[GHC] #13900: Core lint in BuildFlavour=perf-llvm

[GHC]

#13900: Core lint in BuildFlavour=perf-llvm
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by dfeuer):

 I can cut down `GHCi.UI.Tags` and still get a similar core lint error. As
 I cut it down, the mismatched types shifted around a bit; unfortunately I
 didn't capture the intermediate steps, so I can't show them. This version
 is cut down about as far as I could cut it:

 {{{#!hs
 module GHCi.UI.Tags (
   createTagsFile
 ) where

 import GHC
 import GHCi.UI.Monad

 data TagsKind
 data TagInfo

 createTagsFile :: TagsKind -> FilePath -> GHCi ()
 createTagsFile _tagskind _tagsFile = do
   _graph <- GHC.getModuleGraph
   mapM_ (listModuleTags . GHC.ms_mod) $ [undefined]

 {-# NOINLINE listModuleTags #-}
 listModuleTags :: GHC.Module -> GHCi [TagInfo]
 listModuleTags m =
   let {-# NOINLINE names #-}
       names = []
       localNames = filter ((m ==) . nameModule) names
   in mapM GHC.lookupName localNames >> return []
 }}}

 Lots of the little bits remaining seem to be necessary: the `filter`, the
 `GHC.getModuleGraph`, the `mapM` in `listModuleTags` rather than `mapM_`.
 I'll keep digging now that I've cut it down, but I figured I should record
 this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13900#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler