Use dispatch.vim's new -dir= and =expr

This enables using :0FocusCommand and :.FocusCommand to see the exact
rake task that will run.

Author: tpope

autoload/rails.vim

@@ -4859,15 +4859,28 @@ function! rails#buffer_setup() abort
   let &l:makeprg = self.app().rake_command('static')
   let &l:errorformat .= ',chdir '.escape(self.app().path(), ',')
 
-  if self.type_name('test', 'spec', 'cucumber')
-    call self.setvar('dispatch', ':Runner')
-  elseif self.name() ==# 'Rakefile'
-    call self.setvar('dispatch', ':Rake --tasks')
-  elseif self.name() =~# '^\%(app\|config\|db\|lib\|log\|README\)'
-    call self.setvar('dispatch', ':Rake')
-  elseif self.name() =~# '^public'
+  if exists(':Dispatch') == 2 && !exists('g:autoloaded_dispatch')
+    runtime! autoload/dispatch.vim
+  endif
+  if exists('*dispatch#dir_opt')
+    let dir = dispatch#dir_opt(self.app().path())
+  endif
+
+  if self.name() =~# '^public'
     call self.setvar('dispatch', ':Preview')
+  elseif self.type_name('test', 'spec', 'cucumber')
+    call self.setvar('dispatch', ':Runner')
+  elseif self.name() =~# '^\%(app\|config\|db\|lib\|log\|README\|Rakefile\)'
+    if exists('dir')
+      call self.setvar('dispatch',
+            \ dir . '-compiler=rails ' .
+            \ self.app().rake_command('static') .
+            \ ' `=rails#buffer(' . self['#'] . ').default_rake_task(v:lnum)`')
+    else
+      call self.setvar('dispatch', ':Rake')
+    endif
   endif
+
   if empty(self.getvar('start'))
     call self.setvar('start', ':Server')
   endif