convert all the policies to DeclarativePolicy

app/models/ability.rb

@@ -56,24 +56,26 @@ class Ability
       end
     end
 
-    def allowed?(user, action, subject = :global)
-      allowed(user, subject).include?(action)
-    end
+    def allowed?(user, action, subject = :global, opts = {})
+      if subject.is_a?(Hash)
+        opts, subject = subject, :global
+      end
 
-    def allowed(user, subject = :global)
-      return BasePolicy::RuleSet.none if subject.nil?
-      return uncached_allowed(user, subject) unless RequestStore.active?
+      policy = policy_for(user, subject)
 
-      user_key = user ? user.id : 'anonymous'
-      subject_key = subject == :global ? 'global' : "#{subject.class.name}/#{subject.id}"
-      key = "/ability/#{user_key}/#{subject_key}"
-      RequestStore[key] ||= uncached_allowed(user, subject).freeze
+      case opts[:scope]
+      when :user
+        DeclarativePolicy.user_scope { policy.can?(action) }
+      when :subject
+        DeclarativePolicy.subject_scope { policy.can?(action) }
+      else
+        policy.can?(action)
+      end
     end
 
-    private
-
-    def uncached_allowed(user, subject)
-      BasePolicy.class_for(subject).abilities(user, subject)
+    def policy_for(user, subject = :global)
+      cache = RequestStore.active? ? RequestStore : {}
+      DeclarativePolicy.policy_for(user, subject, cache: cache)
     end
   end
 end

app/models/project_feature.rb

@@ -57,8 +57,11 @@ class ProjectFeature < ActiveRecord::Base
   end
 
   def feature_available?(feature, user)
-    access_level = public_send(ProjectFeature.access_level_attribute(feature))
-    get_permission(user, access_level)
+    get_permission(user, access_level(feature))
+  end
+
+  def access_level(feature)
+    public_send(ProjectFeature.access_level_attribute(feature))
   end
 
   def builds_enabled?

app/policies/base_policy.rb

-class BasePolicy
-  class RuleSet
-    attr_reader :can_set, :cannot_set
-    def initialize(can_set, cannot_set)
-      @can_set = can_set
-      @cannot_set = cannot_set
-    end
+require 'declarative_policy'
 
-    delegate :size, to: :to_set
+class BasePolicy < DeclarativePolicy::Base
+  desc "User is an instance admin"
+  with_options scope: :user, score: 0
+  condition(:admin) { @user&.admin? }
 
-    def self.empty
-      new(Set.new, Set.new)
-    end
+  with_options scope: :user, score: 0
+  condition(:external_user) { @user.nil? || @user.external? }
 
-    def self.none
-      empty.freeze
-    end
-
-    def can?(ability)
-      @can_set.include?(ability) && !@cannot_set.include?(ability)
-    end
-
-    def include?(ability)
-      can?(ability)
-    end
-
-    def to_set
-      @can_set - @cannot_set
-    end
-
-    def merge(other)
-      @can_set.merge(other.can_set)
-      @cannot_set.merge(other.cannot_set)
-    end
-
-    def can!(*abilities)
-      @can_set.merge(abilities)
-    end
-
-    def cannot!(*abilities)
-      @cannot_set.merge(abilities)
-    end
-
-    def freeze
-      @can_set.freeze
-      @cannot_set.freeze
-      super
-    end
-  end
-
-  def self.abilities(user, subject)
-    new(user, subject).abilities
-  end
-
-  def self.class_for(subject)
-    return GlobalPolicy if subject == :global
-    raise ArgumentError, 'no policy for nil' if subject.nil?
-
-    if subject.class.try(:presenter?)
-      subject = subject.subject
-    end
-
-    subject.class.ancestors.each do |klass|
-      next unless klass.name
-
-      begin
-        policy_class = "#{klass.name}Policy".constantize
-
-        # NOTE: the < operator here tests whether policy_class
-        # inherits from BasePolicy
-        return policy_class if policy_class < BasePolicy
-      rescue NameError
-        nil
-      end
-    end
-
-    raise "no policy for #{subject.class.name}"
-  end
-
-  attr_reader :user, :subject
-  def initialize(user, subject)
-    @user = user
-    @subject = subject
-  end
-
-  def abilities
-    return RuleSet.none if @user && @user.blocked?
-    return anonymous_abilities if @user.nil?
-    collect_rules { rules }
-  end
-
-  def anonymous_abilities
-    collect_rules { anonymous_rules }
-  end
-
-  def anonymous_rules
-    rules
-  end
-
-  def rules
-    raise NotImplementedError
-  end
-
-  def delegate!(new_subject)
-    @rule_set.merge(Ability.allowed(@user, new_subject))
-  end
-
-  def can?(rule)
-    @rule_set.can?(rule)
-  end
-
-  def can!(*rules)
-    @rule_set.can!(*rules)
-  end
-
-  def cannot!(*rules)
-    @rule_set.cannot!(*rules)
-  end
-
-  private
-
-  def collect_rules(&b)
-    @rule_set = RuleSet.empty
-    yield
-    @rule_set
-  end
+  with_options scope: :user, score: 0
+  condition(:can_create_group) { @user&.can_create_group }
 end

