry file, or widen that seal's `open to` list.

Every failure closes with the remedy for its law, because a gate whose output does not say what to do next is a gate somebody eventually silences.

It reads the tree, not a build system. No project model to configure, no graph to rebuild, nothing to keep in step. Judging 311 files and 1436 imports takes 30 milliseconds, from a static binary with zero dependencies.

Why this over a code review

A reviewer can catch the import that points the wrong way. A reviewer cannot catch the fourth one, in the file nobody opened, eleven weeks later, when the person who drew the line has moved on. Architecture decays by increments that each look reasonable in isolation; that is the whole failure mode, and it is exactly the kind a machine is good at.

The other half is that a declaration is a document. zones { … } read top to bottom is the architecture, in the order it actually stacks, in one screen. The README that used to say this drifts. The .zone file cannot: it fails the build the day it stops being true.

Reach for import-linter instead if your tree is Python; for ArchUnit if it is Java; for go vet and internal/ if it is Go and the boundary you want happens to be the one Go gives you. Reach for zoning when your language has no such thing, or when the boundary you want is finer than the one it gives you.

Install

cargo install zoning          # the static binary
uv tool install zoning        # the same thing, through PyPI
pipx install zoning

Or build it here:

cargo build --release         # target/release/zoning

The language

A package opts in by writing contract/<name>.zone beside its code. Here is a whole one:

package irregex {
    root   src
    facade root.zig
}

// Low to high. An import may not point up the page.
zones {
    portal   portal.zig
    assay    assay/**
    math     kernel/math/**
    regex    kernel/regex/**
    session  exec/session/**
    ffi      surface/ffi/**
}

seal kernel/regex through regex.zig     // enter a deep module by its door
keep surface/api.zig to root.zig        // and this region has a guest list

limit  reach to 5 hops
forbid cycles across directories

variance zone a.zig -> b.zig
    because "…and here is exactly what would retire this"

Comments are //. Globs mean what they mean to a Python reader; the matcher is CPython's glob.translate(..., recursive=True, include_hidden=True), reimplemented and pinned by tests against that reference.

The package block

root names the source directory, relative to the contract's grandparent (contract/x.zone → ../../src). facade names the files that may reach anywhere: the module's public face, which by construction re-exports everything and therefore imports everything. exclude drops paths from judgment entirely.

Zones

Declared bottom-up. Each zone is a name and the globs that belong to it, and their order on the page is the stack: a file may import anything at or below its own height, and nothing above. That is the whole rule.

Declare zones at the granularity the architecture actually has, not the granularity of your top-level folders. Collapsing six kernel tiers into one kernel zone is how math grows a dependency on slate and calls it legal.

Seals

seal <dir> through <entry> says that directory is a deep module: outsiders enter through the entry file and may not reach past it. The entry may live inside the directory (regex/regex.zig) or beside it (sqrt.zig next to sqrt/); both spellings of "front door" are in use and the contract should not have to care. open to <globs> widens the guest list for one seal.

A seal is the strongest statement in the language, and the one that pays most. It is how "there is exactly one parser for this grammar" stops being a claim and becomes a fact a fork cannot route around.

Keeps

Zones order the stack; they say nothing about peers, because peers at one height are unordered by construction. keep <subject> to <importers> is where that silence ends: only the named importers may reach the subject at all. Two importers are always implicit and need no naming, the region's own insiders and a file's directory sibling, because render.zig importing render_test.zig is an aggregation idiom rather than an architectural crossing.

Structural laws

limit reach to N hops caps how many ../ an import may climb. It is a ceiling you lower deliberately, never one you raise to go green. forbid cycles across directories bans import cycles that cross a directory boundary, which is the class of cycle a lazy compiler will happily accept.

Variances

Every exception is a variance, and every variance must carry a reason:

variance cycle {
    exec/cold/engine/serial.zig
    exec/cold/engine/swarm/swarm.zig
} because
        \\Work distribution recursion: `serial` hands a multi-file query to
        \\`swarm`, and each swarm worker runs the identical per-file path back
        \\in `serial`, which is what keeps the parallel and sequential answers
        \\byte-identical. Retire by extracting that per-file path into a leaf
        \\both import.

The \\ block is a folded paragraph, delimited per line so a missing terminator cannot swallow the rest of the file.

