Gray out ignored variables

This commit grays out variables whose name start with `_`. These
variables are meant to be ignored in the program. The fontification of
the single underscore matcher (`_`) and pseudo-variables like
`__MODULE__` & company where not modified.

See #292.

Fixes #292

Author: rranelli

elixir-mode.el

@@ -112,6 +112,16 @@
   "For use with atoms & map keys."
   :group 'font-lock-faces)
 
+(defvar elixir-ignored-var-face 'elixir-ignored-var-face)
+(defface elixir-ignored-var-face
+  '((((class color) (min-colors 88) (background light))
+     :foreground "#424242")
+    (((class color) (background dark))
+     (:foreground "#616161"))
+    (t nil))
+  "For use with ignored variables (starting with underscore)."
+  :group 'font-lock-faces)
+
 (eval-when-compile
   (defconst elixir-rx-constituents
     `(
@@ -405,6 +415,14 @@ is used to limit the scan."
                      (one-or-more "\n")))
      1 font-lock-variable-name-face)
 
+    ;; Gray out variables starting with "_"
+    (,(elixir-rx symbol-start
+                 (group (and "_"
+                             (any "A-Z" "a-z" "0-9"))
+                        (zero-or-more (any "A-Z" "a-z" "0-9" "_"))
+                        (optional (or "?" "!"))))
+     1 elixir-ignored-var-face)
+
     ;; Map keys
     (,(elixir-rx (group (and (one-or-more identifiers) ":")))
      1 elixir-atom-face)

test/elixir-mode-font-test.el

@@ -426,6 +426,33 @@ end
 "
    (should (eq (elixir-test-face-at 33) 'font-lock-string-face))))
 
+(ert-deftest elixir-mode-syntax-table/gray-out-ignored-var ()
+  "https://github.com/elixir-lang/emacs-elixir/issues/292"
+  :tags '(fontification syntax-table)
+  (elixir-test-with-temp-buffer
+   "variable
+    _var
+    _x
+    _x!
+    _x?
+    _
+    __MODULE__
+"
+   (should (eq (elixir-test-face-at 4) nil))
+   (should (eq (elixir-test-face-at 14) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 15) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 23) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 24) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 30) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 32) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 38) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 40) 'font-lock-comment-face))
+   (should (eq (elixir-test-face-at 46) 'font-lock-constant-face))
+   (should (eq (elixir-test-face-at 46) 'font-lock-constant-face))
+   (should (eq (elixir-test-face-at 52) 'font-lock-constant-face))
+   (should (eq (elixir-test-face-at 53) 'font-lock-constant-face))
+   (should (eq (elixir-test-face-at 55) 'font-lock-constant-face))))
+
 (provide 'elixir-mode-font-test)
 
 ;;; elixir-mode-font-test.el ends here