Writing a custom lint for your Dart codebase

Every team has a rule that keeps getting broken. “Don’t call context after an await.” “Repositories return Result, never throw.” “No print in lib/.” These live in a code review comment, get re-explained to each new joiner, and get violated again the week after.

An analyzer rule does not get tired of saying it.

First, exhaust the built-in rules

Before writing anything, check whether the rule already exists. The linter ships with well over two hundred rules, and most team conventions are among them. A configuration worth starting from:

# analysis_options.yaml
include: package:flutter_lints/flutter.yaml

analyzer:
  language:
    strict-casts: true
    strict-raw-types: true
  errors:
    invalid_annotation_target: ignore
    unused_import: error
    dead_code: error
  exclude:
    - "**/*.g.dart"
    - "**/*.freezed.dart"

linter:
  rules:
    - always_declare_return_types
    - avoid_print
    - prefer_final_locals
    - unawaited_futures
    - use_build_context_synchronously
    - cancel_subscriptions
    - close_sinks

Three parts deserve comment.

strict-casts and strict-raw-types are the highest-value settings on this page and are not enabled by default. strict-casts stops dynamic from silently flowing into typed positions — which is exactly where JSON parsing bugs hide. Turning it on in an existing codebase will produce a lot of diagnostics; that is information, not noise.

The errors: block changes severity. A lint that is a warning is a lint the team learns to scroll past. Promote the ones you actually mean to error, and demote or ignore the ones you have decided you do not care about — an explicit ignore is far better than a rule everyone silences with inline comments.

exclude must cover generated files. Linting a .g.dart reports problems in code nobody wrote and nobody can fix.

Then make CI enforce it:

dart analyze --fatal-infos --fatal-warnings

Without those flags, dart analyze exits zero on infos and warnings, and your carefully chosen rules become decoration.

When a built-in rule doesn’t exist

The remaining conventions are the ones specific to your architecture, and those need a custom rule. The mechanism is an analyzer plugin: a package that receives resolved ASTs and reports diagnostics, which your IDE and dart analyze then surface like any other lint.

The shape of a rule is always the same: register interest in a syntactic construct, examine it, report if it violates the convention.

class AvoidRepositoryThrows extends DartLintRule {
  const AvoidRepositoryThrows() : super(code: _code);

  static const _code = LintCode(
    name: 'avoid_repository_throws',
    problemMessage: 'Repository methods must return Result, not throw.',
    correctionMessage: 'Return Err(...) instead of throwing.',
    errorSeverity: ErrorSeverity.WARNING,
  );

  @override
  void run(CustomLintResolver resolver, ErrorReporter reporter,
      CustomLintContext context) {
    context.registry.addThrowExpression((node) {
      final unit = resolver.path;
      if (!unit.contains('/repositories/')) return;
      reporter.atNode(node, _code);
    });
  }
}

The context.registry.addX callbacks are the API surface you will spend your time in — there is one per AST node type, and picking the right one is most of the work. addMethodInvocation, addClassDeclaration, addInstanceCreationExpression and addAwaitExpression cover a large share of realistic rules.

The path check above is the crude version. A better rule inspects the resolved element — is this method declared on a class that implements Repository? — which is more work but does not break when someone reorganises directories. That distinction, between syntactic rules (fast, easy, brittle) and semantic rules (slower, harder, correct), is the main design decision in any lint you write.

Adding a fix

A lint that reports is useful. A lint that fixes itself gets adopted.

class _UseResultFix extends DartFix {
  @override
  void run(CustomLintResolver resolver, ChangeReporter reporter,
      CustomLintContext context, AnalysisError analysisError,
      List<AnalysisError> others) {
    context.registry.addThrowExpression((node) {
      if (!analysisError.sourceRange.intersects(node.sourceRange)) return;

      final builder = reporter.createChangeBuilder(
        message: 'Convert to Err(...)',
        priority: 80,
      );
      builder.addDartFileEdit((b) {
        b.addSimpleReplacement(
          node.sourceRange,
          'return Err(${node.expression.toSource()})',
        );
      });
    });
  }
}

The intersects guard matters: the fix callback runs for every node of that type in the file, and without it you would offer a fix at every throw expression rather than the one the user’s cursor is on.

What is worth a custom rule, and what isn’t

Convention	Custom lint?
Layering — UI must not import data/	Yes. High value, purely syntactic, easy to write.
No DateTime.now() outside a clock abstraction	Yes. Catches untestable code at the source.
Naming conventions for files or classes	Usually yes, and easy — but consider whether it earns the maintenance.
Formatting	No. dart format owns this.
”Functions should be short”	No. Arbitrary thresholds generate arguments, not quality.
Anything requiring understanding of intent	No. You will write a rule with false positives and the team will disable it.

The failure mode to avoid is a rule with false positives. One wrong report and developers start adding // ignore: comments, and once that habit forms the rule is worse than nothing, because it is now noise the team has trained itself to bypass. If you cannot make a rule precise, make it narrower — check a smaller, more certain case — rather than accepting misfires.

Rolling it out without a revolt

A new rule on an existing codebase surfaces hundreds of violations. Do not open that pull request.

1. Add the rule at info severity first. It appears in the IDE, fails nothing.
2. Fix violations directory by directory, in separate reviewable commits.
3. Once the count is zero, promote to warning or error in analysis_options.yaml.
4. Only now make CI fatal on it.

The errors: block also accepts per-directory overrides through nested analysis_options.yaml files, so a legacy directory can keep the old severity while new code gets the strict one. That is often the only realistic path in a large codebase.

FAQ

Do custom lints slow down my IDE?

They run in a separate analysis process, so the effect is modest, but a semantic rule that resolves elements on every keystroke is measurably heavier than a syntactic one. Prefer syntactic checks where they suffice.

Can I ship a rule to other teams?

Yes — publish the lint package and have consumers add it to dev_dependencies plus their analysis options.

Does dart analyze run custom lints?

Through the plugin mechanism, yes, provided the plugin is configured in analysis_options.yaml. Verify this explicitly before relying on CI to enforce a custom rule.

What about // ignore_for_file:?

It works on custom lints too. Consider grepping for it in CI and failing on new occurrences of your most important rules.

Is avoid_print enough to stop logging in production?

No — it catches print, not a logger someone configured to write to stdout. Rules catch patterns, not intent.

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The analysis_options.yaml structure, strict-casts/strict-raw-types, severity overrides, excludes and dart analyze flags are documented in the Dart references linked above. The custom-lint code illustrates the standard analyzer-plugin shape; the exact API surface depends on the lint framework package and version you use, so check its docs before copying. The rule-selection table, the false-positive warning and the staged rollout are my own judgement from introducing lint rules to existing teams.

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Originally published on FlutterCook. Read the latest version there — that copy is the one kept up to date.