var!/2

.ex

defmodule Foo do

  defmacro foo(do: block) do
    quote do
      var!(foo, Foo) = "foo"
      unquote(block)
    end
  end

end

defmodule Runner do
  require Foo

  def fun do
    Foo.foo do
      IO.puts var!(foo, Foo)
    end
    IO.puts var!(foo, Foo)
  end

  def run do
    IO.puts var!(foo, Foo) # (CompileError) expected var "foo" (context Foo) to expand to an existing variable or be part of a match
  end

end

Runner.fun
Runner.run