feat(font): fontify arrow fn arguments (wip)

Author: Fuco1

typescript-mode.el

@@ -1015,7 +1015,7 @@ lines."
                         (progn
                           (forward-comment most-positive-fixnum)
                           (memq (char-after) '(?\, ?\; ?\] ?\) ?\}))))
-              do (forward-sexp)))
+                 do (forward-sexp)))
    while (and (eq (char-after) ?\n)
               (save-excursion
                 (forward-char)
@@ -1686,6 +1686,33 @@ point of view of font-lock.  It applies highlighting directly with
   ;; Matcher always "fails"
   nil)
 
+(defun typescript--function-argument-matcher (limit)
+  (condition-case nil
+      (save-restriction
+        (widen)
+        (narrow-to-region (point-min) limit)
+        (while (re-search-forward (rx (group (regexp typescript--name-re))
+                                      (* whitespace)
+                                      (group (or "," ":" ")")))
+                                  nil t)
+          (font-lock-apply-highlight '(1 font-lock-variable-name-face t))
+
+          (let ((ender (match-string 2)))
+            ;; we need to skip the type specification
+            (when (equal ender ":")
+              (when (re-search-forward (rx (or (and "," eol)
+                                               (and ")")
+                                               (and ","
+                                                    (* whitespace)
+                                                    (regexp typescript--name-re)
+                                                    (group (or ":" ")")))))
+                                       nil t)
+                (goto-char (match-beginning 0)))))))
+    ;; Conditions to handle
+    (scan-error nil)
+    (end-of-buffer nil))
+  nil)
+
 (defun typescript--in-documentation-comment-p ()
   "Reports whether point is inside a documentation comment."
   (let ((parse (syntax-ppss)))
@@ -1864,7 +1891,27 @@ and searches for the next token to be highlighted."
         (prog1 (save-excursion (re-search-forward ")" nil t))
           (backward-char))
         nil
-        (1 font-lock-variable-name-face))))
+        (1 font-lock-variable-name-face)))
+
+    ;; formal parameters in arrow function
+    ,(list
+      (rx (group "=>") (* whitespace) (? eol) (* whitespace) "{")
+      '(1 font-lock-keyword-face)
+      `(;; ,(rx (group (regexp typescript--name-re))
+        ;;      (* whitespace)
+        ;;      (? ":" (* whitespace) (regexp typescript--name-re) (* whitespace))
+        ;;      (or "," ")" eol))
+        typescript--function-argument-matcher
+        (prog1 (progn
+                 (backward-char)
+                 (typescript--backward-to-parameter-list)
+                 (point))
+          (backward-sexp))
+        (re-search-forward "{" nil t)
+        nil
+        ;; (1 font-lock-variable-name-face t t)
+        ))
+    )
   "Level three font lock for `typescript-mode'.")
 
 (defun typescript--flyspell-mode-predicate ()