mix format raises File.Error when using .formatter.exs with import_deps in a different folder

[sascha-wolf]

Precheck

• Do not use the issues tracker for help or support (try Elixir Forum, Stack Overflow, IRC, etc.) ✅
• For bugs, do a quick search and make sure the bug has not yet been reported ✅
• Finally, be nice and have fun! ✅

Environment

• Elixir 1.6.1
• Erlang OTP20
• macOS High Sierra (10.13.2)

Current behavior

Running mix format using --dot-formatter with a .formatter.exs in a different folder - containing a import_deps configuration - results in the following error:

** (File.Error) could not read file "<PATH_TO_CURRENT_FOLDER>/_build/dev/.mix/cached_formatter_deps": no such file or directory
    (elixir) lib/file.ex:310: File.read!/1
    (mix) lib/mix/tasks/format.ex:193: Mix.Tasks.Format.read_deps_manifest/1
    (mix) lib/mix/tasks/format.ex:173: Mix.Tasks.Format.fetch_deps_opts/1
    (mix) lib/mix/tasks/format.ex:129: Mix.Tasks.Format.run/1
    (mix) lib/mix/task.ex:314: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:80: Mix.CLI.run_task/2

Expected behavior

mix format should format the file(s) using the configuration from .formatter.exs and respect :import_deps.

Research

I've tried to track the cause of the issue and it seems the root cause might be this line:

elixir/lib/mix/lib/mix/tasks/format.ex

Line 166 in ae8502d

deps_manifest = Path.join(Mix.Project.manifest_path(), @deps_manifest)

It seems that Mix.Project.manifest_path/0 always takes the current folder to expand the path. This is fairly easy to reproduce in iex.

# Let's assume we're in the root of an umbrella project
iex> Mix.Project.manifest_path
"<path-to-project>/_build/<env>"
iex> cd "apps"
iex> Mix.Project.manifest_path
"<path-to-project>/apps/_build/<env>"

This then results in mix format trying to read <path-to-project>/apps/_build/<env>/cached_formatter_deps which obviously can not be found and raises a File.Error.

[josevalim]

Can you please provide a sample app that reproduces the error? This will make it easier for us to track it down and write a patch. Thanks.