Fix method signatures being overwritten by prepended modules

[DaneHarrison]

Motivation

Fixes #2072 - When a module is prepended onto a class, Tapioca generates the RBI using the prepended module's signature instead of its own.

Implementation

compile_method receives methods obtained with Module#instance_method, which follows the ancestor chain. When a
module is prepended to a class and defines the method, instance_method returns the prepended module's UnboundMethod rather than the class's own, so compile_method was compiling the wrong method.

• method_owned_by_constant? becomes method_defined_by_constant. It walks super_method and returns the method the constant itself defines.

• New signature_defined_by_constant finds the sig declared on the constant's own method. Sorbet can file it under a prepended method instead, so it checks those too and only accepts a sig whose signature.method.owner matches.

• The attr_accessor writer inference resolves the constant's own reader first, so it works in either evaluation order.

• visibility_defined_by_constant gets the visibility from the constant's own method, so a prepended module's visibility doesn't leak into what we pass to compile_method.

Further Considerations:

• If both the class and the prepended module have a sig for the same name, Sorbet keeps only one per key. Whichever block runs last wins - see wrap_method_if_needed and unwrap_method
• There are other callers to signature_of i.e under dsl, I suspect those will need to be updated as well
  • Let me know if you'd like me to do a follow up PR on this

Tests

• No sig: a prepended module in front of the method, and multiple prepended modules
• Cross-gem prepend matching the issue, checking both gems' RBIs
• Sig with a prepended module that has different parameters
• Only the prepended module has a sig (the class's method must not take it)
• Sig evaluated before the prepend, and between two prepends
• attr_accessor writer inference with the reader's sig evaluated before and after the prepend
• A prepended module's sig that fails to load is reported once, against the module
• Skipped: both the class and the prepended module have a sig (Sorbet keeps only one)
• Sig on a singleton method with a module prepended to its singleton class
• A private method with a public prepended module in front of it (the class's method must stay private)

[DaneHarrison]

I have signed the CLA!

[KaanOzkan]

I tried to do this for DSL generation but it was too complex. I think this is acceptable.

[KaanOzkan]

Agent brought up that the method visibility can also be incorrectly attributed to the prepended method, could you add a test and a fix for this?

it "uses the constant's own method visibility instead of the prepended method's visibility" do
  add_ruby_file("foo.rb", <<~RUBY)
    module Foo
      def bar(x); end
    end

    class Baz
      prepend Foo

      private

      def bar; end
    end
  RUBY

  output = template(<<~RBI)
    class Baz
      include ::Foo

      private

      def bar; end
    end

    module Foo
      def bar(x); end
    end
  RBI

  assert_equal(output, compile)
end

[KaanOzkan]

I'm in support of removing this test, I don't see it being changed in the future.

[KaanOzkan]

Is this second call necessary?

[DaneHarrison]

I forgot to remove it after changing original_method. Fixed and good catch

[KaanOzkan]

This and the next test shouldn't say "signature" IMO since it's not Sorbet signatures. Maybe method definition?

[KaanOzkan]

Could you add a singleton equivalent for this?

it "compiles a singleton method using its own signature through a prepended module" do
  add_ruby_file("foo.rb", <<~RUBY)
    module Wrapper
      def bar(x, y); end
    end

    class Baz
      extend T::Sig

      sig { params(x: Integer).returns(Integer) }
      def self.bar(x)
        x
      end

      singleton_class.prepend(Wrapper)
    end
  RUBY

  output = template(<<~RBI)
    class Baz
      extend ::Wrapper

      class << self
        sig { params(x: ::Integer).returns(::Integer) }
        def bar(x); end
      end
    end

    module Wrapper
      def bar(x, y); end
    end
  RBI

  assert_equal(output, compile)
end

[DaneHarrison]

Thank you for the review! I addressed all comments in my last commit.
I'd like to continue contributing to the project. There are a bunch of help wanted issues, any you'd like me to pick up next?