Dynamically load rule documentation

spec/ameba/base_spec.cr

@@ -67,5 +67,11 @@ module Ameba::Rule
         rule.excluded?(Source.new "", "source.cr").should be_false
       end
     end
+
+    describe ".parsed_doc" do
+      it "returns the parsed rule doc" do
+        DummyRule.parsed_doc.should eq "Dummy Rule which does nothing."
+      end
+    end
   end
 end

spec/spec_helper.cr

@@ -2,6 +2,7 @@ require "spec"
 require "../src/ameba"
 
 module Ameba
+  # Dummy Rule which does nothing.
   struct DummyRule < Rule::Base
     properties do
       description : String = "Dummy rule that does nothing."

src/ameba/rule/base.cr

@@ -120,6 +120,56 @@ module Ameba::Rule
     protected def self.subclasses
       {{ @type.subclasses }}
     end
+
+    macro inherited
+      protected def self.path_to_source_file
+        __FILE__
+      end
+    end
+
+    # Returns documentation for this rule if any.
+    #
+    # ```
+    # module Ameba
+    #   # This is a test rule.
+    #   # Does nothing.
+    #   struct MyRule < Ameba::Rule::Base
+    #     def test(source)
+    #     end
+    #   end
+    # end
+    #
+    # MyRule.parsed_doc # => "This is a test rule.\nDoes nothing."
+    # ```
+    def self.parsed_doc
+      source = File.read(path_to_source_file)
+      nodes = Crystal::Parser.new(source).tap(&.wants_doc = true).parse
+      type_name = rule_name.split("/").last?
+      DocFinder.new(nodes, type_name).doc
+    end
+
+    # :nodoc:
+    private class DocFinder < Crystal::Visitor
+      getter doc : String?
+      getter type_name : String?
+
+      def initialize(nodes, @type_name)
+        self.accept(nodes)
+      end
+
+      def visit(node : Crystal::ASTNode)
+        return false if @doc
+
+        if node.responds_to?(:name) &&
+           (name = node.name) &&
+           name.is_a?(Crystal::Path) &&
+           name.names.last? == @type_name
+          @doc = node.doc
+        end
+
+        true
+      end
+    end
   end
 
   # Returns a list of all available rules.