If I understand right you want to format in dev mode and therefore you can access Mix. In this case you can use this:

> iex -S mix
iex(1)> {formatter, _opts} = Mix.Tasks.Format.formatter_for_file("source.ex")
{#Function<19.89531025/1 in Mix.Tasks.Format.find_formatter_for_file/2>,
 [
   sigils: [],
   plugins: [FreedomFormatter],
   trailing_comma: true,
   inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
   locals_without_parens: [noop: 1]
 ]}
iex(2)> formatter.("""
...(2)> defmodule    Foo do
...(2)> def foo do
...(2)> noop 5
...(2)> [
...(2)> 1,
...(2)> 2,
...(2)> ]
...(2)> end
...(2)> end
...(2)> """) |> IO.puts()
defmodule Foo do
  def foo do
    noop 5

    [
      1,
      2,
    ]
  end
end

:ok

The "source.ex" given to formatter_for_file does not have to be an existing file.