I’m curious about this question, too.

Just created a small demo:

defmodule Demo do
  @topic "chat:#{chat_id}"

  def debug() do
    IO.inspect(@topic)
  end
end

It shows:

$> iex test.ex
Erlang/OTP 25 [erts-13.0.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]

warning: variable "chat_id" does not exist and is being expanded to "chat_id()", please use parentheses to remove the ambiguity or change the variable name
  test.ex:2: Demo

** (CompileError) test.ex:2: undefined function chat_id/0 (there is no such import)
    (elixir 1.13.4) expanding macro: Kernel.to_string/1
    test.ex:2: Demo (module)
    (elixir 1.13.4) expanding macro: Kernel.@/1
    test.ex:2: Demo (module)