app/policies/ci/build_policy.rb

 module Ci
   class BuildPolicy < CommitStatusPolicy
-    alias_method :build, :subject
-
-    def rules
-      super
-
-      # If we can't read build we should also not have that
-      # ability when looking at this in context of commit_status
-      %w[read create update admin].each do |rule|
-        cannot! :"#{rule}_commit_status" unless can? :"#{rule}_build"
-      end
-
-      if can?(:update_build) && protected_action?
-        cannot! :update_build
-      end
-    end
-
-    private
-
-    def protected_action?
-      return false unless build.action?
+    condition(:protected_action) do
+      next false unless @subject.action?
 
       !::Gitlab::UserAccess
-        .new(user, project: build.project)
-        .can_merge_to_branch?(build.ref)
+        .new(@user, project: @subject.project)
+        .can_merge_to_branch?(@subject.ref)
     end
+
+    rule { protected_action }.prevent :update_build
   end
 end

app/policies/ci/pipeline_policy.rb

 module Ci
   class PipelinePolicy < BasePolicy
-    def rules
-      delegate! @subject.project
-    end
+    delegate { @subject.project }
   end
 end

app/policies/ci/runner_policy.rb

 module Ci
   class RunnerPolicy < BasePolicy
-    def rules
-      return unless @user
+    with_options scope: :subject, score: 0
+    condition(:shared) { @subject.is_shared? }
 
-      can! :assign_runner if @user.admin?
+    with_options scope: :subject, score: 0
+    condition(:locked, scope: :subject) { @subject.locked? }
 
-      return if @subject.is_shared? || @subject.locked?
+    condition(:authorized_runner) { @user.ci_authorized_runners.include?(@subject) }
 
-      can! :assign_runner if @user.ci_authorized_runners.include?(@subject)
-    end
+    rule { anonymous }.prevent_all
+    rule { admin | authorized_runner }.enable :assign_runner
+    rule { ~admin & shared }.prevent :assign_runner
+    rule { ~admin & locked }.prevent :assign_runner
   end
 end

app/policies/ci/trigger_policy.rb

 module Ci
   class TriggerPolicy < BasePolicy
-    def rules
-      delegate! @subject.project
-
-      if can?(:admin_build)
-        can! :admin_trigger if @subject.owner.blank? ||
-            @subject.owner == @user
-        can! :manage_trigger
-      end
-    end
+    delegate { @subject.project }
+
+    with_options scope: :subject, score: 0
+    condition(:legacy) { @subject.legacy? }
+
+    with_score 0
+    condition(:is_owner) { @user && @subject.owner_id == @user.id }
+
+    rule { ~can?(:admin_build) }.prevent :admin_trigger
+    rule { legacy | is_owner }.enable :admin_trigger
+
+    rule { can?(:admin_build) }.enable :manage_trigger
   end
 end

