Add Loc#source to display the source content form a RBI location

Signed-off-by: Alexandre Terrasa <alexandre.terrasa@shopify.com>

lib/rbi/loc.rb

@@ -32,5 +32,20 @@ def initialize(file: nil, begin_line: nil, end_line: nil, begin_column: nil, end
     def to_s
       "#{file}:#{begin_line}:#{begin_column}-#{end_line}:#{end_column}"
     end
+
+    sig { returns(T.nilable(String)) }
+    def source
+      file = self.file
+      return nil unless file
+      return nil unless ::File.file?(file)
+
+      return ::File.read(file) unless begin_line && end_line
+
+      string = String.new
+      ::File.foreach(file).with_index do |line, line_number|
+        string << line if line_number + 1 >= begin_line && line_number + 1 <= end_line
+      end
+      string
+    end
   end
 end

test/rbi/loc_test.rb

@@ -0,0 +1,69 @@
+# typed: true
+# frozen_string_literal: true
+
+require "test_helper"
+
+module RBI
+  class LocTest < Minitest::Test
+    extend T::Sig
+
+    TEST_FILE_PATH = "tmp/rbi/tests/source.rbi"
+
+    TEST_FILE_CONTENT = <<~RBI
+      # typed: true
+
+      class Foo
+        def foo; end
+      end
+
+      class Bar; end
+      class Baz; end
+    RBI
+
+    def setup
+      dir = ::File.dirname(TEST_FILE_PATH)
+      FileUtils.mkdir_p(dir)
+      ::File.write(TEST_FILE_PATH, TEST_FILE_CONTENT)
+    end
+
+    def teardown
+      FileUtils.rm_rf(TEST_FILE_PATH)
+    end
+
+    def test_loc_source_without_file_returns_nil
+      loc = Loc.new(file: nil)
+      assert_nil(loc.source)
+    end
+
+    def test_loc_source_with_unexisting_file_returns_nil
+      loc = Loc.new(file: "tmp/rbi/tests/not_found.rbi")
+      assert_nil(loc.source)
+    end
+
+    def test_loc_source_from_file_without_lines
+      loc = Loc.new(file: TEST_FILE_PATH)
+      assert_equal(TEST_FILE_CONTENT, loc.source)
+    end
+
+    def test_loc_source_from_file_between_lines
+      loc = Loc.new(file: TEST_FILE_PATH, begin_line: 3, end_line: 5)
+      assert_equal(<<~RBI, loc.source)
+        class Foo
+          def foo; end
+        end
+      RBI
+    end
+
+    def test_loc_source_from_file_at_line
+      loc = Loc.new(file: TEST_FILE_PATH, begin_line: 7, end_line: 7)
+      assert_equal(<<~RBI, loc.source)
+        class Bar; end
+      RBI
+    end
+
+    def test_loc_source_from_file_out_of_range
+      loc = Loc.new(file: TEST_FILE_PATH, begin_line: -10, end_line: 100)
+      assert_equal(TEST_FILE_CONTENT, loc.source)
+    end
+  end
+end