buffer-local-variables is a built-in function in `C source code'.

(buffer-local-variables &optional BUFFER)

Return an alist of variables that are buffer-local in BUFFER.
Most elements look like (SYMBOL . VALUE), describing one variable.
For a symbol that is locally unbound, just the symbol appears in the value.
Note that storing new VALUEs in these elements doesn't change the variables.
No argument or nil as argument means use current buffer as BUFFER.