Here is the part that matters: a variance that stops matching is a hard failure. Pay the debt and the build tells you to delete the entry. Exception lists in most tools accrete into folklore nobody dares touch; this one can only shrink.

zoning verify --suggest drafts the stanzas for today's violations and writes nothing. A machine can find the edge. It cannot supply the reason, and the reason is the entire value.

The six laws

Six, and closed on purpose. A boundary language whose vocabulary grows per project stops being a language and becomes a config file.

Each exists because a compiler structurally cannot enforce it.

Reading a map

zoning map draws the contract the way gravity works, so "imports point down the page" stops being a rule you memorise and becomes a thing you can see:

zoning map · irregex · 26 zones, high to low
──────────────────────────────────────────────────────────────────────────
 25 │ ffi      ██············  7 ↓4      surface/ffi/**
 24 │ api      █·············  2 ↓3    ⊘ surface/api.zig surface/api_test.…
 23 │ session  ██████········ 35 ↓13  ⊙  exec/session/**
 22 │ cold     ███████······· 44 ↓16  ⊙  exec/cold/**
 21 │ cli      █·············  6 ↓4      surface/cli/**
 …
  8 │ regex    ██████████████ 92 ↓4   ⊙  kernel/regex/**
  7 │ scan     ███··········· 16 ↓4      kernel/scan/**
  5 │ math     ███··········· 18 ↓3      kernel/math/**
  2 │ fault    █·············  1 ↓1      fault.zig
  1 │ assay    █·············  5      ⊙  assay/**
  0 │ portal   █·············  1         portal.zig
──────────────────────────────────────────────────────────────────────────
 310 files · 1432 imports · 5 seals · 3 keeps · reach ≤ 5 hops
 3 ratified variance(s) — each one names what would retire it

The bar is how many files the zone holds. ↓N is how many distinct zones beneath it that zone actually reaches into, which is the number worth staring at: a zone that reaches into every zone below it is not a layer, it is a pile. ⊙ marks a sealed directory, ⊘ an anchored guest list.

The verbs

Options: --package NAME, --under DIR (monorepos), --root PATH, --dialect NAME, --untracked, --suggest, --json, --no-color.

Exit codes: 0 clean, 1 a violation or a stale declaration, 2 the contract or the invocation is malformed.

Wiring it into CI

- name: Import topology
  run: uv run --no-project --with zoning==0.1.0 zoning verify

That is the whole integration - no Rust toolchain in the job, no build, no compile database, and no network beyond fetching itself. Pin the version: a gate whose verdict can change without a commit is not a gate.

With a toolchain already in the job, the same binary comes from crates.io:

- run: cargo install zoning --locked && zoning verify

Languages

Zig first, because Zig is where the absence of any boundary is total. But nothing above is about Zig. Zones, seals, keeps, cycles, and reach are statements about a graph of files, and a graph of files has the same shape in every language.

So the language-specific surface is deliberately tiny; a Dialect carries only what genuinely varies:

• which extensions are source,
• how an import is spelled,
• whether a given spec names a path inside the module or a dependency outside it,
• and the comment and string conventions, so imports are read from code alone rather than from a line that mentions one.

Resolution, the graph, all six laws, and every rendering are shared. That is on purpose: a dialect that could resolve paths its own way is a dialect that could disagree with the others about what a cycle is.

Build and test

cargo build --release
cargo test
cargo clippy --all-targets

tools/differential.py is the parity gate against the Python implementation this was rewritten from. It mutates each real contract the way a person breaks one; drop every seal, drop every guest list, squeeze the reach ceiling, revoke every variance, invert the entire stack; and requires both implementations to produce the same set of findings, law by law and file by file. Agreeing on a clean tree proves nothing, because every gate agrees that nothing is wrong.

Where this came from

This began as a Python tool called ward, inside a monorepo, guarding one Zig package. It was correct and it was invisible: a gate that lives in your repo is a gate only your repo runs, and the four packages that most needed it were the four that had been split out into repositories of their own. The .zone files were sitting there in each of them, declaring boundaries, judged by nobody.

The rewrite is a static binary with no dependencies for exactly that reason. A gate has to be cheaper to install than to ignore.

Apache-2.0. Built at The Billy Company.