lexical-binding

lexical-binding is a variable defined in `C source code'.
Its value is

nil

• Automatically becomes buffer-local when set.
• This variable is safe as a file local variable if its value
  satisfies the predicate `booleanp'.

Documentation:
Whether to use lexical binding when evaluating code.
Non-nil means that the code in the current buffer should be evaluated
with lexical binding.
This variable is automatically set from the file variables of an
interpreted Lisp file read using `load'. Unlike other file local
variables, this must be set in the first line of a file.