[GHC] #14757: ghc recompilation check doesn't take into account headers directly included by CApiFFI

[GHC]

#14757: ghc recompilation check doesn't take into account headers directly included
by CApiFFI
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:  capi capiffi   |  Operating System:  Unknown/Multiple
  cpp                                |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Continuing from #14756, given the following simple module

 {{{#!hs
 {-# LANGUAGE CPP, CApiFFI #-}

 #include "Bar.hs.inc"

 module Foo where

 foreign import capi unsafe "cbits.h foo" c_foo :: Int -> IO ()
 }}}

 After having compiled `ghc -c Foo.hs` successfully, and second invocation
 of `ghc -c Foo.hs`

 - ...will properly recompile when `Bar.hs.inc` changes (or any of its
 recursive `#include`s!) (good!)
 - ...will fail to detect when `cbits.h` changes, and thus claim
 `compilation IS NOT required` (BAD!)

 Ideally, we should always run CPP for the CApiFFI-included, in order to
 detect when any recursive `#include`s inside `cbits.h` change, and use a
 recorded checksum to detect whether recompilation of the C wrapper (and
 thus re-linking of the haskell module; which currently forces
 resynthesizing the object files) is necessary.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14757>
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