app/policies/commit_status_policy.rb

 class CommitStatusPolicy < BasePolicy
-  def rules
-    delegate! @subject.project
+  delegate { @subject.project }
+
+  %w[read create update admin].each do |action|
+    rule { ~can?(:"#{action}_commit_status") }.prevent :"#{action}_build"
   end
 end

app/policies/deploy_key_policy.rb

 class DeployKeyPolicy < BasePolicy
-  def rules
-    return unless @user
+  with_options scope: :subject, score: 0
+  condition(:private_deploy_key) { @subject.private? }
 
-    can! :update_deploy_key if @user.admin?
+  condition(:has_deploy_key) { @user.project_deploy_keys.exists?(id: @subject.id) }
 
-    if @subject.private? && @user.project_deploy_keys.exists?(id: @subject.id)
-      can! :update_deploy_key
-    end
-  end
+  rule { anonymous }.prevent_all
+
+  rule { admin }.enable :update_deploy_key
+  rule { private_deploy_key & has_deploy_key }.enable :update_deploy_key
 end

app/policies/deployment_policy.rb

 class DeploymentPolicy < BasePolicy
-  def rules
-    delegate! @subject.project
-  end
+  delegate { @subject.project }
 end

app/policies/environment_policy.rb

 class EnvironmentPolicy < BasePolicy
-  alias_method :environment, :subject
+  delegate { @subject.project }
 
-  def rules
-    delegate! environment.project
-
-    if can?(:create_deployment) && environment.stop_action?
-      can! :stop_environment if can_play_stop_action?
-    end
+  condition(:stop_action_allowed) do
+    @subject.stop_action? && can?(:update_build, @subject.stop_action)
   end
 
-  private
-
-  def can_play_stop_action?
-    Ability.allowed?(user, :update_build, environment.stop_action)
-  end
+  rule { can?(:create_deployment) & stop_action_allowed }.enable :stop_environment
 end

app/policies/external_issue_policy.rb

 class ExternalIssuePolicy < BasePolicy
-  def rules
-    delegate! @subject.project
-  end
+  delegate { @subject.project }
 end

app/policies/global_policy.rb

 class GlobalPolicy < BasePolicy
-  def rules
-    return unless @user
+  desc "User is blocked"
+  with_options scope: :user, score: 0
+  condition(:blocked) { @user.blocked? }
 
-    can! :create_group if @user.can_create_group
-    can! :read_users_list
+  desc "User is an internal user"
+  with_options scope: :user, score: 0
+  condition(:internal) { @user.internal? }
 
-    unless @user.blocked? || @user.internal?
-      can! :log_in unless @user.access_locked?
-      can! :access_api
-      can! :access_git
-      can! :receive_notifications
-      can! :use_quick_actions
-    end
+  desc "User's access has been locked"
+  with_options scope: :user, score: 0
+  condition(:access_locked) { @user.access_locked? }
+
+  rule { anonymous }.prevent_all
+
+  rule { default }.policy do
+    enable :read_users_list
+    enable :log_in
+    enable :access_api
+    enable :access_git
+    enable :receive_notifications
+    enable :use_quick_actions
+  end
+
+  rule { blocked | internal }.policy do
+    prevent :log_in
+    prevent :access_api
+    prevent :access_git
+    prevent :receive_notifications
+    prevent :use_quick_actions
+  end
+
+  rule { can_create_group }.policy do
+    enable :create_group
+  end
+
+  rule { access_locked }.policy do
+    prevent :log_in
   end
 end

app/policies/group_label_policy.rb

 class GroupLabelPolicy < BasePolicy
-  def rules
-    delegate! @subject.group
-  end
+  delegate { @subject.group }
 end

app/policies/group_member_policy.rb

 class GroupMemberPolicy < BasePolicy
-  def rules
-    return unless @user
+  delegate :group
 
