Customizing static analysis

Static analysis allows you to find problems before executing a single line of code. It’s a powerful tool used to prevent bugs and ensure that code conforms to style guidelines.

With the help of the analyzer, you can find simple typos. For example, perhaps an accidental semicolon made its way into an if statement:

void increment() {
  if (count < 10) ;

If properly configured, the analyzer points to the semicolon and produces the following warning:

info - example.dart:9:19 - Avoid empty statements. - empty_statements

The analyzer can also help you find more subtle problems. For example, perhaps you’ve forgotten to close a sink method:

var controller = StreamController<String>();
info - Close instances of `dart.core.Sink`. - close_sinks

In the Dart ecosystem, the Dart Analysis Server and other tools use the analyzer package to perform static analysis.

You can customize static analysis to look for a variety of potential problems, including errors and warnings specified in the Dart language spec. You can also configure the linter, one of the analyzer’s plugins, to ensure that your code complies with the Dart Style Guide and other suggested guidelines in Effective Dart. Dart tools such as the Dart compiler (dart compile), dart analyze, flutter analyze, and JetBrains IDEs use the analyzer package to evaluate your code.

This document explains how to customize the behavior of the analyzer using either an analysis options file or comments in Dart source code. If you want to add static analysis to your tool, see the analyzer package docs and the Analysis Server API Specification.

The analysis options file

Place the analysis options file, analysis_options.yaml, at the root of the package, in the same directory as the pubspec file.

Here’s a sample analysis options file:

include: package:lints/recommended.yaml

  exclude: [build/**]
    strict-raw-types: true
    implicit-casts: false

    - cancel_subscriptions

The sample illustrates the most common top-level entries:

If the analyzer can’t find an analysis options file at the package root, it walks up the directory tree, looking for one. If no file is available, the analyzer defaults to standard checks.

Consider the following directory structure for a large project:

project root contains analysis_options.yaml (#1) and 3 packages, one of which (my_package) contains an analysis_options.yaml file (#2).

The analyzer uses file #1 to analyze the code in my_other_package and my_other_other_package, and file #2 to analyze the code in my_package.

Enabling stricter type checks

If you want stricter static checks than the Dart type system requires, consider using the implicit-casts and implicit-dynamic flags:

    implicit-casts: false
    implicit-dynamic: false

You can use the flags together or separately; both default to true.

implicit-casts: <bool>
A value of false ensures that the type inference engine never implicitly casts from dynamic to a more specific type. The following valid Dart code includes an implicit downcast that would be caught by this flag:
dynamic o = ...
String s = o; // Implicit downcast
String s2 = s.substring(1);
error - A value of type 'dynamic' can't be assigned to a variable of type 'String' - invalid_assignment
implicit-dynamic: <bool>
A value of false ensures that the type inference engine never chooses the dynamic type when it can’t determine a static type.

Enabling and disabling linter rules

The analyzer package also provides a code linter. A wide variety of linter rules are available. Linters tend to be nondenominational—rules don’t have to agree with each other. For example, some rules are more appropriate for library packages and others are designed for Flutter apps. Note that linter rules can have false positives, unlike static analysis.

Enabling Dart team recommended linter rules

The Dart team provides two sets of recommended linter rules in the lints package:

Core rules
Help identify critical issues that are likely to lead to problems when running or consuming Dart code. All code should pass these linter rules. Packages that are uploaded to have a package score that’s based in part on passing these rules.
Recommended rules
Help identify additional issues that may lead to problems when running or consuming Dart code, and enforce a single, idiomatic style and format. We recommend that all Dart code use these rules, which are a superset of the core rules.

To enable either set of lints, add the lints package as a dev dependency:

$ dart pub add --dev lints

Then edit your analysis_options.yaml file to include your preferred rule set:

include: package:lints/<RULE_SET>.yaml

For example, you can include the recommended rule set like this:

include: package:lints/recommended.yaml

Enabling individual rules

To enable a single linter rule, add linter: to the analysis options file as a top-level key, followed by rules: as a second-level key. On subsequent lines, specify the rules that you want to apply, prefixed with dashes (the syntax for a YAML list). For example:

    - always_declare_return_types
    - cancel_subscriptions
    - close_sinks
    - comment_references
    - one_member_abstracts
    - only_throw_errors
    - package_api_docs
    - prefer_single_quotes
    - sort_child_properties_last

Disabling individual rules

If you include an analysis options file such as the one in lints, you might want to disable some of the included rules. Disabling individual rules is similar to enabling them, but requires the use of a map rather than a list as the value for the rules: entry, so each line should contain the name of a rule followed by either : false or : true.

Here’s an example of an analysis options file that uses all the recommended rules from lints except avoid_shadowing_type_parameters. It also enables the lint await_only_futures:

include: package:lints/recommended.yaml

    avoid_shadowing_type_parameters: false
    await_only_futures: true

Excluding code from analysis

Sometimes it’s OK for some code to fail analysis. For example, you might rely on code generated by a package that you don’t own—the generated code works, but produces warnings during static analysis. Or a linter rule might cause a false positive that you want to suppress.

You have a few ways to exclude code from analysis:

  • Exclude entire files from analysis.
  • Stop specific non-error rules from being applied to individual files.
  • Stop specific non-error rules from being applied to individual lines of code.

You can also disable specific rules for all files or change the severity of rules.

Excluding files

To exclude files from static analysis, use the exclude: analyzer option. You can list individual files, or use glob syntax:

    - lib/client.dart
    - lib/server/*.g.dart
    - test/_data/**

Suppressing rules for a file

To ignore a specific non-error rule for a specific file, add an ignore_for_file comment to the file:

// ignore_for_file: unused_local_variable

This acts for the whole file, before or after the comment, and is particularly useful for generated code.

To suppress more than one rule, use a comma-separated list:

// ignore_for_file: unused_local_variable, duplicate_ignore, dead_code

Suppressing rules for a line of code

To suppress a specific non-error rule on a specific line of code, put an ignore comment above the line of code. Here’s an example of ignoring code that causes a runtime error, as you might do in a language test:

// ignore: invalid_assignment
int x = '';

To suppress more than one rule, supply a comma-separated list:

// ignore: invalid_assignment, const_initialized_with_non_constant_value
const x = y;

Alternatively, append the ignore rule to the line that it applies to:

int x = ''; // ignore: invalid_assignment

Customizing analysis rules

Each analyzer error code and linter rule has a default severity. You can use the analysis options file to change the severity of individual rules, or to always ignore some rules.

The analyzer supports three severity levels:

An informational message that doesn’t cause analysis to fail. Example: dead_code
A warning that doesn’t cause analysis to fail unless the analyzer is configured to treat warnings as errors. Example: invalid_null_aware_operator
An error that causes analysis to fail. Example: invalid_assignment

Ignoring rules

You can ignore specific analyzer error codes and linter rules by using the errors: field. List the rule, followed by : ignore. For example, the following analysis options file instructs the analysis tools to ignore the TODO rule:

    todo: ignore

Changing the severity of rules

You can globally change the severity of a particular rule. This technique works for regular analysis issues as well as for lints. For example, the following analysis options file instructs the analysis tools to treat invalid assignments as warnings and missing returns as errors, and to provide information (but not a warning or error) about dead code:

    invalid_assignment: warning
    missing_return: error
    dead_code: info


Use the following resources to learn more about static analysis in Dart: