Introduce :rigid parsing mode

lib/liquid/environment.rb

@@ -34,7 +34,7 @@ module Liquid
       # @param file_system The default file system that is used
       #  to load templates from.
       # @param error_mode [Symbol] The default error mode for all templates
-      #  (either :strict, :warn, or :lax).
+      #  (either :strict, :warn, :lax, or :rigid).
       # @param exception_renderer [Proc] The exception renderer that is used to
       #   render exceptions.
       # @yieldparam environment [Environment] The environment instance that is being built.

lib/liquid/parse_context.rb

@@ -51,6 +51,26 @@ module Liquid
     end
 
     def parse_expression(markup)
+      if @error_mode == :rigid
+        parser = new_parser(markup)
+
+        # Return nil immediately if the markup is empty or contains only
+        # whitespaces
+        return if parser.look(:end_of_string)
+
+        expression_string = parser.expression
+
+        # In rigid mode, verify that all tokens have been consumed
+        #
+        # Extra tokens remaining after the expression indicate invalid syntaxes,
+        # such as: "product title" (instead of "product.title")
+        parser.consume(:end_of_string) unless parser.look(:end_of_string)
+
+        # Use Parser for strict token validation, but still return
+        # Expression objects for compatibility with the rendering pipeline.
+        markup = expression_string
+      end
+
       Expression.parse(markup, @string_scanner, @expression_cache)
     end
 

lib/liquid/parser_switching.rb

@@ -8,6 +8,8 @@ module Liquid
       case parse_context.error_mode
       when :strict
         raise
+      when :rigid
+        raise
       when :warn
         parse_context.warnings << e
       end
@@ -17,6 +19,7 @@ module Liquid
     def parse_with_selected_parser(markup)
       case parse_context.error_mode
       when :strict then strict_parse_with_error_context(markup)
+      when :rigid  then strict_parse_with_error_context(markup)
       when :lax    then lax_parse(markup)
       when :warn
         begin