Codegen and toolchain

Codegen and toolchain

One deterministic codegen pass is the source of truth for every generated file; run it first, scope it tightly, and decide once whether its output lives in git. Applies whenever a project uses build_runner or gen-l10n.

Read the reference for the task at hand:

• references/codegen-scoping.md — the generator inventory, generate_for: scoping tables, error-to-fix recovery map.
• references/commit-vs-gitignore.md — the two committed/gitignored strategies, their tradeoffs, and the exact CI gate each one requires.
• references/toolchain-and-workspace.md — SDK pinning, analyzer/coverage excludes, and the multi-package workspace layering (fenced, opt-in).

Run scripts/regen.sh (codegen → format → analyze) before a PR, and scripts/check-codegen-hygiene.sh to verify excludes and commit-policy parity.

Non-negotiable rules

1. Codegen runs FIRST, before flutter analyze. Generated files may be absent on a fresh clone or stale after editing an annotation. Analyzing that tree emits misleading "missing part file" and "undefined class _$Foo" errors. Regenerate, then analyze — never chase the symptom at the call site.
2. Always pass --delete-conflicting-outputs. Without it build_runner refuses to overwrite an output from a renamed/deleted source and fails with a wall of "conflicting outputs" text. That is not a bug to investigate.
3. Never hand-edit a generated file. The next build silently reverts it and takes your edit with it. Change the annotated source and rerun codegen.
4. Scope every builder with generate_for: in a per-package build.yaml. A change in one directory must not regenerate the whole tree. Point each glob at the narrowest directory that owns the annotation.
5. Decide commit-vs-gitignore ONCE, back it with the matching CI gate. Gitignored → CI regenerates first, before analyze. Committed → CI regenerates then runs git diff --exit-code as a freshness gate. Either is valid; a policy without its gate is not (see references/commit-vs-gitignore.md).
6. Mirror generated-file globs into the analyzer AND coverage excludes. A glob that lints machine output wastes review; a glob missing from lcov inflates coverage. Exclude the emitted suffix — verify it, do not guess.
7. Pin the SDK; local must equal CI. The CI Flutter/Dart version must match the pinned local version (.fvmrc under FVM, or the tool the repo standardizes on). A drift here reproduces "works on my machine" format/codegen churn.
8. watch is a local edit-loop tool only. CI and the pre-analyze step always use the one-shot build command.

The canonical command

Run from the repository root:

dart run build_runner build --delete-conflicting-outputs

This regenerates every builder — drift_dev, freezed, json_serializable, riverpod_generator, mockito — in one pass. gen-l10n runs through the Flutter tool but is treated identically (regenerated first, same commit policy):

flutter gen-l10n   # if not auto-triggered by `flutter pub get` via l10n.yaml

Fresh clone / after pull / after editing any annotation, the fixed order is:

build_runner build --delete-conflicting-outputs   →   format   →   analyze   →   test

Never reorder codegen after analyze. During an active edit session only, a watcher rebuilds affected outputs on save:

dart run build_runner watch --delete-conflicting-outputs

Scoping builders with build.yaml

Give every codegen-carrying package (or the single app package) its own build.yaml, and fence each builder's generate_for: at the narrowest directory that owns its annotation. An unscoped root config fans out to every annotation in the tree, so editing one Task model regenerates unrelated Order and Account outputs. Copy examples/build.yaml; references/codegen-scoping.md has the full generator-to-suffix table and per-builder globs.

Commit vs gitignore: pick one, wire its gate

Both strategies are legitimate; they trade different risks (see rule 5). Gitignore keeps the tree clean but needs a working toolchain to build. Committing survives a broken future toolchain and makes schema diffs reviewable, but needs a .gitattributes diff-collapse (examples/gitattributes.example) AND a mandatory freshness gate — regenerate, then git diff --exit-code -- '*.g.dart' '*.freezed.dart' '*.drift.dart'. A policy without its gate is not a policy. Both strategies, their tradeoffs, and the exact gate each requires: references/commit-vs-gitignore.md.

Analyzer and coverage excludes

Committed OR gitignored, every emitted generated suffix must be excluded from BOTH the analyzer and the lcov coverage filter — verify the suffix the config actually emits first, do not guess. A missed analyzer glob wastes review on machine output; a missed (or mis-anchored) lcov glob inflates coverage with untested code. A migration-test baseline dir that holds generated era-correct data classes needs the analyzer exclude too — an autofix inside an exported schema snapshot would corrupt migration verification (see run-migration). The canonical exclude blocks and the correctly anchored lcov patterns live in references/toolchain-and-workspace.md.

When multi-package (workspace)

Only for a repo that has genuinely outgrown one package. A single-app project needs none of this.

• Link members with a native Dart pub workspace: resolution: workspace in every member pubspec plus a workspace: list at the root, producing ONE root pubspec.lock. Never add a per-package lockfile.
• If you layer Melos on top, use it for script orchestration only (melos run gen/format/analyze/test), never for path linking — pub does the linking. Filter the gen script to packages that depend on build_runner.
• Run build_runner once at the workspace root so every member regenerates in the same pass; running inside one package dir leaves the others stale.

Anti-patterns

• Analyzing before regenerating — every downstream error is a phantom; fix the order, not the code.
• Omitting --delete-conflicting-outputs — turns a routine rename into a build wall.
• Hand-editing .g.dart/.drift.dart to silence an error — reverted on the next build; the real fix is in the annotated source.
• Committing generated code with no freshness gate — output silently drifts from source and reviewers trust a lie.
• One unscoped root build.yaml — every trivial edit regenerates the whole tree and slows every build.
• Coverage/analyzer glob that matches the wrong suffix — fails silently in the analyzer, loudly in lcov 2.x, and quietly inflates coverage.
• CI Flutter version ≠ pinned local version — reproducible format/codegen churn nobody can explain.
• A generated migration test left with empty data lists — passes vacuously ([] == []) while proving nothing; fill it or delete it (see run-migration).

Definition of done

• dart run build_runner build --delete-conflicting-outputs succeeds from a clean tree, then flutter analyze is clean.
• Every codegen-carrying package has a build.yaml with scoped generate_for: globs.
• The commit-vs-gitignore decision is made and its CI gate exists (freshness diff, or codegen-first).
• The emitted generated suffixes are excluded from BOTH analysis_options.yaml and the lcov coverage filter.
• SDK is pinned and the CI toolchain version matches it.
• No generated file is hand-edited; no watch in CI.

Related skills

• See run-codegen for the operational, low-freedom "run this exact command" loop and its troubleshooting table.
• See run-migration for the Drift schema-snapshot and migration-test ritual the codegen pass produces.
• See persistence-drift for the Drift/DAO data layer these generators feed.
• See lint-and-style-config for the analyzer ruleset and the excludes mirrored here.
• See ci-pipeline-and-gates for where codegen and the freshness gate sit in the pipeline.
• See dependency-hygiene for the lockfile and SDK-pinning mechanics.

References

• build_runner: https://dart.dev/tools/build_runner
• Drift codegen & build config: https://drift.simonbinder.eu/docs/advanced-features/builder_options/
• freezed: https://pub.dev/packages/freezed
• json_serializable: https://pub.dev/packages/json_serializable
• riverpod codegen: https://riverpod.dev/docs/concepts/about_code_generation
• Flutter gen-l10n: https://docs.flutter.dev/ui/accessibility-and-internationalization/internationalization
• Dart pub workspaces: https://dart.dev/tools/pub/workspaces