Change to mix.exs directory before format

[michaelvobrien]

• Set the default-directory to where mix.exs is located, because mix
  is meant to be run from the project root directory. Also, mix format
  looks for .formatter.exs files in subdirectories.

• If mix.exs is NOT found, the original default-directory and
  default formatter are used for mix format.

• Create the temp file in the current directory, because mix format
  uses path patterns.

See:

• Fix - Allow to use a dot_formatter config file in a different folder which imports dependencies elixir-lang/elixir#7328 (comment)

• Allow subdirectories in .formatter.exs elixir-lang/elixir#7398 (comment)

[Trevoke]

This is related to #415.

[axelson]

Will this work with an umbrella project where the .formatter.exs to be used is in the umbrella root? I ask because it seems like the code will pick up on the apps/web_project/mix.exs without looking for the the root mix.exs.

[Trevoke]

This issue is mostly related to #415 and #421 -- in #415, it came up that the root formatter in an umbrella app had an umbrella option that could be created. This indicates that the problem may be a tad more complex: if there are two formatter.exs, one in the umbrella app and one in the root, which one should take precedence?

Looking at the code now I'm a tad worried because default-directory is an emacs-provided variable and I don't much like modifying them. We need to look into what can be done to not rely on that value.

[Trevoke]

1. As per the issue mentioned by @axelson I wrote something on the plane today which may solve the problem, assuming we want to give the top-most formatter precedence: https://github.com/Trevoke/lsp-elixir/blob/master/lsp-elixir.el#L71

2. As for not overriding default-directory, I think it's slightly cleaner to do something like this:

(with-temp-buffer
  (find-file directory-to-visit)
  (do-stuff))