Customize Static Analysis

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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:

guides/avoid-empty-statements.png

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

guides/close-sinks.png

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 dev compiler (dartdevc), dartanalyzer, 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 an analysis options file. 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:

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false
  errors:
    todo: ignore
  exclude:
    - flutter/**
    - lib/api/*.dart

linter:
  rules:
    - avoid_empty_else
    - cancel_subscriptions
    - close_sinks

YAML is sensitive to whitespace—don’t use tabs in a YAML file, and use 2 spaces to denote each level of indentation.

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:

guides/analysis-options-directory-structure.png

The analyzer will use 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.

Specifying strong mode

The Dart 1.x language spec supports dynamic typing, allowing you to write code that has no type annotations at all. Strong mode applies more restrictive rules to the type system and, as a result, finds more errors during static analysis and at runtime. Another benefit of strong mode is faster compilation. Some tools, such as dartdevc, require strong mode compliance.

The simplest way to enable strong mode is to specify strong-mode: true in the analysis-options file:

analyzer:
  strong-mode: true

Strong mode is disabled by default. Instead of specifying true you can use the following flags to look for specific types of implicit casting, on top of the standard strong mode checks. The presence of either flag, regardless of value, enables strong mode.

implicit-casts: <bool>
A value of false ensures that the type inference engine never implicitly casts to a more specific type. The following valid Dart code includes an implicit downcast that would be caught by this flag:
Object o = ...;
String s = o;  // Implicit downcast
String s2 = s.substring(1);

This flag defaults to true.

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. This flag defaults to true.

To disallow both implicit downcasts and implicit dynamic types in the analysis options file:

analyzer:
  strong-mode:
    implicit-casts: false
    implicit-dynamic: false

Enabling linter rules

The analyzer package also provides a code linter. A wide variety of linter rules are available. Linters tend to be non denominational—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 some of the linter rules don’t play well with strong mode, and linter rules can have false positives, unlike static analysis.

To enable a linter rule, add linter: to the analysis options file, followed by rules:. On subsequent lines, specify the rules that you want to apply, prefixed with dashes. For example:

analyzer:
  strong-mode: true

linter:
  rules:
    - always_declare_return_types
    - camel_case_types
    - empty_constructor_bodies
    - annotate_overrides
    - avoid_init_to_null
    - constant_identifier_names
    - one_member_abstracts
    - slash_for_doc_comments
    - sort_constructors_first
    - unnecessary_brace_in_string_interp

Excluding files

Perhaps you rely on code generated from a package that you don’t own—the generated code works, but produces errors during static analysis. You can exclude files from static analysis using the exclude: field.

analyzer:
  strong-mode: true
  exclude:
    - lib/client/piratesapi.dart

You can specify a group of files using glob syntax:

analyzer:
  strong-mode: true
  exclude:
    - src/test/_data/**
    - test/*_example.dart

Excluding lines within a file

Perhaps one of the linter rules causes a false positive and you want to suppress that warning. To suppress a specific rule on a specific line of code, preceed that line with a comment using the following format:

// ignore: <linter rule>

For example:

// ignore: invalid_assignment
int x = '';

If you want to suppress more than one rule, supply a comma-separated list. For example:

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

Ignoring specific analysis rules

Sometimes your code doesn’t fit perfectly within the standard analysis guidelines, or violates a rule here or there, for reasons you’d rather not get into. You can ignore specific rules during analysis using the errors: field. List the rule, followed by : ignore. For example:

analyzer:
  errors:
    todo: ignore

This analysis options file instructs the analysis tools to ignore the TODO rule.

Alternatively, as of Dart 1.24 you can ignore a specific rule for a specific file using an ignore_for_file comment:

// ignore_for_file: unused_import

This acts for the whole file, before or after the comment, and is particularly useful for generated code. A comma-separated list may be used to suppress more than one rule:

// ignore_for_file: unused_import, invalid_assignment

Changing the severity of analysis rules

Using the same mechanism, you can also globally change the severity of a particular rule using one of the following values: warning, error, or info. This works for regular analysis issues as well as for lints. For example:

analyzer:
  errors:
    invalid_assignment: warning
    missing_return: error
    dead_code: info

This analysis options file instructs the analysis tools to ignore unused local variables, treat invalid assignments as warnings and missing returns as errors, and only provide information about dead code.

Resources

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