[GHC] #657: add WARNING pragma

[GHC]

#657: add WARNING pragma
-------------------------+--------------------------------------------------
    Reporter:  ijones    |        Owner:             
        Type:  task      |       Status:  new        
    Priority:  normal    |    Milestone:             
   Component:  Compiler  |      Version:  6.4.1      
    Severity:  normal    |     Keywords:             
          Os:  Unknown   |   Difficulty:  Easy (1 hr)
Architecture:  Unknown   |  
-------------------------+--------------------------------------------------
It might be nice to have a WARNING pragma in place of or in addition
 to the DEPRECATED pragma that just allows you to issue a generic
 warning:
 {{{
 {-# WARNING f, C, T "These are deprecated" #-}
 {-# WARNING g "This has terrible performance" #-}
 {-# WARNING T "We had to expose this type for bad reasons, don't use it."
 #-}
 {-# WARNING preConf "This is an unstable interface. May become
 deprecated." #-}
 {-# WARNING fromJust "Calls the error function.  use maybe for safety" #-}
 module Wibble {-# WARNING "Use Wibble_1 instead" #-}
 }}}

 This is more general than the DEPRECATED pragma.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/657>
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