[GHC] #9352: Allow `State# s` argument/result types in `ccall` FFI imports

[GHC]

#9352: Allow `State# s` argument/result types in `ccall` FFI imports
-------------------------------------+-------------------------------------
              Reporter:  hvr         |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:  #9218
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by hvr):

 As first proof-of-concept, I made the type-checker allow `State# s` to see
 if it would just-work(tm), here's the Cmm output resulting from the `foo`
 declaration:

 {{{#!c
 [section "data" {
      M.c_foo_closure:
          const M.c_foo_info;
  },
  M.c_foo_entry() //  [R2]
          { info_tbl: [(c1Cx,
                        label: M.c_foo_info
                        rep:HeapRep static { Fun {arity: 2 fun_type:
 ArgSpec 4} })]
            stack_info: arg_space: 8 updfr_space: Just 8
          }
      {offset
        c1Cx:
            _s1Cl::I64 = R2;
            goto c1Cz;
        c1Cz:
            _c1Cv::I64 = foo;
            _c1Cw::I64 = _s1Cl::I64;
            call "ccall" arg hints:  [‘signed’]  result hints:  []
 (_c1Cv::I64)(_c1Cw::I64);
            call (P64[Sp])() args: 8, res: 0, upd: 8;
      }
  }]
 }}}


 Compiling `bar` however, results in

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.9.20140723 for x86_64-unknown-linux):
         resultWrapper (# State# s, Int# #)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9352#comment:1>
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