-    target_user = @subject.user
-    group = @subject.group
+  with_scope :subject
+  condition(:last_owner) { @subject.group.last_owner?(@subject.user) }
 
-    return if group.last_owner?(target_user)
+  desc "Membership is users' own"
+  with_score 0
+  condition(:is_target_user) { @user && @subject.user_id == @user.id }
 
-    can_manage = Ability.allowed?(@user, :admin_group_member, group)
+  rule { anonymous }.prevent_all
+  rule { last_owner }.prevent_all
 
-    if can_manage
-      can! :update_group_member
-      can! :destroy_group_member
-    elsif @user == target_user
-      can! :destroy_group_member
-    end
-
-    additional_rules!
+  rule { can?(:admin_group_member) }.policy do
+    enable :update_group_member
+    enable :destroy_group_member
   end
 
-  def additional_rules!
-    can_override = Ability.allowed?(@user, :override_group_member, @subject.group)
-
-    if can_override
-      can! :override_group_member if @subject.ldap?
-      can! :update_group_member unless @subject.ldap? && !@subject.override?
-    end
+  rule { is_target_user }.policy do
+    enable :destroy_group_member
   end
 end

app/policies/group_policy.rb

 class GroupPolicy < BasePolicy
-  prepend EE::GroupPolicy
-
-  def rules
-    can! :read_group if @subject.public?
-    return unless @user
-
-    globally_viewable = @subject.public? || (@subject.internal? && !@user.external?)
-    access_level = @subject.max_member_access_for_user(@user)
-    owner = access_level >= GroupMember::OWNER
-    master = access_level >= GroupMember::MASTER
-    reporter = access_level >= GroupMember::REPORTER
-
-    can_read = false
-    can_read ||= globally_viewable
-    can_read ||= @user.admin?
-    can_read ||= @user.auditor?
-    can_read ||= access_level >= GroupMember::GUEST
-    can_read ||= GroupProjectsFinder.new(group: @subject, current_user: @user).execute.any?
-    can! :read_group if can_read
-
-    if reporter
-      can! :admin_label
-    end
-
-    # Only group masters and group owners can create new projects
-    if master
-      can! :create_projects
-      can! :admin_milestones
-    end
-
-    # Only group owner and administrators can admin group
-    if owner
-      can! :admin_group
-      can! :admin_namespace
-      can! :admin_group_member
-      can! :change_visibility_level
-      can! :create_subgroup if @user.can_create_group
-    end
-
-    if globally_viewable && @subject.request_access_enabled && access_level == GroupMember::NO_ACCESS
-      can! :request_access
-    end
-  end
+  desc "Group is public"
+  with_options scope: :subject, score: 0
+  condition(:public_group) { @subject.public? }
+
+  with_score 0
+  condition(:logged_in_viewable) { @user && @subject.internal? && !@user.external? }
+
+  condition(:has_access) { access_level != GroupMember::NO_ACCESS }
 
-  def can_read_group?
-    return true if @subject.public?
-    return true if @user.admin?
-    return true if @user.auditor?
-    return true if @subject.internal? && !@user.external?
-    return true if @subject.users.include?(@user)
+  condition(:guest) { access_level >= GroupMember::GUEST }
+  condition(:owner) { access_level >= GroupMember::OWNER }
+  condition(:master) { access_level >= GroupMember::MASTER }
+  condition(:reporter) { access_level >= GroupMember::REPORTER }
 
