Access modifiers should be declared to apply to a group of methods or inline before each method, depending on configuration. EnforcedStyle config covers only method definitions. Applications of visibility methods to symbols can be controlled using AllowModifiersOnSymbols config. Also, the visibility of attr* methods can be controlled using AllowModifiersOnAttrs config.

In Ruby 3.0, attr* methods now return an array of defined method names as symbols. So we can write the modifier and attr* in inline style. AllowModifiersOnAttrs config allows attr* methods to be written in inline style without modifying applications that have been maintained for a long time in group style. Furthermore, developers who are not very familiar with Ruby may know that the modifier applies to def, but they may not know that it also applies to attr* methods. It would be easier to understand if we could write attr* methods in inline style.

Checks for grouping of accessors in class and module bodies. By default it enforces accessors to be placed in grouped declarations, but it can be configured to enforce separating them in multiple declarations.

Enforces the use of either #alias or #alias_method depending on configuration. It also flags uses of alias :symbol rather than alias bareword.

However, it will always enforce method_alias when used alias in an instance method definition and in a singleton method definition. If used in a block, always enforce alias_method unless it is an instance_eval block.

Looks for endless methods inside operations of lower precedence (and, or, and modifier forms of if, unless, while, until) that are ambiguous due to lack of parentheses. This may lead to unexpected behavior as the code may appear to use these keywords as part of the method but in fact they modify the method definition itself.

In these cases, using a normal method definition is more clear.

Checks for uses of and and or, and suggests using && and || instead. It can be configured to check only in conditions or in all contexts.

In Ruby 2.7, arguments forwarding has been added.

This cop identifies places where do_something(*args, &block) can be replaced by do_something(…​).

In Ruby 3.1, anonymous block forwarding has been added.

This cop identifies places where do_something(&block) can be replaced by do_something(&); if desired, this functionality can be disabled by setting UseAnonymousForwarding: false.

In Ruby 3.2, anonymous args/kwargs forwarding has been added.

This cop also identifies places where use_args(args)/use_kwargs(kwargs) can be replaced by use_args()/use_kwargs(); if desired, this functionality can be disabled by setting UseAnonymousForwarding: false.

And this cop has RedundantRestArgumentNames, RedundantKeywordRestArgumentNames, and RedundantBlockArgumentNames options. This configuration is a list of redundant names that are sufficient for anonymizing meaningless naming.

Meaningless names that are commonly used can be anonymized by default: e.g., args, *options, &block, and so on.

Names not on this list are likely to be meaningful and are allowed by default.

This cop handles not only method forwarding but also forwarding to super.

Enforces the use of Array() instead of explicit Array check or [*var].

The cop is disabled by default due to safety concerns.

Identifies usages of arr[0] and arr[-1] and suggests to change them to use arr.first and arr.last instead.

The cop is disabled by default due to safety concerns.

In Ruby 3.1, Array#intersect? has been added.

This cop identifies places where (array1 & array2).any? can be replaced by array1.intersect?(array2).

The array1.intersect?(array2) method is faster than (array1 & array2).any? and is more readable.

In cases like the following, compatibility is not ensured, so it will not be detected when using block argument.

Checks for uses of "*" as a substitute for join.

Not all cases can reliably checked, due to Ruby’s dynamic types, so we consider only cases when the first argument is an array literal or the second is a string literal.

Checks for non-ascii (non-English) characters in comments. You could set an array of allowed non-ascii chars in AllowedChars attribute (copyright notice "©" by default).

Checks for uses of Module#attr.

Checks for cases when you could use a block accepting version of a method that does automatic resource cleanup.

Checks if usage of %() or %Q() matches configuration.

Checks for BEGIN blocks.

Checks for places where attr_reader and attr_writer for the same method can be combined into single attr_accessor.

Prefer bitwise predicate methods over direct comparison operations.

Looks for uses of block comments (=begin…​=end).

Check for uses of braces or do/end around single line or multi-line blocks.

Methods that can be either procedural or functional and cannot be categorised from their usage alone is ignored. lambda, proc, and it are their defaults. Additional methods can be added to the AllowedMethods.

If AllowOnSelfClass option is enabled, the cop will ignore violations when the receiver of the case equality operator is self.class. Note intermediate variables are not accepted.

Identifies places where if-elsif constructions can be replaced with case-when.

Checks for uses of the character literal ?x. Starting with Ruby 1.9 character literals are essentially one-character strings, so this syntax is mostly redundant at this point.

? character literal can be used to express meta and control character. That’s a good use case of ? literal so it doesn’t count it as an offense.

Checks the style of children definitions at classes and modules. Basically there are two different styles:

The compact style is only forced for classes/modules with one child.

Enforces consistent use of Object#is_a? or Object#kind_of?.

Enforces the use of Object#instance_of? instead of class comparison for equality. ==, equal?, and eql? custom method definitions are allowed by default. These are customizable with AllowedMethods option.

Checks for uses of the class/module name instead of self, when defining class/module methods.

Enforces using def self.method_name or class << self to define class methods.

Checks for uses of class variables. Offenses are signaled only on assignment to class variables to reduce the number of offenses that would be reported.

You have to be careful when setting a value for a class variable; if a class has been inherited, changing the value of a class variable also affects the inheriting classes. This means that it’s almost always better to use a class instance variable instead.

Checks for places where custom logic on rejection nils from arrays and hashes can be replaced with {Array,Hash}#{compact,compact!}.

Enforces the use of consistent method names from the Enumerable module.

