emit $eof token with eof...eof+1 range to properly render diagnostic

lib/parser/lexer.rl

@@ -300,7 +300,7 @@ class Parser::Lexer
       [ false, [ '$error'.freeze, range(p - 1, p) ] ]
     else
       eof = @source_pts.size
-      [ false, [ '$eof'.freeze,   range(eof, eof) ] ]
+      [ false, [ '$eof'.freeze,   range(eof, eof + 1) ] ]
     end
   end
 

test/test_diagnostic.rb

@@ -92,4 +92,16 @@ def test_bug_error_on_newline
       '(string):2:                                ^'
     ], diag.render)
   end
+
+  def test_render_eof_error
+    location = Parser::Source::Range.new(@buffer, @buffer.source.length, @buffer.source.length + 1)
+
+    diag = Parser::Diagnostic.new(:error, :unexpected_token, { token: '$end' }, location, [])
+
+    assert_equal([
+      '(string):1:34: error: unexpected token $end',
+      '(string):1: if (this is some bad code + bugs)',
+      '(string):1:                                  ^'
+    ], diag.render)
+  end
 end

test/test_lexer.rb

@@ -2620,7 +2620,7 @@ def test_float_suffix
   def test_eof
     assert_scanned("self",
                    :kSELF, "self", [0, 4])
-    assert_equal([false, ["$eof", Parser::Source::Range.new(@lex.source_buffer, 4, 4)]],
+    assert_equal([false, ["$eof", Parser::Source::Range.new(@lex.source_buffer, 4, 5)]],
                  @lex.advance)
   end