+  condition(:has_projects) do
     GroupProjectsFinder.new(group: @subject, current_user: @user).execute.any?
   end
+
+  with_options scope: :subject, score: 0
+  condition(:request_access_enabled) { @subject.request_access_enabled }
+
+  rule { public_group }      .enable :read_group
+  rule { logged_in_viewable }.enable :read_group
+  rule { guest }             .enable :read_group
+  rule { admin }             .enable :read_group
+  rule { has_projects }      .enable :read_group
+
+  rule { reporter }.enable :admin_label
+
+  rule { master }.policy do
+    enable :create_projects
+    enable :admin_milestones
+  end
+
+  rule { owner }.policy do
+    enable :admin_group
+    enable :admin_namespace
+    enable :admin_group_member
+    enable :change_visibility_level
+  end
+
+  rule { owner & can_create_group }.enable :create_subgroup
+
+  rule { public_group | logged_in_viewable }.enable :view_globally
+
+  rule { default }.enable(:request_access)
+
+  rule { ~request_access_enabled }.prevent :request_access
+  rule { ~can?(:view_globally) }.prevent   :request_access
+  rule { has_access }.prevent              :request_access
+
+  def access_level
+    return GroupMember::NO_ACCESS if @user.nil?
+
+    @access_level ||= @subject.max_member_access_for_user(@user)
+  end
 end

app/policies/issuable_policy.rb

 class IssuablePolicy < BasePolicy
-  def action_name
-    @subject.class.name.underscore
-  end
+  delegate { @subject.project }
 
-  def rules
-    if @user && @subject.assignee_or_author?(@user)
-      can! :"read_#{action_name}"
-      can! :"update_#{action_name}"
-    end
+  desc "User is the assignee or author"
+  condition(:assignee_or_author) do
+    @user && @subject.assignee_or_author?(@user)
+  end
 
-    delegate! @subject.project
+  rule { assignee_or_author }.policy do
+    enable :read_issue
+    enable :update_issue
+    enable :read_merge_request
+    enable :update_merge_request
   end
 end

app/policies/issue_policy.rb

@@ -3,25 +3,17 @@ class IssuePolicy < IssuablePolicy
   # Make sure to sync this class checks with issue.rb to avoid security problems.
   # Check commit 002ad215818450d2cbbc5fa065850a953dc7ada8 for more information.
 
-  def issue
-    @subject
+  desc "User can read confidential issues"
+  condition(:can_read_confidential) do
+    @user && IssueCollection.new([@subject]).visible_to(@user).any?
   end
 
-  def rules
-    super
+  desc "Issue is confidential"
+  condition(:confidential, scope: :subject) { @subject.confidential? }
 
-    if @subject.confidential? && !can_read_confidential?
-      cannot! :read_issue
-      cannot! :update_issue
-      cannot! :admin_issue
-    end
-  end
-
-  private
-
-  def can_read_confidential?
-    return false unless @user
-
-    IssueCollection.new([@subject]).visible_to(@user).any?
+  rule { confidential & ~can_read_confidential }.policy do
+    prevent :read_issue
+    prevent :update_issue
+    prevent :admin_issue
   end
 end

app/policies/namespace_policy.rb

 class NamespacePolicy < BasePolicy
-  def rules
-    return unless @user
+  rule { anonymous }.prevent_all
 
-    if @subject.owner == @user || @user.admin?
-      can! :create_projects
-      can! :admin_namespace
-    end
+  condition(:owner) { @subject.owner == @user }
+
+  rule { owner | admin }.policy do
+    enable :create_projects
+    enable :admin_namespace
   end
 end

app/policies/nil_policy.rb

+class NilPolicy < BasePolicy
+  rule { default }.prevent_all
+end

app/policies/note_policy.rb

 class NotePolicy < BasePolicy
-  def rules
-    delegate! @subject.project
+  delegate { @subject.project }
 
-    return unless @user
+  condition(:is_author) { @user && @subject.author == @user }
+  condition(:for_merge_request, scope: :subject) { @subject.for_merge_request? }
+  condition(:is_noteable_author) { @user && @subject.noteable.author_id == @user.id }
 
-    if @subject.author == @user
-      can! :read_note
-      can! :update_note
-      can! :admin_note
-      can! :resolve_note
-    end
+  condition(:editable, scope: :subject) { @subject.editable? }
 
