Add fn for comparing line numbers.

Elixir is quite flexible wrt syntax around the `->` token. It can follow
`do` or `fn`, either on the same line or separately. Because of this, a
function that can detect whether it's on the same line as its parent was
needed.

After adding this function, all tests passed. I'm skeptical about the
robustness of this function but it will have to be battle hardened by
users reporting bugs.

Author: mattdeboard

elixir-smie.el

@@ -106,6 +106,10 @@
       (progn (move-end-of-line 1)
              (looking-back elixir-smie--block-operator-regexp)))))
 
+(defun elixir-smie--same-line-as-parent (parent-pos child-pos)
+  "Return non-nil if `child-pos' is on same line as `parent-pos'."
+  (= (line-number-at-pos parent-pos) (line-number-at-pos child-pos)))
+
 (defun elixir-smie-forward-token ()
   (cond
    ;; If there is nothing but whitespace between the last token and eol, emit
@@ -184,8 +188,29 @@
        (smie-rule-parent elixir-smie-indent-basic))))
 
     (`(:after . "->")
-     (if (smie-rule-hanging-p)
-         elixir-smie-indent-basic))
+     (cond
+      ;; This first condition is kind of complicated so I'll try to make this
+      ;; comment as clear as possible.
+
+      ;; "If `->' is the last thing on the line, and its parent token
+      ;; is `fn' ..."
+      ((and (smie-rule-hanging-p)
+            (smie-rule-parent-p "fn"))
+       ;; "... and if:
+
+       ;; 1. `smie--parent' is non-nil
+       ;; 2. the `->' token in question is on the same line as its parent (if
+       ;;    the logic has gotten this far, its parent will be `fn')
+
+       ;; ... then indent the line after the `->' aligned with the
+       ;; parent, offset by `elixir-smie-indent-basic'."
+       (if (and smie--parent (elixir-smie--same-line-as-parent
+                              (nth 1 smie--parent)
+                              (point)))
+           (smie-rule-parent elixir-smie-indent-basic)))
+      ;; Otherwise, if just indent by two.
+      ((smie-rule-hanging-p)
+       elixir-smie-indent-basic)))
 
     (`(:before . ";")
      (cond