Macro hygiene bypass for empty atom variable

[lukaszsamson]

Elixir and Erlang/OTP versions

Erlang/OTP 28 [erts-16.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.20.0-rc.4 (c1256f1) (compiled with Erlang/OTP 28)

Operating system

any

Current behavior

When translating var names to erlang if the variable is :"" it gets the same erlang name as _

elixir/lib/elixir/src/elixir_erl_var.erl

Lines 48 to 49 in 5d794ab

	build_name('_', Count) -> list_to_atom("_@" ++ integer_to_list(Count));
	build_name(Name, Count) -> list_to_atom("_" ++ atom_to_list(Name) ++ "@" ++ integer_to_list(Count)).

Which can lead to collision and macro hygiene bypass

This bug can be triggered in

defmodule Repro do
  defmacro collide(expr) do
    empty = Macro.var(:"", nil)
    quote do
      (fn x ->
        unquote(empty) = String.Chars.to_string(x)
        {unquote(empty), x}
      end).(unquote(expr))
    end
  end
end

defmodule UseRepro do
  require Repro
  def run(x), do: Repro.collide(x)
end

UseRepro.run(:abc)
** (MatchError) no match of right hand side value:

    "abc"

    iex:5: UseRepro.run/1
    iex:4: (file)

the same code works correctly if Macro.var(:"nonempty", nil) is used instead of Macro.var(:"", nil)

Expected behavior

Either forbid empty atom at AST expansion/translation time or normalize it