Adds Rubocop rule for line break after guard clause

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

2 files changed, 101 insertions, 0 deletions

diff --git a/rubocop/cop/line_break_after_guard_clauses.rb b/rubocop/cop/line_break_after_guard_clauses.rb
new file mode 100644
index 00000000000..67477f064ab
--- /dev/null
+++ b/rubocop/cop/line_break_after_guard_clauses.rb
@@ -0,0 +1,100 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # Ensures a line break after guard clauses.
+    #
+    # @example
+    #   # bad
+    #   return unless condition
+    #   do_stuff
+    #
+    #   # good
+    #   return unless condition
+    #
+    #   do_stuff
+    #
+    #   # bad
+    #   raise if condition
+    #   do_stuff
+    #
+    #   # good
+    #   raise if condition
+    #
+    #   do_stuff
+    #
+    #   Multiple guard clauses are allowed without
+    #   line break.
+    #
+    #   # good
+    #   return unless condition_a
+    #   return unless condition_b
+    #
+    #   do_stuff
+    #
+    #   Guard clauses in case statement are allowed without
+    #   line break.
+    #
+    #   # good
+    #   case model
+    #     when condition_a
+    #       return true unless condition_b
+    #     when
+    #       ...
+    #   end
+    #
+    #   Guard clauses before end are allowed without
+    #   line break.
+    #
+    #   # good
+    #   if condition_a
+    #     do_something
+    #   else
+    #     do_something_else
+    #     return unless condition
+    #   end
+    #
+    #   do_something_more
+    class LineBreakAfterGuardClauses < RuboCop::Cop::Cop
+      MSG = 'Add a line break after guard clauses'
+
+      def_node_matcher :guard_clause_node?, <<-PATTERN
+        [{(send nil? {:raise :fail :throw} ...) return break next} single_line?]
+      PATTERN
+
+      def on_if(node)
+        return unless node.single_line?
+        return unless guard_clause?(node)
+        return if next_line(node).blank? || clause_last_line?(next_line(node)) || guard_clause?(next_sibling(node))
+
+        add_offense(node, :expression, MSG)
+      end
+
+      def autocorrect(node)
+        lambda do |corrector|
+          corrector.insert_after(node.loc.expression, "\n")
+        end
+      end
+
+      private
+
+      def guard_clause?(node)
+        return false unless node.if_type?
+
+        guard_clause_node?(node.if_branch)
+      end
+
+      def next_sibling(node)
+        node.parent.children[node.sibling_index + 1]
+      end
+
+      def next_line(node)
+        processed_source[node.loc.line]
+      end
+
+      def clause_last_line?(line)
+        line =~ /^\s*(?:end|elsif|else|when|rescue|ensure)/
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 4ebbe010e90..0992168e1ff 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -3,6 +3,7 @@ require_relative 'cop/active_record_serialize'
 require_relative 'cop/custom_error_class'
 require_relative 'cop/gem_fetcher'
 require_relative 'cop/in_batches'
+require_relative 'cop/line_break_after_guard_clauses'
 require_relative 'cop/polymorphic_associations'
 require_relative 'cop/project_path_helper'
 require_relative 'cop/redirect_with_status'