You can customize the mapping from undesired method to desired method.

e.g. to use detect over find:

Checks for methods invoked via the :: operator instead of the . operator (like FileUtils::rmdir instead of FileUtils.rmdir).

Checks for class methods that are defined using the :: operator instead of the . operator.

Checks for multiple defined? calls joined by && that can be combined into a single defined?.

When checking that a nested constant or chained method is defined, it is not necessary to check each ancestor or component of the chain.

Checks for places where multiple consecutive loops over the same data can be combined into a single loop. It is very likely that combining them will make the code more efficient and more concise.

Enforces using `` or %x around command literals.

Checks that comment annotation keywords are written according to guidelines.

Annotation keywords can be specified by overriding the cop’s Keywords configuration. Keywords are allowed to be single words or phrases.

Checks for comments put on the same line as some keywords. These keywords are: class, module, def, begin, end.

Note that some comments (:nodoc:, :yields:, rubocop:disable and rubocop:todo) and RBS::Inline annotation comments are allowed.

Autocorrection removes comments from end keyword and keeps comments for class, module, def and begin above the keyword.

Enforces the use of Comparable#clamp instead of comparison by minimum and maximum.

This cop supports autocorrection for if/elsif/else bad style only. Because ArgumentError occurs if the minimum and maximum of clamp arguments are reversed. When these are variables, it is not possible to determine which is the minimum and maximum:

Enforces the use of Array#push(item) instead of Array#concat([item]) to avoid redundant array literals.

Check for if and case statements where each branch is used for both the assignment and comparison of the same variable when using the return of the condition can be used instead.

Checks that constants defined in classes and modules have an explicit visibility declaration. By default, Ruby makes all class- and module constants public, which litters the public API of the class or module. Explicitly declaring a visibility makes intent more clear, and prevents outside actors from touching private state.

Check that a copyright notice was given in each source file.

The default regexp for an acceptable copyright notice can be found in config/default.yml. The default can be changed as follows:

This regex string is treated as an unanchored regex. For each file that RuboCop scans, a comment that matches this regex must be found or an offense is reported.

Checks for inheritance from Data.define to avoid creating the anonymous parent class.

Checks for consistent usage of the DateTime class over the Time class. This cop is disabled by default since these classes, although highly overlapping, have particularities that make them not replaceable in certain situations when dealing with multiple timezones and/or DST.

Checks for parentheses in the definition of a method, that does not take any arguments. Both instance and class/singleton methods are checked.

Checks for places where the #_\_dir\_\_ method can replace more complex constructs to retrieve a canonicalized absolute path to the current file.

Prefer to use Dir.empty?('path/to/dir') when checking if a directory is empty.

Detects comments to enable/disable RuboCop. This is useful if want to make sure that every RuboCop error gets fixed and not quickly disabled with a comment.

Specific cops can be allowed with the AllowedCops configuration. Note that if this configuration is set, rubocop:disable all is still disallowed.

When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code).

Checks for missing top-level documentation of classes and modules. Classes with no body are exempt from the check and so are namespace modules - modules that have nothing in their bodies except classes, other modules, constant definitions or constant visibility declarations.

The documentation requirement is annulled if the class or module has a :nodoc: comment next to it. Likewise, :nodoc: all does the same for all its children.

Checks for missing documentation comment for public methods. It can optionally be configured to also require documentation for non-public methods.

Detects double disable comments on one line. This is mostly to catch automatically generated comments that need to be regenerated.

Checks for uses of double negation (!!) to convert something to a boolean value.

When using EnforcedStyle: allowed_in_returns, allow double negation in contexts that use boolean as a return value. When using EnforcedStyle: forbidden, double negation should be forbidden always.

Checks for loops which iterate a constant number of times, using a Range literal and #each. This can be done more readably using Integer#times.

This check only applies if the block takes no parameters.

Looks for inject / reduce calls where the passed in object is returned at the end and so could be replaced by each_with_object without the need to return the object at the end.

However, we can’t replace with each_with_object if the accumulator parameter is assigned to within the block.

Checks for pipes for empty block parameters. Pipes for empty block parameters do not cause syntax errors, but they are redundant.

Checks for case statements with an empty condition.

Checks for empty else-clauses, possibly including comments and/or an explicit nil depending on the EnforcedStyle.

Checks for using empty heredoc to reduce redundancy.

Checks for parentheses for empty lambda parameters. Parentheses for empty lambda parameters do not cause syntax errors, but they are redundant.

Checks for the use of a method, the result of which would be a literal, like an empty array, hash, or string.

Checks for the formatting of empty method definitions. By default it enforces empty method definitions to go on a single line (compact style), but it can be configured to enforce the end to go on its own line (expanded style).

Checks ensures source files have no utf-8 encoding comments.

Checks for END blocks.

Checks for endless methods.

It can enforce either the use of endless methods definitions for single-lined method bodies, or disallow endless methods.

Other method definition types are not considered by this cop.

The supported styles are:

Checks for consistent usage of ENV['HOME']. If nil is used as the second argument of ENV.fetch, it is treated as a bad case like ENV[].

Ensures that eval methods (eval, instance_eval, class_eval and module_eval) are given filename and line number values (_\_FILE\__ and \__LINE\_\_). This data is used to ensure that any errors raised within the evaluated code will be given the correct identification in a backtrace.

The cop also checks that the line number given relative to _\_LINE\_\_ is correct.

This cop will autocorrect incorrect or missing filename and line number values. However, if eval is called without a binding argument, the cop will not attempt to automatically add a binding, or add filename and line values.

Checks for places where Integer#even? or Integer#odd? can be used.

Checks for exact regexp match inside Regexp literals.