avoid posting when body only has mentions it

app/lib/bangtags.rb

@@ -1001,7 +1001,7 @@ class Bangtags
     text.strip!
     text = text.split("\n").map { |chunk| chunk.strip }.join("\n") if @strip_lines
 
-    if text.blank?
+    if text.blank? || has_only_mentions?(text)
       RemoveStatusService.new.call(@status)
     else
       status.text = text
@@ -1090,6 +1090,10 @@ class Bangtags
     end
   end
 
+  def has_only_mentions?(text)
+    text.match?(/^(?:\s*@((#{Account::USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?))+$/i)
+  end
+
   def add_tags(to_status, *tags)
     valid_name = /^[[:word:]:._\-]*[[:alpha:]:._·\-][[:word:]:._\-]*$/
     tags = tags.select {|t| t.present? && valid_name.match?(t)}.uniq

app/services/post_status_service.rb

@@ -55,6 +55,7 @@ class PostStatusService < BaseService
 
     raise Mastodon::LengthValidationError, I18n.t('statuses.replies_rejected') if recipient_rejects_replies?
     raise Mastodon::LengthValidationError, I18n.t('statuses.kicked') if kicked?
+    raise Mastodon::LengthValidationError, I18n.t('statuses.no_body') if has_only_mentions?
 
     return idempotency_duplicate if idempotency_given? && idempotency_duplicate?
 
@@ -98,6 +99,10 @@ class PostStatusService < BaseService
     @in_reply_to.present? && ConversationKick.where(account_id: @account.id, conversation_id: @in_reply_to.conversation_id).exists?
   end
 
+  def has_only_mentions?
+    @options[:nomentions].blank? && @text.match?(/^(?:\s*@((#{Account::USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?))+$/i)
+  end
+
   def mark_recipient_known
     @in_reply_to.account.mark_known! unless !Setting.auto_mark_known || @in_reply_to.account.known?
   end

config/locales/en.yml

@@ -976,6 +976,7 @@ en:
       vote: Vote
     replies_rejected: 'The author is not accepting replies to this roar.'
     kicked: "You do not have the author's permission to reply to this thread."
+    no_body: "Your post has mentions without any body text."
     show_more: Show more
     sign_in_to_participate: Sign in to participate in the conversation
     title: '%{name}: "%{quote}"'