transient--expand-define-args: Make available at compile-time

That makes it possible to use `transient-define-' in "transient.el"
itself; as we started to do in the previous commit.

lisp/transient.el

@@ -1002,30 +1002,31 @@ example, sets a variable, use `transient-define-infix' instead.
 (put 'transient--default-infix-command 'interactive-only t)
 (put 'transient--default-infix-command 'command-modes (list 'not-a-mode))
 
-(defun transient--expand-define-args (args &optional arglist)
-  (unless (listp arglist)
-    (error "Mandatory ARGLIST is missing"))
-  (let (class keys suffixes docstr)
-    (when (stringp (car args))
-      (setq docstr (pop args)))
-    (while (keywordp (car args))
-      (let ((k (pop args))
-            (v (pop args)))
-        (if (eq k :class)
-            (setq class v)
-          (push k keys)
-          (push v keys))))
-    (while (let ((arg (car args)))
-             (or (vectorp arg)
-                 (and arg (symbolp arg))))
-      (push (pop args) suffixes))
-    (list (if (eq (car-safe class) 'quote)
-              (cadr class)
-            class)
-          (nreverse keys)
-          (nreverse suffixes)
-          docstr
-          args)))
+(eval-and-compile
+  (defun transient--expand-define-args (args &optional arglist)
+    (unless (listp arglist)
+      (error "Mandatory ARGLIST is missing"))
+    (let (class keys suffixes docstr)
+      (when (stringp (car args))
+        (setq docstr (pop args)))
+      (while (keywordp (car args))
+        (let ((k (pop args))
+              (v (pop args)))
+          (if (eq k :class)
+              (setq class v)
+            (push k keys)
+            (push v keys))))
+      (while (let ((arg (car args)))
+               (or (vectorp arg)
+                   (and arg (symbolp arg))))
+        (push (pop args) suffixes))
+      (list (if (eq (car-safe class) 'quote)
+                (cadr class)
+              class)
+            (nreverse keys)
+            (nreverse suffixes)
+            docstr
+            args))))
 
 (defun transient--parse-child (prefix spec)
   (cl-etypecase spec