class Sass::Script::Parser

def is_safe_value?(expr)

or a list whose last element is also safe.
It's safe as long as the previous expression is an identifier or number,

interpolation.
Returns whether `expr` is safe as the value immediately before an 
def is_safe_value?(expr)
  return is_safe_value?(expr.elements.last) if expr.is_a?(Script::Tree::ListLiteral)
  return false unless expr.is_a?(Script::Tree::Literal)
  expr.value.is_a?(Script::Value::Number) ||
    (expr.value.is_a?(Script::Value::String) && expr.value.type == :identifier)
end