Allow file template :trigger to be a function

modules/feature/file-templates/autoload.el

@@ -18,9 +18,13 @@ these properties:
     Provides a secondary predicate. This function takes no arguments and is
     executed from within the target buffer. If it returns nil, this rule will be
     skipped over.
-  :trigger
-    The yasnippet trigger keyword used to trigger the target snippet. If
-    omitted, `+file-templates-default-trigger' is used.
+  :trigger STRING|FUNCTION
+    If a string, this is the yasnippet trigger keyword used to trigger the
+      target snippet.
+    If a function, this function will be run in the context of the buffer to
+      insert a file template into. It is given no arguments and must insert text
+      into the current buffer manually.
+    If omitted, `+file-templates-default-trigger' is used.
   :mode SYMBOL
     What mode to get the yasnippet snippet from. If omitted, either PRED (if
     it's a major-mode symbol) or the mode of the buffer is used.
@@ -69,6 +73,8 @@ evil is loaded and enabled)."
         (user-error "Couldn't determine mode for %s file template" pred))
       (unless trigger
         (setq trigger +file-templates-default-trigger))
+      (if (functionp trigger)
+          (funcall trigger)
         (require 'yasnippet)
         (unless yas-minor-mode
           (yas-minor-mode-on))
@@ -81,7 +87,7 @@ evil is loaded and enabled)."
                    (and yas--active-field-overlay
                         (overlay-buffer yas--active-field-overlay)
                         (overlay-get yas--active-field-overlay 'yas--field)))
-        (evil-initialize-state 'insert)))))
+          (evil-initialize-state 'insert))))))
 
 ;;;###autoload
 (defun +file-templates-get-short-path ()