-    if @subject.for_merge_request? &&
-        @subject.noteable.author == @user
-      can! :resolve_note
-    end
+  rule { ~editable | anonymous }.prevent :edit_note
+  rule { is_author | admin }.enable :edit_note
+  rule { can?(:master_access) }.enable :edit_note
+
+  rule { is_author }.policy do
+    enable :read_note
+    enable :update_note
+    enable :admin_note
+    enable :resolve_note
+  end
+
+  rule { for_merge_request & is_noteable_author }.policy do
+    enable :resolve_note
   end
 end

app/policies/personal_snippet_policy.rb

 class PersonalSnippetPolicy < BasePolicy
-  def rules
-    can! :read_personal_snippet if @subject.public?
-    return unless @user
+  condition(:public_snippet, scope: :subject) { @subject.public? }
+  condition(:is_author) { @user && @subject.author == @user }
+  condition(:internal_snippet, scope: :subject) { @subject.internal? }
 
-    if @subject.public?
-      can! :comment_personal_snippet
-    end
+  rule { public_snippet }.policy do
+    enable :read_personal_snippet
+    enable :comment_personal_snippet
+  end
 
-    if @subject.author == @user
-      can! :read_personal_snippet
-      can! :update_personal_snippet
-      can! :destroy_personal_snippet
-      can! :admin_personal_snippet
-      can! :comment_personal_snippet
-    end
+  rule { is_author }.policy do
+    enable :read_personal_snippet
+    enable :update_personal_snippet
+    enable :destroy_personal_snippet
+    enable :admin_personal_snippet
+    enable :comment_personal_snippet
+  end
 
-    unless @user.external?
-      can! :create_personal_snippet
-    end
+  rule { ~anonymous }.enable :create_personal_snippet
+  rule { external_user }.prevent :create_personal_snippet
 
-    if @subject.internal? && !@user.external?
-      can! :read_personal_snippet
-      can! :comment_personal_snippet
-    end
+  rule { internal_snippet & ~external_user }.policy do
+    enable :read_personal_snippet
+    enable :comment_personal_snippet
   end
+
+  rule { anonymous }.prevent :comment_personal_snippet
 end

app/policies/project_label_policy.rb

 class ProjectLabelPolicy < BasePolicy
-  def rules
-    delegate! @subject.project
-  end
+  delegate { @subject.project }
 end

app/policies/project_member_policy.rb

 class ProjectMemberPolicy < BasePolicy
-  def rules
-    # anonymous users have no abilities here
-    return unless @user
+  delegate { @subject.project }
 
-    target_user = @subject.user
-    project = @subject.project
+  condition(:target_is_owner, scope: :subject) { @subject.user == @subject.project.owner }
+  condition(:target_is_self) { @user && @subject.user == @user }
 
-    return if target_user == project.owner
+  rule { anonymous }.prevent_all
+  rule { target_is_owner }.prevent_all
 
-    can_manage = Ability.allowed?(@user, :admin_project_member, project)
-
-    if can_manage
-      can! :update_project_member
-      can! :destroy_project_member
-    end
-
-    if @user == target_user
-      can! :destroy_project_member
-    end
+  rule { can?(:admin_project_member) }.policy do
+    enable :update_project_member
+    enable :destroy_project_member
   end
+
+  rule { target_is_self }.enable :destroy_project_member
 end

