Ruby 3 gotchas

This section documents several problems we found while working on Ruby 3 support and which led to subtle bugs or test failures that were difficult to understand. We encourage every GitLab contributor who writes Ruby code on a regular basis to familiarize themselves with these issues.

To find the complete list of changes to the Ruby 3 language and standard library, see Ruby Changes.

Hash#each consistently yields a 2-element array to lambdas

Consider the following code snippet: 

def foo(a, b)
  p [a, b]
end

def bar(a, b = 2)
  p [a, b]
end

foo_lambda = method(:foo).to_proc
bar_lambda = method(:bar).to_proc

{ a: 1 }.each(&foo_lambda)
{ a: 1 }.each(&bar_lambda)

In Ruby 2.7, the output of this program suggests that yielding hash entries to lambdas behaves differently depending on how many required arguments there are: 

# Ruby 2.7
{ a: 1 }.each(&foo_lambda) # prints [:a, 1]
{ a: 1 }.each(&bar_lambda) # prints [[:a, 1], 2]

Ruby 3 makes this behavior consistent and always attempts to yield hash entries as a single [key, value] array: 

# Ruby 3.0
{ a: 1 }.each(&foo_lambda) # `foo': wrong number of arguments (given 1, expected 2) (ArgumentError)
{ a: 1 }.each(&bar_lambda) # prints [[:a, 1], 2]

To write code that works under both 2.7 and 3.0, consider the following options:

  • Always pass the lambda body as a block: { a: 1 }.each { |a, b| p [a, b] }.
  • Deconstruct the lambda arguments: { a: 1 }.each(&->((a, b)) { p [a, b] }).

We recommend always passing the block explicitly, and prefer two required arguments as block parameters.

To learn more, see Ruby issue 12706.

Symbol#to_proc returns signature metadata consistent with lambdas

A common idiom in Ruby is to obtain Proc objects using the &:<symbol> shorthand and pass them to higher-order functions: 

[1, 2, 3].each(&:to_s)

Ruby desugars &:<symbol> to Symbol#to_proc. We can call it with the method receiver as its first argument (here: Integer), and all method arguments (here: none) as its remaining arguments.

This behaves the same in both Ruby 2.7 and Ruby 3. Where Ruby 3 diverges is when capturing this Proc object and inspecting its call signature. This is often done when writing DSLs or using other forms of meta-programming: 

p = :foo.to_proc # This usually happens via a conversion through `&:foo`

# Ruby 2.7: prints [[:rest]] (-1)
# Ruby 3.0: prints [[:req], [:rest]] (-2)
puts "#{p.parameters} (#{p.arity})"

Ruby 2.7 reports zero required and one optional parameter for this Proc object, while Ruby 3 reports one required and one optional parameter. Ruby 2.7 is incorrect: the first argument must always be passed, as it is the receiver of the method the Proc object represents, and methods cannot be called without a receiver.

Ruby 3 corrects this: the code that tests Proc object arity or parameter lists might now break and has to be updated.

To learn more, see Ruby issue 16260.

OpenStruct does not evaluate fields lazily

The OpenStruct implementation has undergone a partial rewrite in Ruby 3, resulting in behavioral changes. In Ruby 2.7, OpenStruct defines methods lazily, when the method is first accessed. In Ruby 3.0, it defines these methods eagerly in the initializer, which can break classes that inherit from OpenStruct and override these methods.

Don’t inherit from OpenStruct for these reasons; ideally, don’t use it at all. OpenStruct is considered problematic. When writing new code, prefer a Struct instead, which is simpler in implementation, although less flexible.

Regexp and Range instances are frozen

It is not necessary anymore to explicitly freeze Regexp or Range instances because Ruby 3 freezes them automatically upon creation.

This has a subtle side-effect: Tests that stub method calls on these types now fail with an error because RSpec cannot stub frozen objects: 

# Ruby 2.7: works
# Ruby 3.0: error: "can't modify frozen object"
allow(subject.function_returning_range).to receive(:max).and_return(42)

Rewrite affected tests by not stubbing method calls on frozen objects. The example above can be rewritten as: 

# Works with any Ruby version
allow(subject).to receive(:function_returning_range).and_return(1..42)

Table tests fail with Ruby 3.0.2

Ruby 3.0.2 has a known bug that causes table tests to fail when table values consist of integer values. The reasons are documented in issue 337614. This problem has been fixed in Ruby and the fix is expected to be included in Ruby 3.0.3.

The problem only affects users who run an unpatched Ruby 3.0.2. This is likely the case when you installed Ruby manually or via tools like asdf. Users of the gitlab-development-kit (GDK) are also affected by this problem.

Build images are not affected because they include the patch set addressing this bug.

Deprecations are not caught in DeprecationToolkit if the method is stubbed

We rely on deprecation_toolkit to fail fast when using functionality that is deprecated in Ruby 2 and removed in Ruby 3. A common issue caught during the transition from Ruby 2 to Ruby 3 relates to the separation of positional and keyword arguments in Ruby 3.0.

Unfortunately, if the author has stubbed such methods in tests, deprecations would not be caught. We run automated detection for this warning in tests via deprecation_toolkit, but it relies on the fact that Kernel#warn emits a warning, so stubbing out this call will effectively remove the call to warn, which means deprecation_toolkit will never see the deprecation warnings. Stubbing out the implementation removes that warning, and we never pick it up, so the build is green.

Please refer to issue 364099 for more context.

Testing in irb and rails console

Another pitfall is that testing in irb/rails c silences the deprecation warning, since irb in Ruby 2.7.x has a bug that prevents deprecation warnings from showing.

When writing code and performing code reviews, pay extra attention to method calls of the form f({k: v}). This is valid in Ruby 2 when f takes either a Hash or keyword arguments, but Ruby 3 only considers this valid if f takes a Hash. For Ruby 3 compliance, this should be changed to one of the following invocations if f takes keyword arguments:

  • f(**{k: v})
  • f(k: v)

RSpec with argument matcher fails for shorthand Hash syntax

Because keyword arguments (“kwargs”) are a first-class concept in Ruby 3, keyword arguments are not converted into internal Hash instances anymore. This leads to RSpec method argument matchers failing when the receiver takes a positional options hash instead of kwargs: 

def m(options={}); end

expect(subject).to receive(:m).with(a: 42)

In Ruby 3 this expectations fails with the following error: 

  Failure/Error:

     #<subject> received :m with unexpected arguments
       expected: ({:a=>42})
            got: ({:a=>42})

This happens because RSpec uses a kwargs argument matcher here, but the method takes a hash. It works in Ruby 2, because a: 42 is converted to a hash first and RSpec will use a hash argument matcher.

A workaround is to not use the shorthand syntax and pass an actual Hash instead whenever we know a method to take an options hash: 

# Note the braces around the key-value pair.
expect(subject).to receive(:m).with({ a: 42 })

For more information, see the official issue report for RSpec.