Avoid an expression match on a dynamic variable

[dylanahsmith]

Problem

Shopify/liquid#1300 introduced an error in liquid-c's unit tests:

  1) Failure:
VariableTest#test_literals [/Users/dylants/src/liquid-c/test/unit/variable_test.rb:37]:
--- expected
+++ actual
@@ -1 +1 @@
-[#<Liquid::VariableLookup:0xXXXXXX @name="", @lookups=[], @command_flags=0>, []]
+[nil, []]

Which is basically saying that [blank] is being parsed as having a nil expression instead of being the expected variable lookup for a blank string.

The problem is that an Expression::LITERALS lookup was being done twice on a square bracket variable lookup in liquid-c, once on the expression inside the square brackets (blank) and another time on the resulting expression. Only the former is done by the liquid gem (in Expression.parse).

Shopify/liquid#1300 caused this test to fail because there is an Expression::LITERALS mapping "blank" => "" and another mapping "" => nil. Previously, the first lookup would return a MethodLiteral object, which there wasn't a second mapping for in Expression::LITERALS.

Note that the assign tag doesn't even allow a variable to be assigned with an empty string for a name, it requires at least one character for the name.

Solution

Use a boolean flag to make sure we are only doing this Expression::LITERALS lookup when we haven't just parsed square brackets around the variable name.

Note that #60 will remove parse_variable method, which is why I wasn't seeing the failure when testing on that liquid-c branch.