Change to mix.exs directory before format

- 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:

- elixir-lang/elixir#7328 (comment)

- elixir-lang/elixir#7398 (comment)

elixir-format.el

@@ -84,8 +84,16 @@
     (setq buffer-read-only nil)
     (erase-buffer)))
 
+(defun elixir-format--temp-file-path ()
+  "Make a temp file in the current directory, because mix format
+applies rules based on path patterns and looks for .formatter.exs
+files in subdirectories."
+  (concat (file-name-sans-extension buffer-file-name)
+          "-emacs-elixir-format."
+          (file-name-extension buffer-file-name)))
+
 (defun elixir-format--run-format (called-interactively-p)
-  (let ((tmpfile (make-temp-file "elixir-format" nil ".ex"))
+  (let ((tmpfile (elixir-format--temp-file-path))
         (our-elixir-format-arguments (list (elixir-format--mix-executable) "format")))
 
     (write-region nil nil tmpfile)
@@ -95,7 +103,7 @@
       (setq our-elixir-format-arguments (append our-elixir-format-arguments elixir-format-arguments)))
     (setq our-elixir-format-arguments (append our-elixir-format-arguments (list tmpfile)))
 
-    (if (zerop (apply #'call-process (elixir-format--elixir-executable) nil (elixir-format--errbuff) nil our-elixir-format-arguments))
+    (if (zerop (elixir-format--from-mix-root (elixir-format--elixir-executable) (elixir-format--errbuff) our-elixir-format-arguments))
         (elixir-format--call-format-command tmpfile)
       (elixir-format--failed-to-format called-interactively-p))
     (delete-file tmpfile)
@@ -194,6 +202,26 @@ Shamelessly stolen from go-mode (https://github.com/dominikh/go-mode.el)"
          (delete-region (progn (forward-visible-line 0) (point))
                         (progn (forward-visible-line arg) (point))))))
 
+
+(defun elixir-format--from-mix-root (elixir-path errbuff format-arguments)
+  "Run mix format where `mix.exs' is located, because mix is
+meant to be run from the project root. Otherwise, run in the
+current directory."
+  (let ((original-default-directory default-directory)
+        (mix-dir (locate-dominating-file buffer-file-name "mix.exs")))
+
+    (when mix-dir
+      (setq default-directory (expand-file-name mix-dir)))
+
+    (message (concat "Run "
+                     (abbreviate-file-name default-directory) ": "
+                     (mapconcat 'identity format-arguments " ")))
+
+    (let ((result (apply #'call-process
+                         elixir-path nil errbuff nil format-arguments)))
+      (setq default-directory original-default-directory)
+      result)))
+
 (provide 'elixir-format)
 
 ;;; elixir-format.el ends here