app/policies/project_snippet_policy.rb

 class ProjectSnippetPolicy < BasePolicy
-  def rules
-    # We have to check both project feature visibility and a snippet visibility and take the stricter one
-    # This will be simplified - check https://gitlab.com/gitlab-org/gitlab-ce/issues/27573
-    return unless @subject.project.feature_available?(:snippets, @user)
-    return unless Ability.allowed?(@user, :read_project, @subject.project)
-
-    can! :read_project_snippet if @subject.public?
-    return unless @user
-
-    if @user && (@subject.author == @user || @user.admin?)
-      can! :read_project_snippet
-      can! :update_project_snippet
-      can! :admin_project_snippet
-    end
-
-    if @user.auditor?
-      can! :read_project_snippet
-    end
-
-    if @subject.internal? && !@user.external?
-      can! :read_project_snippet
-    end
-
-    if @subject.project.team.member?(@user)
-      can! :read_project_snippet
-    end
+  delegate :project
+
+  desc "Snippet is public"
+  condition(:public_snippet, scope: :subject) { @subject.public? }
+  condition(:private_snippet, scope: :subject) { @subject.private? }
+  condition(:public_project, scope: :subject) { @subject.project.public? }
+
+  condition(:is_author) { @user && @subject.author == @user }
+
+  condition(:internal, scope: :subject) { @subject.internal? }
+
+  # We have to check both project feature visibility and a snippet visibility and take the stricter one
+  # This will be simplified - check https://gitlab.com/gitlab-org/gitlab-ce/issues/27573
+  rule { ~can?(:read_project) }.policy do
+    prevent :read_project_snippet
+    prevent :update_project_snippet
+    prevent :admin_project_snippet
+  end
+
+  # we have to use this complicated prevent because the delegated project policy
+  # is overly greedy in allowing :read_project_snippet, since it doesn't have any
+  # information about the snippet. However, :read_project_snippet on the *project*
+  # is used to hide/show various snippet-related controls, so we can't just move
+  # all of the handling here.
+  rule do
+    all?(private_snippet | (internal & external_user),
+         ~project.guest,
+         ~admin,
+         ~is_author)
+  end.prevent :read_project_snippet
+
+  rule { internal & ~is_author & ~admin }.policy do
+    prevent :update_project_snippet
+    prevent :admin_project_snippet
+  end
+
+  rule { public_snippet }.enable :read_project_snippet
+
+  rule { is_author | admin }.policy do
+    enable :read_project_snippet
+    enable :update_project_snippet
+    enable :admin_project_snippet
   end
 end

app/policies/user_policy.rb

 class UserPolicy < BasePolicy
   include Gitlab::CurrentSettings
 
-  def rules
-    can! :read_user if @user || !restricted_public_level?
+  desc "The application is restricted from public visibility"
+  condition(:restricted_public_level, scope: :global) do
+    current_application_settings.restricted_visibility_levels.include?(Gitlab::VisibilityLevel::PUBLIC)
+  end
 
-    if @user
-      if @user.admin? || @subject == @user
-        can! :destroy_user
-      end
+  desc "The current user is the user in question"
+  condition(:user_is_self, score: 0) { @subject == @user }
 
-      cannot! :destroy_user if @subject.ghost?
-    end
-  end
+  desc "This is the ghost user"
+  condition(:subject_ghost, scope: :subject, score: 0) { @subject.ghost? }
 
-  def restricted_public_level?
-    current_application_settings.restricted_visibility_levels.include?(Gitlab::VisibilityLevel::PUBLIC)
-  end
+  rule { ~restricted_public_level }.enable :read_user
+  rule { ~anonymous }.enable :read_user
+
+  rule { user_is_self | admin }.enable :destroy_user
+  rule { subject_ghost }.prevent :destroy_user
 end

lib/gitlab/allowable.rb

 module Gitlab
   module Allowable
-    def can?(user, action, subject = :global)
-      Ability.allowed?(user, action, subject)
+    def can?(*args)
+      Ability.allowed?(*args)
     end
   end
 end

spec/policies/project_policy_spec.rb

@@ -167,8 +167,8 @@ describe ProjectPolicy, models: true do
         end
 
         it do
-          is_expected.not_to include(:read_build)
-          is_expected.to include(:read_pipeline)
+          expect_disallowed(:read_build)
+          expect_allowed(:read_pipeline)
         end
       end
     end

spec/policies/project_snippet_policy_spec.rb

@@ -129,7 +129,7 @@ describe ProjectSnippetPolicy, models: true do
     context 'snippet author' do
       let(:snippet) { create(:project_snippet, :private, author: regular_user, project: project) }
 
-      subject { described_class(regular_user, snippet) }
+      subject { described_class.new(regular_user, snippet) }
 
       it do
         expect_allowed(:read_project_snippet)