Suggestion: Treat `ActiveModel::Attribute` as non-nilable if a PresenceValidator is present

class Foo
  include ActiveModel::Model

  attribute :name, :string
  validates :name, presence: true
end

Under normal circumstances, Foo#name should never be nil.

Yes, it's possible to get a nil like Foo.new.name, but I'd argue that returning a type error would make sense here.

I ended up implementing this tapioca extension in my codebase:

# sorbet/tapioca/compilers/active_model_presence_validator.rb

require "active_model/attributes"
require "tapioca/dsl/compilers/active_model_attributes"

module Tapioca
  module Compilers
    module ActiveModelPresenceValidator
      def attribute_methods_for_constant
        return super unless constant.is_a?(ActiveModel::Validations::ClassMethods)

        attribute_methods = super.to_h # Convert to hash for easier manipulation

        attribute_methods.each do |method, type|
          has_unconditional_validator = constant
            .validators_on(method)
            .grep(ActiveModel::Validations::PresenceValidator)
            # Skip if any options would mean that the attribute is not always non-nilable
            .any? { |v| v.options.slice(:if, :unless, :on, :allow_nil, :allow_blank).empty? }

          next unless has_unconditional_validator

          type = as_non_nilable_type(type)
          attribute_methods[method] = type
          attribute_methods["#{method}="] = type
        end

        attribute_methods.to_a
      end
    end

    Dsl::Compilers::ActiveModelAttributes.prepend(ActiveModelPresenceValidator)
  end
end

It's hacky, but it works 😅

Start with sorbet/tapioca/compilers/active_model_presence_validator.rb and the Tapioca::Dsl::Compilers::ActiveModelAttributes entry point shown in the issue. Inspect how validators and conditional options are represented, then verify generated RBI output for attributes with unconditional and conditional PresenceValidator instances. Done means unconditional presence validation produces non-nilable getter and setter types without changing conditional cases.