Formatter: mix format - (stdin mode) pollutes stdout with compilation output

[rhblind]

Hello,

When I'm using mix format - to format code from stdin. If dependencies needs to be compiled, the compilation output is written to stdout alongside the formatted code.
This problem is very visible with editor integrations that relies on piping the output back into the file that's to be formatted (I'm using apheleia for Emacs.), and only expects the formatted code on stdout.

I'm not quite sure if this really is a Mix issue or editor formatting system issue, but I suspect that apheleia is not the only formatter that relies on pipes for getting the formatted code.

Elixir and Erlang/OTP versions

Elixir version

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

Elixir 1.19.4 (compiled with Erlang/OTP 28)

Operating system

macOS Tahoe 26.4 (25E246)

Current behavior

Steps to reproduce

mix phx.new foobar
cd foobar
mix deps.get
echo 'defmodule Test do end' | mix format -
...
1> Generated phoenix_live_reload app
3> Generated ecto_sql app
4> ==> swoosh
4> Compiling 59 files (.ex)
4> Generated swoosh app
2> Generated phoenix_live_view app
2> ==> phoenix_live_dashboard
2> Compiling 36 files (.ex)
2> Generated phoenix_live_dashboard app
defmodule Test do
end

• Compilation messages appears on both stdiout and stderr
• stdout contains both compilation output + the formatted code
• Makes it very hard to use piping for integrating with editors

Expected behavior

I'm not sure what the correct solution is, but maybe compilation messages could go only on stderr while the formatted code goes to stdout?

Another solution could be to add a flag to the format task that suppress all messages except the formatted code.

[rhblind]

Perfect, thanks ❤️